Method of charging secondary batteries

Abstract: In a method of charging secondary battery, the charging control is performed when it is detected that the secondary differential value of the detected voltage curve changes from positive value to a negative value at the point C or its near portion.

Description: The present invention relates to a method of charging secondary batteries. Charging voltage curves show a specific pattern as shown in Fig. 1 (a), having a relatively small battery voltage increase till the intermediate period of the charging period independent of the various charging conditions such as an amount of the charging current or room temperature while in the end of the full charging period, the battery voltage abruptly increases, reaching a peak, thereafter decreasing gradually. There always appears a curve point C on the charging voltage curve from the intermediate to end of the charging period and there appears a period in which the change of the battery voltage varies from zero to minus maximum value. By analyzing the first derivation of the voltage curve, which is a change of the voltage per unit time dV/dt, a positive (plus) projecting peak (curve point C) just before the peak A as shown in Fig. 1 (a) may be detected, the change of the battery voltage before and after the projected peak being sufficiently large. The gradient d 2 V/dt 2 of the change of the battery voltage, that is a second derivation varies from plus to minus abruptly as shown in Fig. 1 (c). It is known that, during the secondary battery charging, there occurs a peak voltage point A in a voltage curve as shown in Fig. 1 (a) at the end of the charging operation. In a first conventional method of charging a secondary battery a charging control is used in which a peak point A measured as a charge potential level is determined to initiate the end of charging (US-A-4 281 277). In a second method a time point where the battery voltage has dropped from the peak voltage A is determined and used to initiate the end of charging. It is further known (Japanese patent publication 53-40695) to detect the voltage of the secondary battery during charging for controlling the charging current when the change of the battery voltage exceeds a predetermined value. However, it is very difficult to detect accurately whether the voltage reaches the peak value A or not by detecting only the voltage change and to avoid a detection error because the voltage change per unit time near the peak point A of the charge voltage and such small voltage change occurs not only near the peak point A but also at various other places. The second method of detecting the time point when the voltage has dropped by ΔV has the advantage that it is easier to detect the timing of the current control than to detect according to the first mentioned method, but there is the disadvantage that the charged value changes from battery to battery because the time point fluctuates for the fixed voltage ΔV due to the fact that charging characteristic curve greatly change by amount of the charging current and room temperature. In particular, in the recent developed nickel hydrogen battery the voltage drop from peak point is small, so that the above mentioned problem still occurs. In a method, in which the gradient of the charging voltage curve is detected, the setting value is settable easier than in the previous methods, but it is difficult to control the charging current optimum by only detecting the period of time when the voltage change exceeds a predetermined value. A further method of charging batteries is disclosed in EP-A-0 005 841. This method uses the detection of inflection points which occur as the electrochemical potential changes with respect to time. Under the condition that the first seconds of charging are disregarded the charging is terminated when two consecutive inflection points have been detected. The phase of charging may be followed by a phase in which an overcharge or surcharge is performed with a relatively low charge rate for a fixed amount of time. As an emergency measure for the case that an already charged battery is subjected to the process the voltage difference is monitored so that in case that the inflection points cannot be detected since they have been passed in the first seconds of charging, the charging phase is terminated upon the detection of a negative voltage difference. The use of inflection points to control the charging and the emergency measure of monitoring a negative voltage difference are also disclosed in FR-A-2 591 822. However, the processes according to both methods mentioned above fail to achieve a full charge of the battery. A further problem occurs when pair batteries are subjected to charging using any of the above mentioned charging methods. According to conventional methods, if a bad battery is mixed in or if one or more batteries with relatively highly charged batteries are mixed, they tend to be overcharged, so that the cyclic charging property of the batteries may be deteriorated. It is the