Abstract:
The disclosed ultrasonic hydrometer determines the specific gravity (density) of the electrolyte of a wet battery, such as a lead-acid battery. The hydrometer utilizes a transducer that when excited emits an ultrasonic impulse that traverses through the electrolyte back and forth between spaced sonic surfaces. The transducer detects the returning impulse, and means measures the time &#34;t&#34; between the initial and returning impulses. Considering the distance &#34;d&#34; between the spaced sonic surfaces and the measured time &#34;t&#34;, the sonic velocity &#34;V&#34; is calculated with the equation &#34;V=2d/t&#34;. The hydrometer also utilizes a thermocouple to measure the electrolyte temperature. A hydrometer database correlates three variable parameters including sonic velocity in and temperature and specific gravity of the electrolyte, for temperature values between 0° and 40° C. and for specific gravity values between 1.05 and 1.30. Upon knowing two parameters (the calculated sonic velocity and the measured temperature), the third parameter (specific gravity) can be uniquely found in the database. The hydrometer utilizes a microprocessor for data storage and manipulation. The disclosed modified battery has a hollow spacer nub on the battery side wall, the sonic surfaces being on the inside of the nub and the electrolyte filling between the surfaces to the exclusion of intervening structure. An accessible pad exposed on the nub wall opposite one sonic surface allows the reliable placement thereagainst of the transducer.

Description:
CONTRACTUAL ORIGIN OF THE INVENTION 
     The United States Government has rights in this invention pursuant to Contract No. W-31-109-ENG-38 between the U.S. Department of Energy and the University of Chicago representing Argonne National Laboratory. 
    
    
     BACKGROUND OF THE INVENTION 
     In a conventional lead-acid battery, a battery case defines interior cells, and spaced plates or electrodes are located within each cell submerged in a battery electrolyte. The lead-acid battery generally uses highly reactive sponge lead for the negative electrode, lead dioxide for the positive electrode, and a liquid solution of sulfuric acid for the electrolyte. The generalized equation for the electrochemical reaction within the cell when discharging and when being charged is: ##STR1## 
     This electrochemical reaction causes a measurable change in the specific gravity or density of the electrolyte. In determining the state-of-charge or operational readiness of a lead-acid battery, information regarding the specific gravity or density of the electrolyte in addition to voltage current, and temperature thus is desired. The specific gravity of the electrolyte in a lead-acid battery changes between a low value of 1.05-1.15 when the battery is in a discharged state and a high value of 1.25-1.35 when the battery is in a fully charged state. 
     Knowing this, it has been commonplace to measure with a hydrometer the specific gravity of the battery electrolyte to determine the vitality of the battery. To use the hydrometor, the battery filler cap must be removed in order to insert a hydrometer tube into the battery cell so as to draw the electrolyte into the hydrometer. The hydrometer has a float that bobs within the electrolyte to a specific depth, and this float is visually sighted against a scale to determine a specific gravity reading. For good accuracy, the reading must be adjusted for the difference between the electrolyte temperature and a normalized temperature, commonly considered to be 25° C., since the specific gravity varies substantially as a function of temperature. 
     Use of the hydrometer can be difficult and/or dangerous and/or tedious. For example, the battery can be located in a vehicle chassis where it could be difficult to obtain the electrolyte sample and/or have good sight of the positioned hydrometer. The battery electrolyte must be taken from the battery so that the possibility exists that it can be spilled onto the vehicle or onto the person, causing damage to either, or it can be contaminated before being put back into the battery. Moreover, as the float must be free to bob or move within the hydrometer, the hydrometer must be precisely oriented in a vertical manner when taking the reading. Once the electrolyte is discharged from the hydrometer back into the battery cell, the reading is lost and there is no permanent record. The required removal of the battery filler caps, etc. can be quite tedious if a large number of batteries must be charged and checked on a regular basis. Moreover, a true test would have to be made on all six battery cells; but this is not done as a matter of course and only a rough approximation of the battery condition is obtained by testing just one of the battery cells. Moreover, a maintenance-free battery does not even have a removable cap so that the float type hydrometer technique of testing the battery cannot even be used. 
     SUMMARY OF THE INVENTION 
     This invention relates to an improved design of a typical wet cell battery, such as a lead-acid battery, that allows the use of improved apparatus for and method of determining this condition of the battery by specifically measuring the speed or velocity of a pulsed sound wave through the liquid electrolyte and calibrating this against a database including a correction for temperature to find the specific gravity of the electrolyte. 
