Abstract:
A method and device for cooling the electrolyte of aqueous battery cells such as lead acid batteries. The method includes the steps of delivering non-saturated gas to the electrolyte of in the battery cell, contacting said non-saturated gas with said electrolyte so that water from said electrolyte evaporates and thereby cools said electrolyte, and allowing the gas to escape from said battery cell, thereby removing heat from the electrolyte. An apparatus for carrying out the method is also provided. A fluid conduit is positioned to deliver the cooling air to the electrolyte within the cell. At least one pump delivers the air to the fluid conduit.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the priority of U.S. Provisional Application No. 60/598,403 filed Aug. 2, 2004, which is hereby incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention is related to aqueous batteries, and more particularly to methods and devices for cooling the electrolyte of such batteries.  
       BACKGROUND  
       [0003]     Aqueous batteries, such as industrial batteries that are typically made up of multiple battery cells connected in series, are used widely in industry in such uses as electric vehicles, e.g., fork lift trucks. As well known in the art, each cell contains positive and negative plates (electrodes) immersed in an electrolyte. Such batteries are used in service until depleted of charge, at which time the battery is removed and replaced with a freshly charged battery. The removed depleted battery is connected to a charger to be recharged. This method of operation requires several batteries for each vehicle, and a charging room with chargers where the depleted batteries can be recharged. This method of operation also requires the swapping of batteries each time a depleted battery is replaced.  
         [0004]     Significant benefits have been obtained with a recent development that provides for fast or “rapid-charging” of industrial batteries, particularly in the material handling industry, e.g., battery powered fork lift trucks. Rapid-charging has eliminated the need for maintaining multiple batteries for each fork lift truck and the need for fixed battery charging rooms and battery charging equipment for the extra batteries. With the rapid charge process, only one battery per truck is required, which is charged quickly while still connected to the truck, thus providing great savings in costs and time.  
         [0005]     Unfortunately, the rapid charge process has created problems that negatively affect the battery. One problem is that rapid charging can cause the battery cells to get very hot, and high cell temperatures cause the batteries to fail prematurely. For instance, batteries that are rapid charged often fail in less than two years whereas their normal life expectancy is 5 to 7 years. Since batteries for trucks can cost several thousands of dollars, early failure is very costly for the battery user. Thus significant improvements can be obtained by cooling the batteries to remove the excess heat, thereby extending the battery life.  
         [0006]     Rapid charge also has a second problem. When a battery is charged at a very high rate, the charging voltage at the bottom of the battery plates is lower than the charging voltage at the top of the plates due to the voltage drop of the plates themselves. This effect tends to undercharge the bottom of the plates and overcharge the top of the plates.  
         [0007]     In lead-acid batteries, this condition is further exacerbated by a third problem, also related to rapid-charge. Batteries used on these chargers are usually “opportunity charged” as the situation permits. This implies partial charging which, in turn, leads to a phenomenon of acid stratification. During a partial charge, strong, dense acid emerging from the plates tends to sink to the bottom of the cell, leaving weak, dilute acid at the top of the cell. In essence, this is like having two half-cells inside the same cell container but with different electrolytes. A cell with strong acid requires a higher charge voltage than a cell with weak acid. Yet, in the case of rapid-charge, the bottom of the plates has a lower charge voltage than the top due to the excessive voltage drop as described in the second problem above. Unless the electrolyte is mixed, it is likely that the charging will be limited to the top of the plates, leaving the bottom of the plates discharged and ultimately seriously damaged. For example, in lead acid batteries, it is known that stratification can cause the active material at the bottom of the negative plates to fall off the grids as lead sulfate. Nickel alkaline cells do not have an electrolyte that stratifies and therefore do not require a mixing system. Nevertheless, such cells also run hot and may benefit from cooling.  
         [0008]     A known solution to the stratification problem of lead acid cells is air mixing of the electrolyte. Each cell is equipped with a small air tube that channels compressed air from an external source into the lower regions of the cell. The resulting air bubbles rise to the surface and mix the acid. The system is generally used to reduce the amount of overcharge required for normal operation. This, in turn, reduces electricity usage in the charging cycle, provides a more uniform current density and thus reduces the amount of water lost by electrolysis—which is the traditional way to mix the cell electrolyte. The amount of air used in this conventional method is deliberately minimized to prevent evaporation of water from the electrolyte.  
         [0009]     While conventional air-mix systems can prevent stratification, they do not supply enough air to cool the battery significantly. Typically with existing air-mix systems, the air flow to the cells is supplied from a pump in the battery charger which also includes the control system, timers, etc., that govern the on and off times for the air flow. The air is delivered via flexible tubes to the individual air-tubes in the cells. In rare cases, air pumps may be mounted on the trucks or even on the batteries themselves.  