object of the present invention to provide a method of charging a secondary battery in which full charging can be easily and correctly performed irrespective of difference of the charging conditions. This object is solved, according to the invention, with the features of claim 1. Further it has been discovered that secondary batteries have a point B which shows a battery voltage change similar to the battery voltage change at the curve point C. The time points B and C correspond to only a timing when the charge value reaches a specific value irrespective of the charging condition. Therefore the time points B or C can be used as a signal for starting the charging control. Similarly, as a result of analysis of the battery voltage curve of pair batteries, when the pair batteries are charged, the inventors have found that the curve point C on the battery voltage curve of the pair batteries generally coincides with the respective curve point C1 and C2 of the individual batteries 13a and 13b so long as the batteries 13a and 13b are both in an initial condition at which both batteries have a similar battery characteristics as shown in Fig. 2 (a). However, when different battery characteristics occur in respective batteries after repetition of the charge and discharge cycles, the curve point C and the peak point A of the pair of batteries are obtained by the curve point C1 and the peak point A1 of the battery which has been least discharged. It is an advantage of the present invention to provide a method of charging secondary battery in which the full charge completion can be detected. The method of charging pair batteries according to the invention prevents overcharging. The second detecting process for detecting the end of the charging period detects the instant of time in which the change of the battery voltage per unit time is a negative maximum value. Such an instant occurs after the first detection operation. The second charging control step is performed in response to the second detecting process. In the first charging control step, the charging operation is continued with a charging current decreased compared with the current used in the period before the first charging control step. It is desired to perform a trickle charging after the first charging control step is finished. As the secondary battery, there may be used pair batteries in which a plurality of batteries are connected in parallel or in series. According to the charging method of the present invention, when the charging operation is started, the battery voltage Vb increases along a similar shape curve irrespective of charging conditions such as the charging voltage or room temperature. When the charge of the secondary battery comes to near the end, there appears a curve point C in which the charging voltage curve abruptly rises up. The first derivation of the battery voltage curve or the change of the battery voltage per unit time changes in a positive projection shape as shown in Fig. 1(b). Further the secondary differential curve of the battery voltage changes in an S-shape from the positive value to the minus value around the curve point C. Therefore, by obtaining the second derivation of the battery voltage and detecting the point where the sign of the value changes from plus to minus, the curve point C can be obtained. It is noted that the charging amount of the secondary battery at the curve point C can be determined univocally for similar kinds of batteries by experiment. After the curve point is detected, charging is performed to charge the battery up to 100 % of the battery capacity as shown in Fig. 1(d). The change of the battery voltage becomes zero at the peak point A on the battery voltage curve, the voltage change becomes negative minimum value at the time point when the battery voltage is dropped by a predetermined value after the peak point A. Accordingly, the time period when the battery is fully charged can be determined by detecting a time point when the battery voltage change becomes a negative minimum value. The invention is described more detailed with respect to the drawings, in which: Figs. 1(a) to 1(e) show graphs for explaining the essential structure of the present invention in which Fig. 1 (a) shows charge characteristic of a battery voltage, 1 (b) is a change per unit time of the battery voltage, 1 (c) shows the change of the secondary differential value of the battery voltage curve 1 (d) shows a control condition of the charging current and 1 (e) shows charged amount value characteristic, Figs. 2(a) to 2(b) is a graph showing characteristics of the pair batteries in which Fig. 2 (a) shows the case in which the pair battery characteristic is equivalent and Fig. 2 (b) is the characteristics of the pair battery is different, Fig. 3 is a block diagram of the circuit arrangement adapted to make use of a method according to the present invention, Fig. 4 is a flow chart showing the operations of the arrangement shown in Fig. 3, and Figs. 5 (a) to 5 (f) show various curves showing the operation of the