     This invention specifically provides, in one preferred embodiment, for determining the specific gravity of the electrolyte in the typical lead-acid battery without directly contacting the electrolyte. This allows the invention to be used both on batteries having removable water filler caps but without removing the caps and/or on maintanance-free batteries formed without removable filler caps. 
     The velocity of sound in a liquid is governed by the specific gravity or density and the compressibility of the liquid, and is given by the equation: ##EQU1## V=Velocity ρ=Density 
     β=Isothermal Compressibility 
     An alternative to the calculation form Eq. (2) involves the experimental generation of a database relating the specific gravity, temperature, and sonic velocity for appropriate concentration of the electrolyte, such as sulfuric acid, and using this database to find a missing value of any one of them when the other two are known. Thus, once a database is available, the specific gravity could be determined by knowing the sonic velocity and temperature of the electrolyte. 
     The properties of sound transmission are being used in many different applications, including level or flow detection of gases and liquids and nondestructive fault detection in solids. The basic theory utilizes the different transmission and reflection characteristics of the sonic wave at a boundary between dissimilar media. When a sound wave strikes the boundary between two different transmission media, part of the energy is reflected and part is transmitted. With a battery case of polypropylene having a sonic surface located below the level of the electrolyte, a pulse-echo technique would work by placing a transducer on the outside case of the battery. The sonic wave would be transmitted through the polypropylene to the electrolyte and would reflect off the opposite boundary, either a spaced second sonic surface or the surface of the electrolyte, to return back toward the first sonic surface to be detected then by the transducer receiver. 
     A specific object of this invention is to provide a battery design having spaced sonic surfaces inside the battery case that are constantly under the level of the battery electrolyte, where there is no intervening structure between these sonic surfaces to allow for the sonic pulse transmission back and forth between the surfaces. The spaced sonic surfaces can be spaced vertically, horizontally, or at an angle. Some existing batteries have pairs of spacer nubs formed on the exterior of the side walls for maintaining battery separation from adJacent structures for air cooling the battery. In one battery embodiment, it is contemplated that these spacer nubs can be enlarged, can be made hollow, and the spaced sonic surfaces could be formed at the opposite interior ends of the nubs. 
     The invention provides further an ultrasonic hydrometer apparatus having a probe that can be positioned against the outside of the battery case wall opposite one of the sonic surfaces. Upon being activated, the probe as positioned would transmit an ultrasonic impulse through the battery case wall and from the one surface through the battery electrolyte to reflect off the spaced boundary and/or sonic surface and travel again through the electrolyte to be detected by the same probe. The lapsed time for this impulse to travel back and forth between the spaced sonic surfaces would be measured, and the sonic velocity in the liquid would be determined. 
     To utilize the pulse-echo method, a 5 MHz transducer is used for both sending and receiving the signal. An initial pulse from a generator drives the transducer, and resets and starts a timer. The pulse propagates through the electrolyte and is reflected back and detected by the transducer. The received echo is conditioned and is used to stop the timer. The time measured is the time required for the pulse to travel twice the distance &#34;d&#34; between the spaced sonic surfaces. The velocity can be determined from this data by using the equation 
     
         V=2d/t                                                     Eq. (3) 
    
     where 
     d=distance between sonic surfaces; 
     t=total pulse transit time. 
     One general object of this invention is to provide a portable ultrasonic hydrometer apparatus to allow an operator (tester) to go to and test a battery; and further whereupon a probe of the apparatus need only be applied against the exterior of the battery case without removing any battery caps or the like. A thermocouple further is located in the probe to measure the temperature of the battery electrolyte so that proper adjustments in the readings can be made to a reference temperature. A modified ultrasonic portable hydrometer apparatus is also disclosed which closely resembles a conventional battery testing hydrometer in that to use it some battery electrolyte is withdrawn from the battery to fill a closed ended housing having the spaced sonic surfaces and transducers; but the operation thereafter of these two embodiments would generally be similar. 
     It is contemplated that after obtaining the two inputs, namely the sonic velocity in and the temperature of the battery electrolyte, these inputs would be compared against an appropriate database so that a single specific gravity can be correlated to these paired inputs, to apprise of the corresponding relative condition of the battery. 
     The database disclosed herein has been obtained experimentally and correlates the sonic velocity of sulfuric acid for specific gravities in the range approximately between 1.05 and 1.30 for temperatures in the range between approximately 0°-40° C. Within these ranges, there is a unique or singular velocity at which sound will travel through the electrolyte for each paired combination of specific gravity and temperature. In other words, having a paired combination input for the sonic velocity and for the temperature, a singular specific gravity can be determined. However, beyond these ranges, it is possible to have dual values of specific gravity for a single input combination of sonic velocity and temperature. To eliminate this dual value phenomenon, the apparatus is constrained to operate only within these specific temperature and electrolyte concentration ranges; which is where the lead acid battery operates anyway. Of interest, at the specific gravity of approximately 1.24, the sonic velocity through the battery electrolyte is generally independent of temperature in this range and is approximately 1565 meters per second. 