         [0010]     Accordingly, it is an object of the present invention to provide a means of cooling the electrolyte to mitigate and eliminate the detrimental effects of rapid cooling. Another object is to provide an electrolyte cooling system the can also eliminate other problems such as stratification.  
       SUMMARY OF THE INVENTION  
       [0011]     The present invention provides a novel method of cooling an aqueous electrolyte battery cell by causing the evaporation of water from the electrolyte. In one form, the method includes delivering non-saturated gas to the electrolyte of the battery, contacting the non-saturated gas with the electrolyte so that water from the electrolyte evaporates and thereby cools the electrolyte, and allowing the gas to escape from the battery cell with the vapor. This process is believed to be an evaporative cooling process whereby the heat of evaporation comes from the electrolyte itself, thereby lowering the temperature. A device for carrying the method is also provided. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The following detailed description will be better understood when read in conjunction with the Figures appended hereto. For the purpose of illustrating the invention, there is shown in the drawings a preferred embodiment. It is understood, however, that this invention is not limited to this embodiment or the precise arrangements shown.  
         [0013]      FIG. 1  is a schematic view of a battery cell in accordance with the present invention;  
         [0014]      FIG. 2  is a top view of an eighteen cell battery illustrating in schematic form an cooling air distribution system and a water distribution system; and  
         [0015]      FIG. 3  is a schematic view of a fork lift truck type vehicle having a battery using an electrolyte cooling system in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]     The present invention provides a novel means for cooling aqueous batteries. Although the invention will be discussed with reference to industrial batteries, and more particularly to lead acid type industrial batteries, it will be appreciated that the present invention can be applied to other types of batteries that can benefit from cooling.  
         [0017]     With reference to  FIG. 1 , a battery cell  10  is illustrated. The battery cell  10  is a single cell that typically is combined with multiple such cells  10  to form a battery. For example, lead acid batteries formed of  12 ,  18  and  24  such cells electrically connected in series are typical. The cell  10  has a housing  12 , a housing cover  14  closing the top of the housing  12  and which has a vent  16  through which gasses from within the cell  10  can vent. An aqueous electrolyte  18  is provided in the housing, such as sulfuric acid for lead acid cells and sodium hydroxide for nickel alkaline cells. Immersed in the liquid electrolyte  18  is a positive electrode  20  and a negative electrode  22 , both of which are typically formed as multiple plates which are electrically connected to their respective terminals  24 ,  26  by a strap as is known in the art.  
         [0018]     A fluid conduit  28  delivers gas used for cooling to the electrolyte  18 . In the preferred embodiment, the conduit  28  takes the form of a tube  30  having a first tube section  32  within the cell  10  extending into the electrolyte  18  to the bottom of the cell  10 , and a second tube section  34  extending into the cell  10 , preferably from above the housing cover  14 , and which is connected to a top end  36  of the first tube  32 .  
         [0019]     The fluid conduit  28  has at least one outlet  38  positioned in a lower portion of the electrolyte  18  to maximize the travel of gas bubbles  40  leaving the openings  38  through the electrolyte  18 . In the present embodiment, four openings  30  are provided in the first tube  32  above a bottom  42  of the cell housing  12  so as to be above any sediment that may build up along the bottom of a typical battery cell  10 . For example, the lower openings  38   a  and  38   b,  180 degrees apart from one another, might be 1¼ inch from the bottom  44  of the tube  30 , and openings  30   c  and  30   d,  also 180 degrees apart from one another and 90 degrees from the openings  30   a  and  30   c,  1½ inches from the bottom  44  of the tube  32 . The bottom  44  of the tube  32  can be open, but as the tube rests against the bottom  42  of the cell  10  and possibly in sediment, little gas may exit there.  
         [0020]     In the preferred embodiment, the first tube  32  is installed near the positive strap  25  corner of the cell  10  during manufacture of the cell and prior to installation and sealing of the cover  14  to the housing  12 . (The positive strap  25 , shown partially and schematically, electrically connects together all of the positive plates as is known in the art). The openings  30  can be drilled into the tube section  32 . The length of the first tube section  32  is chosen so that the top  36  of the first tube section  32  is preferably above the strap  25  of the cell  10 , and to provide the best fit for the first tube  32  in the cell  10 .  