arrangement shown in Fig. 4. As shown in Fig. 3 the charging device comprises a charging circuit 12 for commutating commercial A.C. power source 11 and for outputting D.C. current I, a switching element 14 for controlling period of time of passing the current of the output of the charging circuit 12 to a secondary battery 13, a A/D converter 15 for converting the terminal voltage of the secondary battery into digital value and a control circuit 16 for controlling the charge operation of the secondary battery 13 by receiving the output data from the A/D converter 15. The charging circuit 12 consists of a constant current source and output current I is changeable in several steps in response to the control signal S1 fed from the control circuit 16. As the switching element 14, a relay or semiconductor switch or the like may be used. The switching element 14 is placed between the charging circuit 12 and secondary battery, being turned on and off in response to the control signal S2 fed from the control circuit 16, thereby controlling the conductive period to the secondary battery 13. The A/D converter 15 samples the voltage Vb of the secondary battery 13 intermittently, converting the analog value of the secondary battery voltage Vb into the digital signal of a predetermined number of bits for inputting the digital signal into the control circuit 16 as the voltage data Vi. As the control circuit 16, a microcomputer provided with a CPU and a memory is used to provide completely or partially the circuit arrangement as shown in Fig. 3 by suitable software or programming stored in a ROM in the microcomputer. The control circuit 16 comprises a setting operation unit for setting various conditions such as charging condition, a data memory for temporarily storing the data of the detected voltage Vi from the A/D converter 15, a comparator 23 for judging whether or not charge in the battery 13 is full by a calculation using change of the charge voltage Vb, a calculation unit 24 for determining the optimum charging time and charging current corresponding to the result of comparison in the comparator 23, a control output unit 25 for outputting the control signals S1, S2 corresponding to the calculation results in the calculation unit 24 and timer units for outputting various timer signals. Operation The operation of the control circuit 16 is explained with reference to the flow chart of Fig. 4 and the diagram shown in Fig. 5. First, before starting charge operation, various initial settings such as charge current Ib before detecting the curve point C, charge current Im after curve point detecting, charge current Is after full charging detection are performed in the setting operation unit 21 in step 51. When a start switch (not shown) is turned on at the period t1, the control signal S1 corresponding to the charge current Ib is fed to the charging circuit 12 from the control output unit 25, whereby the output current I from the charge circuit 12 is set to Ib. Simultaneously, control signal S2 for indicating to turn on the switching element 14 is given from the control output unit 25 and the charging circuit 12 and the secondary battery 13 conduct and a rapid charging begins with a predetermined charge current at step 52. It is noted that there are two kinds of batteries with respect to the voltage change in the initial charging period. These two kinds of battery are identified by suffix 1 and 2 and the value dV/dt is shown in Fig. 1 (b) and d 2 V/dt 2 shown in Fig. 1 (c). In step 53 the timer units 26 are operated, so that before reading in the battery voltage Vb of the secondary battery 13 in the first time, a waiting time Tw (min.) is intentionally set. Since the change of the battery voltage Vb is large and unstable directly after charge starting as shown in Fig. 5 (a), the waiting time Tw is expressed as Tw = Kw/Ib wherein Kw is a suitable coefficient corresponding to the charge condition, in the present example, Kw is set to 5 and the unit of Ib is the charging rate C which shows that charging is completed 100 % for an hour. An erroneous detection of the curve point C can be prevented by waiting for reading the voltage for the above time Tw. It is noted that for the batteries as shown in Fig. 1 (c 2 ) in which the sign of the value d 2 V/dt 2 is reversed at the time of the charge start, there is no need to provide such a waiting time (step 53) as shown in dotted lines X. When it is judged that the waiting time Tw is lapsed, the battery voltage Vb is read in the calculation unit 24 as Vi through the A/D converter 15 in step 54, the variation n and Vc are initialized then the first detecting process for detecting the curve point C is performed. In step 55 the battery voltage Vi and the difference voltage Vc are stored in the data memory 22. In addition, in step 56 the time interval Ts (min.) for reading in the battery voltage Vb is set. That is, the voltage change per unit time (dV/dt) of the battery voltage Vb is proportional to the charging current Ib. In the present embodiment, the voltage