     A specific microprocessor located in the ultrasonic hydrometer apparatus would have the necessary database and program for correlating lapsed time of the sonic impulses, and the temperature and the specific gravity of the electrolyte in question. 
     The invention allows the specific gravity of the battery electrolyte to be measured quickly and accurately--to the same, if not better degree of accuracy than that of a conventional float type hydrometer. Moreover, the ultrasonic hydrometer apparatus after once having been calibrated, could be operated by untrained personnel. There is no need for sight tubes and/or for maintaining a free floating bob, as in the hydrometer. With the basic invention, the measuring apparatus probe need not contact the battery electrolyte, and the specific gravity output could be given directly in digital format and retained until it is intentionally cancelled. Even the hydrometer type ultrasonic apparatus would have advantages over the conventional float hydrometer, since the tube need not be removed from the battery cell (less chances of spillage, damage or contamination) in order to obtain a reading, and the reading could be obtained almost instantaneously without the need for a specific vertical orientation of the apparatus. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of one embodiment of the subject ultrasonic hydrometer apparatus shown in operative association with a wet battery for determining the specific gravity of the battery electrolyte or liquid; 
     FIG. 2 is a sectional view as seen generally from line 2--2 in FIG. 1; 
     FIG. 3 is a sectional view as seen generally from line 3--3 in FIG. 1, illustrating a modification that can be made to a battery to make it particularly suited for allowing the use of the subject invention; 
     FIG. 4 is a perspective view of an alternate embodiment of the subject ultrasonic hydrometer apparatus shown in operative association with a wet battery having removable water filler caps, where some battery electrolyte is moved from the battery into the apparatus for determining the specific gravity of the electrolyte; 
     FIG. 5 is a schematic block diagram of the operative control used in the probe apparatus disclosed herein; 
     FIG. 6 is a graphic illustration of the database, used in the practice of the subject invention, correlating the sonic velocity in and specific gravity and temperature of the battery electrolyte. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In FIG. 1, a battery 10 is illustrated having an exterior case 12 divided by separators into three cells 13, 14 and 15 as illustrated. Battery plates 16p and 16n are alternately spaced adjacent one another in each cell and a battery liquid or electrolyte fills each cell covering the plates. In the battery illustrated, the positive and negative plates 16p and 16n respectively, of the three cells 13, 14 and 15 would typically be connected together internally of the case by buss bars 17p and 17n and ultimately exposed at positive and negative terminals 18p and 18n, respectively. In a three-cell battery, the battery output between the terminals would be approximately 6 volts. 
     The case 12 is typically formed of polypropylene. This case construction is resistant against the electrolyte, further is liquid-tight to confine the electrolyte within each cell, and further effectively transmits ultrasonic impulses. The battery case 12 illustrated specifically has a top wall 20, a bottom wall 22, opposed pairs of side walls 23, 24 and 25 and 26, and the separator walls 27 and 28 located specifically between the battery cells. 
     Opposing surfaces are formed on the battery case walls, under the level of the electrolyte, and can be used for practicing the particular invention. As illustrated in FIGS. 1, 2 and 3, in cell 13 there are opposed surfaces 30a and 30b that define therebetween a sonic path 30, and opposed surfaces 31a and 31b that define therebetween a sonic path 31; whereas cell 14 shows opposed surfaces 32a and 32b defining therebetween sonic path 32, and opposed surfaces 33a and 33b defining therebetween sonic path 33; while cell 15 shows three sets of opposed surfaces, surfaces 34a and 34b defining therebetween sonic path 34, surfaces 35a and 35b defining therebetween sonic path 35, and surfaces 36a and 36b defining sonic path 36. 
     Referring also now to FIGS. 2 and 3, the sonic path 32 extends between the opposing surfaces 32a and 32b formed on the top wall 20 and bottom wall 22, respectively, and there is no intervening structure therebetween, such as any of battery plates 16p and 16n, or wall structure. A pad area 40 is preferably defined on the exterior of top wall 20 proximate the sonic path 32, operable to highlight the location of the sonic path 32 and to allow for good sonic conductivity as will be noted. 
     An ultrasonic hydrometer apparatus 42, illustrated in FIGS. 1, 2 and 5, includes a probe head 44 having therein a transducer 45 and a thermocouple 46. The apparatus further includes a housing 47 within which is located a pulser 48, battery or power source 49, conditioning electronics package 50, timer 51, and microprocessor 52 including a memory for a database and a control program. The apparatus further has in the housing 47 a mode selector switch 53, a thumb wheel switch 54, digital displays 55g, 55d, 55t for indicating the specific gravity, distance and temperature, and an on/off switch 56. A cable 58 having the appropriate connecting conductors therein extends between the housing 47 and the probe head 44. 
     As illustrated schematically in FIG. 5, the pulser 48 is connected to the transducer 45 and when energized would excite the transducer to emit then an ultrasonic signal. The transducer 45 would be mounted in the probe head 44 so that its output face could be pressed flush against the pad area 40 (see FIGS. 2 and 3) for transmitting this signal effectively to the battery case wall. The sound would be transmitted then through the battery case wall 20, and from sonic surface 32a through the battery electrolyte to reflect off the opposite sonic surface 32b and travel back through the electrolyte to be picked up by the transducer 45, now acting as a receiver. The pulser 48 also is connected to the timer 51 to start the timing cycle when the transducer is initially activated, and the transducer 45 is also connected through an electronic package 50 to the timer so that the returning signal detected by the transducer 45 is also impulsed to the timer to stop the timer. The electronics package 50 (not shown in detail) would include a blocking gate to preclude the initial transducer pulse from passing on to the timer stop control but which thereafter would open to allow the echo pulse through; filter and amplifier means to sort out and amplify the returning signals and a comparator to pass only the real impulse signal which will be much stronger than any secondary or background type return pulses the transducer will be receiving. The timer 51 thus measures the time it takes for the sonic impulse to travel through the electrolyte back and forth (or twice the distance between) the spaced sonic surfaces 32a and 32b, and this determination is passed on as an input to the microprocessor 52. The thermocouple 46 on the probe head 44 measures the battery case temperature, which under steady conditions would approximate the battery electrolyte temperature, and inputs this also to the microprocessor 52. 
     Thus, two inputs are obtained: the transit time &#34;t&#34; for the sonic pulse to travel twice the distance &#34;d&#34; between the spaced sonic surfaces 30a and 30b, and the temperature of the battery electrolyte. Knowing the distance &#34;d&#34; between the spaced surfaces 32a and 32b(or the surfaces of any of the other sonic paths), the sonic velocity can be directly determined by the equation 
     
         V=2d/t                                                     Eq. (3) 
    
     where 
     V=sonic velocity in the electrolyte, 
     d=distance between the sonic surfaces, 
     t=total time between sending and receiving impulses. 
     The microprocessor 52 would thereupon process this calculation to obtain the sonic velocity through the electrolyte. The temperature of the electrolyte would be used as another input value. The microprocessor further would have in memory a database uniquely correlating the sonic velocity in the electrolyte at varying temperatures in the range between approximately 0° and 40° and at varying specific densities or gravities in the range between approximately 1.05 and 1.30 for the electrolyte of sulfuric acid (H 2  SO 4 ). The graphic representation of this database is shown in FIG. 6, and specific values are arranged in matrix format in Table I. Having this database and two parameter inputs of the three interrelated parameters, the third parameter can be determined. 
     
                                           TABLE I__________________________________________________________________________Database Values Correlating Sonic Velocity, Specific Gravity andTemperature in Electrolyte H.sub.2 SO.sub.4TEMP(DEG    SPECIFIC GRAVITYC.)   1.026   1.050        1.080             1.108                  1.131                       1.160                            1.177                                 1.198                                      1.228                                           1.251                                                1.274                                                     1.304__________________________________________________________________________0  1403.167   1410.267        1429.753             1449.969                  1471.640                       1498.049                            1513.135                                 1532.051                                      1551.864                                           1568.343                                                1577.938                                                     1585.7581  1407.052   1414.538        1432.480             1451.796                  1473.774                       1499.544                            1515.391                                 1532.594                                      1552.003                                           1568.200                                                1577.506                                                     1585.0302  1411.765   1418.486        1435.574             1455.403                  1475.662                       1501.108                            1516.455                                 1533.138                                      1552.142                                           1568.058                                                1577.218                                                     1584.1583  1415.813   1423.042        1439.041             1458.228                  1477.807                       1502.217                            1517.254                                 1533.682                                      1552.282                                           1567.916                                                1576.930                                                     1583.7234  1420.352   1427.156        1442.404             1461.064                  1479.832                       1503.655                            1518.320                                 1534.363                                      1552.421                                           1567.774                                                1576.642                                                     1583.2885  1424.685   1430.937        1445.783             1464.159                  1481.990                       1505.227                            1519.254                                 1534.909                                      1552.561                                           1567.631                                                1576.211                                                     1582.8526  1429.044   1435.216        1449.178             1467.018                  1484.154                       1506.670                            1520.324                                 1535.454                                      1552.700                                           1567.489                                                1575.923                                                     1582.4187  1433.074   1439.400        1452.467             1469.450                  1486.004                       1508.117                            1521.395                                 1536.000                                      1552.840                                           1567.347                                                1575.492                                                     1581.9838  1437.245   1443.006        1455.771             1472.142                  1488.180                       1509.434                            1522.467                                 1536.820                                      1553.049                                           1567.205                                                1575.205                                                     1581.5499  1441.441   1446.630        1458.967             1474.780                  1490.041                       1510.754                            1523.407                                 1537.367                                      1553.258                                           