differences Vc between each detected voltage Vi are adapted substantially to be equal in spite of the charging current Ib by reading intermittently the battery voltage vb in the time interval obtained by the equation Ts = Ks/Ib, wherein Ks is a coefficient corresponding to the charge condition and in the present example Ks is 5. When the time Ts is lapsed the battery voltage Vi is read in step 57, thereafter, the vpltage difference Vc between the voltage Vi detected this time and the voltage Vm detected the last time and stored in the data memory 22 is calculated in the calculation unit 24 in step 58. In step 59, the voltage difference Vs between the voltage difference Vc calculated at this time and the voltage difference Vd calculated at the last time is calculated thereby obtaining secondary differential value in the battery voltage curve can be calculated. The secondary differential value in the voltage curve changes abruptly from a positive sign projection to a negative sign projection about the curve point C as shown in Fig. 1 (c). Accordingly, in the present embodiment it is judged that the battery voltage Vb has passed the curve point C when the secondary differential value Vs changes to the negative value. If a point is judged as the curve point C just when the value Vs becomes negative there may occur an error by a noise. In order to avoid this error, there are set times n that the value Vs repeats negative sign, so,that when the sign is positive the value n is kept 0 in step 61. On the other hand, every time negative sign is detected n is increased by 1 in step 62, when, for example, two consecutive negative signs are detected (at time t2) in step 63, the process goes to the second detecting process. In the second detecting process, the difference voltage Vc is detected in steps from 64 to 67 similar to those in the first detecting process. Thereafter, when the battery voltage abruptly drops after the battery voltage reaches the peak value, the control circuit 16 is arranged to detect the time point t4 during which the difference voltage becomes the negative maximum value (minimum of the first voltage derivation dV/dt detected in step 68) as the second detecting process. That is, the value obtained by further differentiating such change shows. from the negative value to the positive value as shown in Fig. 5 (d). The control signal S2 is then fed to the switching element 14 from the control unit 25 after given charge control processes are performed in step 69, the charge operation is finished in step 70. Accordingly, by differentiating similar to the first detecting process, and obtaining the time when this value changes from negative value to the positive value a more accurate full charging period can be detected. The charge control process 69 is performed corresponding to the contents of the setting operation unit 21 and detection of the charging condition in response to the charge conditions such as charging current value. In the present embodiment, the charging current I is decreased from Ib to Im which is slightly smaller than Ib after detection time tl of the curve point C, thereafter, the charging current I is further decreased to a current Is which is smaller than the current Im after detection of the peak point A so as to perform a trickle charging.

Claims: Method of charging secondary batteries with different charge rates comprising the steps of detecting the battery voltage during charging of a secondary battery, detecting in a first detection process that the second derivation of the detected battery voltage curve (d 2 V/dt 2 ) changes from a positive value to a negative value (point C), performing a first charge control step in response to the detection in the first detection process, the charge rate being changed from a rapid charge rate to a reduced charge rate in the first charge control step, and performing a second charge control step, the charge rate being changed from the reduced charge rate to charge stop or trickle charge in the second charge control step characterized by performing a second detection process for detecting a condition that the change of the battery voltage per unit time of the battery voltage is a negative maximum value, said second detecting process being started in response to the detection in the first detecting process, and performing the second charge control step in response to the detection in the second detection process. Method according to claim 1 wherein the secondary battery is a pair of batteries comprising a plurality of secondary batteries. Method according to one of claims 1-2 wherein the second charge control step performs a trickle charging at a low current. Method according to one of claims 1-3, characterized by its use for charging a nickel hydrogen battery.

Inventors: KAJITA KOZO, KIZU MASANOBU, NAGAI RYO, OHIWA TSUNEMI, OSHIMA TOSHIO, KAJITA, KOZO, KIZU, MASANOBU, NAGAI, RYO, OHIWA, TSUNEMI, OSHIMA, TOSHIO
Applicants: HITACHI MAXELL, HITACHI MAXELL LTD.
Application year: 1991
Main classification: H02J7
IPC codes: H02J7