1567.134                                                1574.774                                                     1581.11510 1445.179   1450.029        1462.051             1477.365                  1492.164                       1512.274                            1524.280                                 1538.051                                      1553.398                                           1567.063                                                1574.631                                                     1580.68111 1448.692   1453.689        1464.779             1479.896                  1494.228                       1513.731                            1525.222                                 1538.736                                      1553.677                                           1566.992                                                1574.488                                                     1580.39112 1452.223   1456.630        1467.516             1482.308                  1496.298                       1515.059                            1526.165                                 1539.421                                      1553.817                                           1566.921                                                1574.344                                                     1579.95813 1455.648   1459.953        1470.388             1484.919                  1497.725                       1516.189                            1527.176                                 1539.970                                      1554.097                                           1566.779                                                1573.914                                                     1579.09214 1458.967   1463.043        1473.146             1486.835                  1499.935                       1517.520                            1527.987                                 1540.656                                      1554.376                                           1566.779                                                1573.914                                                     1579.09215 1462.548   1466.707        1475.914             1489.655                  1501.499                       1518.854                            1528.933                                 1541.343                                      1554.656                                           1566.708                                                1573.627                                                     1578.80316 1465.649   1469.388        1478.693             1491.262                  1503.197                       1519.989                            1529.948                                 1541.894                                      1554.936                                           1566.636                                                1573.484                                                     1578.37117 1469.138   1472.769        1481.228             1493.453                  1504.768                       1521.328                            1530.897                                 1542.582                                      1555.216                                           1566.565                                                1573.341                                                     1577.93818 1472.393   1475.662        1484.026             1495.651                  1506.473                       1522.534                            1531.711                                 1543.133                                      1555.496                                           1566.494                                                1573.054                                                     1577.65019 1475.536   1478.566        1486.452             1498.894                  1508.182                       1523.742                            1532.798                                 1543.546                                      1555.776                                           1566.452                                                1572.911                                                     1577.36220 1478.566   1481.609        1488.885             1500.496                  1510.028                       1524.953                            1533.546                                 1544.236                                      1556.409                                           1566.409                                                1572.625                                                     1577.07421 1481.481   1484.409        1491.198             1502.347                  1511.877                       1526.030                            1534.363                                 1544.788                                      1556.336                                           1566.352                                                1572.339                                                     1576.64222 1484.281   1487.091        1493.647             1504.113                  1513.334                       1526.974                            1535.181                                 1545.341                                      1556.476                                           1566.338                                                1572.195                                                     1576.35523 1487.219   1489.912        1496.233             1505.817                  1515.191                       1528.189                            1536.000                                 1545.894                                      1556.757                                           1566.310                                                1571.909                                                     1575.92324 1489.784   1492.357        1498.569             1507.788                  1516.654                       1529.271                            1536.956                                 1546.586                                      1557.037                                           1566.281                                                1571.766                                                     1575.63625 1492.744   1495.068        1500.782             1509.500                  1518.253                       1530.423                            1537.777                                 1547.139                                      1557.318                                           1566.253                                                1571.624                                                     1575.20526 1495.198   1497.400        1502.870             1511.348                  1519.722                       1531.508                            1538.462                                 1547.694                                      1557.458                                           1566.225                                                1571.338                                                     1574.77427 1497.790   1499.606        1505.227             1513.069                  1521.328                       1532.730                            1539.352                                 1548.248                                      1557.739                                           1566.211                                                1571.052                                                     1574.48828 1500.130   1501.956        1506.802             1515.125                  1522.668                       1533.682                            1540.176                                 1548.803                                      1558.020                                           1566.182                                                1570.909                                                     1574.20129 1502.739   1504.178        1509.039             1516.787                  1524.146                       1534.636                            1541.137                                 1549.359                                      1558.301                                           1566.154                                                1570.766                                                     1573.91430 1505.227   1506.408        1511.018             1518.587                  1525.626                       1535.591                            1541.825                                 1549.776                                      1558.582                                           1566.125                                                1570.624                                                     1573.48431 1507.327   1508.643        1412.870             1519.923                  1527.109                       1536.410                            1542.513                                 1550.332                                      1558.863                                           1566.097                                                1570.481                                                     1573.19732 1509.830   1510.622        1514.992             1521.596                  1528.324                       1537.503                            1543.339                                 1550.749                                      1559.145                                           1566.069                                                1570.195                                                     1572.91133 1511.943   1512.473        1516.854             1523.138                  1529.677                       1538.462                            1544.167                                 1551.117                                      1559.285                                           1566.040                                                1570.053                                                     1572.62534 1513.798   1514.328        1518.587             1524.482                  1530.626                       1539.352                            1544.857                                 1551.864                                      1559.356                                           1566.012                                                1569.767                                                     1572.33935 1516.055   1516.322        1520.019             1525.963                  1531.983                       1540.313                            1545.617                                 1552.282                                      1559.567                                           1565.983                                                1569.625                                                     1571.90936 1517.920   1518.187        1521.931             1527.581                  1533.274                       1541.137                            1546.309                                 1552.700                                      1559.708                                           1565.955                                                1569.482                                                     1571.62437 1519.789   1520.090        1523.541             1528.933                  1520.500                       1542.169                            1547.139                                 1553.258                                      1560.060                                           1565.927                                                1569.340                                                     1571.33838 1521.529   1521.797        1525.155             1530.287                  1535.454                       1542.995                            1547.694                                 1553.677                                      1559.989                                           1565.856                                                1569.055                                                     1570.90939 1523.407   1523.541        1526.772             1531.643                  1536.683                       1543.822                            1548.387                                 1554.097                                      1560.130                                           1565.785                                                1568.912                                                     1570.62440 1525.155   1525.424        1528.392             1532.866                  1537.778                       1544.788                            1549.081                                 1554.516                                      1560.271                                           1565.714                                                1568.628                                                     1570.19541 1526.772   1527.041        1529.610             1533.955                  1538.599                       1545.479                            1549.637                                 1554.936                                      1560.553                                           1565.643                                                1568.485                                                     1569.76742 1528.527   1528.527        1531.236             1535.113                  1539.695                       1546.309                            1550.193                                 1555.216                                      1560.533                                           1565.572                                                1568.343                                                     1569.34043 1529.881   1530.016        1532.594             1536.341                  1540.656                       1546.790                            1550.749                                 1555.636                                      1560.694                                           1565.501                                                1568.058                                                     1568.91244 1531.643   1531.643        1533.955             1537.435                  1541.618                       1547.486                            1551.306                                 1555.916                                      1560.694                                           1565.430                                                1567.774                                                     1568.62845 1553.002   1533.138        1535.181             1538.667                  1542.375                       1548.318                            1551.864                                 1556.336                                      1561.117                                           1565.288                                                1567.489                                                     1568.34346 1534.363   1534.363        1536.273             1539.627                  1543.408                       1548.803                            1552.282                                 1556.617                                      1560.976                                           1565.217                                                1567.347                                                     1568.05847 1535.727   1535.591        1537.503             1540.656                  1544.236                       1549.498                            1552.700                                 1556.897                                      1560.976                                           1565.076                                                1567.063                                                     1567.63148 1536.956   1536.683        1538.599             1541.687                  1544.857                       1550.054                            1553.258                                 1557.178                                      1561.187                                           1564.934                                                1566.921                                                     1567.20549 1538.325   1538.051        1539.558             1542.513                  1545.686                       1550.610                            1553.677                                 1557.458                                      1561.328                                           1564.792                                                1566.636                                                     1566.77950 1539.421   1539.147        1540.519             1543.477                  1546.378                       1551.167                            1554.097                                 1557.739                                      1561.117                                           1564.651                                                1566.352                                                     1566.352Sonic Velocities - meters per second__________________________________________________________________________ 
    
     Thus, with the temperature of and sonic velocity in the electrolyte known or measured, the specific gravity of the electrolyte can be determined from the comparison with the database. Because of the matrix nature of the database, the actual value of the third parameter, for example specific gravity, most accurately would be determined by interpolation between the numerical values given in Table I for the two input parameters, for example temperature and sonic velocity. 
     The disclosed ultrasonic hydrometer 42 can be used on most any conventional lead acid battery where no variation or modification need be made thereto; the only criteria being to find a reliable &#34;sonic path&#34;. The &#34;sonic path&#34; must have the spaced sonic surfaces disposed under the level of the electrolyte, the electrolyte must extend continuously between the spaced sonic surfaces, no intervening structure can be disposed between the spaced sonic surfaces, and good sound transmitting contact must be made between the probe head and battery case wall. The sonic paths 30, 32, 34 and 36 have been illustrated of different possible locations and angles that might be used on a battery that has not otherwise been modified. To use the sonic paths 30 or 32, the probe head will be placed against the top wall 20 or against the bottom wall 22. In like manner, to use the sonic path 34, the probe head can be placed against the near side wall 24 or the remote side wall 23; while to use sonic path 36, the probe head can be disposed against the end wall 26. In this regard, the sonic path 36 extends between the end wall 26 and the partition wall 28 between the individual cells 14 and 15. 
     Once specific &#34;sonic paths&#34; have been found on a conventional battery, the corresponding contact pad areas 40 can be marked at the specific locations on the battery. This would indicate where on the battery the probe head could be placed to have a good sonic path. These modifications to commercially available batteries, although not needed, would be desirable to enhance the appeal and accuracy of this invention. 
     A battery could be specifically modified to have the pad areas 40 and sonic surfaces designed into the battery, as illustrated with respect to sonic paths 31, 33 and 35. Thus spacer nubs 71, 73 and 75 are formed on the side walls of the battery, protruding beyond the normal contour of the side wall. Each spacer nub is hollow and has a curved side wall 78 and spaced upper and lower sonic surfaces formed on the inside of corresponding upper and lower walls 79 and 80. A cavity defined therebetween is intentionally free of any intervening structure and becomes filled with the battery electrolyte. The upper wall 79 opposite the upper sonic surface has a contact pad area 40 against which the probe head can be placed. The distance between the spaced sonic surfaces of any one sonic path could be accurately known for easy use of the subject apparatus. 
     With this design, at least one spacer nub (only nubs 71, 73 and 75 being shown) could be formed on each side wall to space the battery a minimum gap from any adjacent structure for providing cooling air circulation around the battery. Specifically, in a three-cell, six-volt battery, the two end cells could have the spacer nubs 71 and 75 located on the opposite end walls, and the interior cell could have a pair of spacer nubs (only nub 73 being shown) formed on the opposite side walls. 
     FIG. 4 illustrates a second embodiment of ultrasonic hydrometer apparatus 42a that can be used with any battery 10a having electrolyte filler openings 85 and removable caps 86 for closing the openings. The hydrometer apparatus has a housing 91 with a hollow and liquid-tight chamber 18a having spaced sonic surfaces 92a and 92b that define therebetween sonic path 92. A flexible squeeze bulb 94 closes the top of the hollow chamber. A tube 96 connected to the hollow chamber near its lower end is designed to fit into a battery cell through the filler opening after removing the filler cap 86. An ultrasonic transducer 95 is located in the housing directly against the underside of the lower wall opposite the sonic surface 92a. No intervening structure is located between the sonic surfaces 92a and 92b. Likewise, a thermocouple 97 is located within the housing to sense the temperature of the liquid electrolyte drawn into the chamber. The control for operating the hydrometer will not be described, inasmuch as it is similar to the hydrometer described with respect to FIGS. 1 and 5, and accordingly will have the similar displays 55g and 55t for the specific gravity and temperature thumb wheel input 54, and mode selector and on-off switches 53 and 56, respectively. Since the sonic surfaces are fixed and will be universally used for every test reading, the distance therebetween will be accurately known and can be inputed into the microprocessor, so no distance display need be used. 
     The ultrasonic hydrometer apparatus 42a illustrated in FIG. 4 could be used on existing batteries having filler caps, free from the possible difficulty in finding a suitable sonic path on the battery. The apparatus 42a would be operated by squeezing and then releasing the bulb 94, to allow the collapsed bulb upon expanding to its original shape to draw the battery electrolyte up into the housing. Activation of the pulser would produce an ultrasonic impulse which would traverse the electrolyte, and the time required to travel through the battery electrolyte back and forth between the sonic surfaces would be measured. The spacing between the sonic surfaces would be accurately known, and the microprocessor would be calibrated accordingly. The ultrasonic hydrometer 42a is thus relatively easy to operate and foolproof in that the operator need not maintain the hydrometer truly vertical when activating the ultrasonic pulse, while further determined specific gravity would be visually indicated and this readout can be retained on the device, if desired, until it were intentionally cancelled out. 
     It can be noted also that the liquid-air interface at the upper surface of the electrolyte in the battery is sufficient to serve as a &#34;sonic surface&#34; in reflecting a sonic pulse coming upon it through the electrolyte. Thus, the transducer probe 44 can be positioned against a lower battery case wall in order to input the ultrasonic pulse from one real sonic surface into the electrolyte in an upward direction where the overlying electrolyte surface will act then to echo the impulse back toward the transducer for detection and timing. 
     FIG. 6 depicts in graphic form part of a database relating the sonic velocity through sulfuric acid as a function of different concentrations (specific gravities) and temperatures. The database is given also in Table I. All values for the specific gravity have been corrected to the standard temperature of 25° C. so that all readings at any other temperatures will reflect this automatically. In the application of the subject invention, the sensitivity of the transducer allows velocity resolutions of ±0.25 meters per second; but considering the overall accuracy in the measurement of distances and temperatures, and the response delays of the control this will be reduced to approximately ±2 meters per second. This corresponds to a determination of the specific gravity of ±0.15%. 
     At the specific gravity (density) of approximately 1.245, no change in velocity occurs over the temperature range of interest, between 0° and 40° C. For specific gravities between approximately 1.05 and 1.30 for temperatures between approximately 0° C. and 40° C., each pair of sonic velocity and temperature values will be associated with only one value of specific gravity. Beyond these ranges, it is possible to have the same sonic velocity for two different values of temperature and/or specific gravity. To eliminate this, the ultrasonic hydrometer should be limited to operation for specific gravities and temperatures only in these approximate ranges. 
     The use of ultrasonic testing apparatus is straightforward if the distance of the sonic path is known. By shifting the mode selector switch 53 to the specific distance parameter, this &#34;distance&#34; can be inputed with the thumb wheel switch (in centimeters, such as 14.78&#34; cms). Thereafter the activation of the impulse signal automatically triggers the transducer and begins the timer. The returning signal stops the timer, so that the time required for the impulse to travel back and forth between the sonic surfaces is measured. The microprocessor then calculates the sonic velocity. The temperature is inputed with the thermocouple 46. The microprocessor then searches the database, having the sonic velocity and temperature inputs, to determine the specific gravity which is automatically corrected to read at 25° C. If the hydrometer 42 is to be used on the same cell with the same sonic path for sustained testing, it is possible that this distance can be retained in memory in the microprocessor so it need not be inputed each time a reading is to be taken. 
     In order to use the hydrometer 42 on a battery where the distance between the spaced sonic surfaces of the battery is not accurately known, it is necessary first to find this distance. This is done by using a conventional float hydrometer to find the specific gravity of the battery electrolyte at the given electrolyte temperature and adjusting this to correspond to the value at 25° C. This value is inputed by positioning the mode selector switch 53 to &#34;specific gravity&#34; and dialing the thumb wheel switch 54. Thereafter the ultrasonic hydrometer can be used at the same battery condition to obtain the transit time along some sonic path between the two sonic surfaces and to obtain the temperature of the electrolyte. Inasmuch as the two known parameters of the specific gravity and temperature uniquely will identify the sonic velocity, having the time known will allow the microprocessor to calculate with Eq. 3, on a one time basis, the distance traversed by the sonic pulse, and this parameter would be illustrated on the readout display 55d. Once a particular sonic path distance has been calibrated, the ultrasonic hydrometer apparatus could thereafter be used on the same path by inputing this distance (or be retaining this distance in memory), whereupon the specific gravity could be determined directly. 
     As the battery charges and discharges, the heavier electrolyte migrates toward the bottom of the battery, leading to stratification of the electrolyte. The ultrasonic method averages localized differences in the density of the electrolyte, since the pulse energy travels through various depth layers or broad lateral stratifications of the electrolyte. This is in contrast to readings obtained with a float hydrometer where only a finite amount of electrolyte is drawn from the battery, usually from the top of the cell, and the differences in densities as between the top and the bottom of the cell will not be sensed. It is more accurate therefore to bubble the cell with the electrolyte withdrawal tube of the hydrometer, or with a bubbler used specifically for this purpose, in order to mix the electrolyte during charge and discharge cycles before taking any readings. The mechanical mixing eliminates the stratification of the electrolyte and allows close correlation of specific gravity readings obtained with the float hydrometer and the ultrasonic hydrometer. 
     It should be noted that this invention would have particular utility in obtaining at frequent intervals the specific gravity readings on a test battery that would be under cyclic charge and discharge conditions, where each cycle may be completed over a several hour period, and further where the voltage and amperage readings can simultaneously be taken. For such a use, the various readings could be recorded also on a computer for complete data retention and analysis.