         [0021]     After the cover  14  is sealed to the housing  12  during manufacture of the cell  10 , a hole  46  is drilled in the corner of the cell  10  above the first tube  32 , e.g. a ⅜ inch hole. A grommet  48 , which acts as a seal between the second tube section  34  and the cell cover  14 , is inserted into the hole  46 . The second tube section  34 , with a connection fitting  50  connected to the top end of the second tube section  34 , and a 45 degree angle cut  52  on its bottom end, is inserted into the hole  46  through the grommet  48  and down into the first tube section  32  until the fitting  50  is fully seated in the grommet  48 . Thus it is appreciated that by installing the tube  32  in the corner of the cell  10 , installation of the second tube section  34  into the first tube section  32  is facilitated. It is also appreciated that the length of the second tube section  34  is chosen to allow it to be inserted into the first tube  32 . The length of the second tube  34  should extend a suitable distance into the first tube and can extend close to the bottom of the first tube  32  if desired. The outer diameter of the second tube  34  should be sized to slidably and snugly fit within the first tube  32  and to minimize air leakage from the top of the tube  32 . The fitting  50  is chosen for the specific type of connection to a gas supply or distribution tube as to be described below. For example, the T fitting shown is useful if the cooling gas supply will be connected to the tube  30  in series with other cells  10  and thus one side of the fitting is for the gas in and the other side for delivery of gas to other cells, an L fitting can be used if this cell is the end of the supply line, or a 90 degree T fitting for corners.  
         [0022]     The cooling gas is preferably supplied by a gas distribution system that can serve all of the cells  10  of a given battery. While any suitable non-saturated gas (not saturated with water vapor) can be used, non saturated air is preferred, and the less saturated (dryer), the better. An example of a preferred cooling air supply and distribution system is illustrated with reference to  FIG. 2  which shows a top view of a battery  54  made up of eighteen cells  10 . The cells  10  are connected in series as known in the art to form a battery  54  such as those used for electric vehicles. Looking down on the covers  14  of the cells  10 , it is seen that the cells  10  have positive and negative terminals  24 ,  26  connected electrically in series to one another, and the battery has a main electrical connector  56  for connecting the battery  54  to the load connector  58 , e.g., the vehicle. Also shown is a watering system for adding water to the batteries as is known in the art, the illustrated water system having water tubing  60  connecting each water flow valve  62  of each cell  10  to a water supply (not shown) via a quick connect connector  64 .  
         [0023]     Although any air or fluid conduit means may be used, the preferred embodiment uses a flexible clear air supply tubing  66  to connect in series each of the cooling air fluid conduits  28  of each cell  10 . Any suitable material, such as PVC tubing may be used. For example, beginning with the cell  10   a  in the upper left corner of the battery  54  as seen in  FIG. 1 , it is seen that an L fitting  68  connects the fluid conduit  28  extending into the cell  10   a  (here the second tube section  34  as seen in  FIG. 1 ) to the supply tubing  66  which is also connected to the T fitting for the adjacent cell  10   b,  which is followed by cell  10   c  (with a T fitting) and so forth until the end or last cell  10   d  is reached where another L fitting is used to terminate the cooling supply line  66 . The cell  10   e  in the upper right hand corner illustrates the use of a 90 degree T fitting  69 . Thus it is seen that the supply tubing  66  connects in series to all the cells  10 .  
         [0024]     Air pumps  70  provide the cooling air to the cooling air supply tubing  66 . In the preferred embodiment, three pumps  70  are provided for redundancy, each pump  70  having two diaphragm pump units (and thus two outlets), and have AC brushless motors for long life. The pumps  70  are preferably driven off of the battery  54  itself, an inverter  72 , shown electrically connected between the pumps  70  and battery  54  by wires  74 , being provided to convert the direct current of the battery  54  to alternating current. The pumps  70  are wired across the battery and not a smaller number of cells  10  so as not to discharge the smaller group of cells. The three pumps  70  can be mounted in a common box and mounted to the battery  54  or preferably on the vehicle and connected to the air supply tubing  66  with quick connects (not shown). Mounting on an electric vehicle  78 , here a fork lift truck, is illustrated in  FIG. 3  with the pumps  70  are wired to the battery  54 . The pumps should preferably be mounted above the elevation of the battery  54  to eliminate the possibility of any electrolyte  18  from the cells  10  entering the pumps  70 .  
         [0025]     It is preferable to connect the various pumps  70  to the cooling supply tubing  66  at different points along the tubing  66 . For example, in the eighteen cell battery  54  illustrated in  FIG. 2 , the three pumps  70  each have their dual supply outlets combined into single supply outputs  76   a,    76   b  and  76   c,  which connect to the supply tubing  66  between cell number  3  and  4 ,  9  and  10 , and  15  and  16  counting from the upper left hand corner of the battery  54  moving to the right, from right to left in the middle row, and from left to right in the bottom row of cells  10 . For a  12  cell battery, the air supply connections are preferably between cell numbers  2  and  3 ,  6  and  7 , and  10  and  11 . For a  24  cell battery, the air supply connections are preferably between cell numbers  4  and  5 , 12  and  13 , and  20  and  21 .  
         [0026]     Having described the various elements and components of the cooling system, the cooling process itself is now described. With reference to  FIG. 1 , it is seen that the gas, preferably air having been delivered to the electrolyte  18  via the pumps  70  and conduit  66 , contacts the electrolyte  18 . In the preferred embodiment, this contact is in the form of intermixing the gas with the electrolyte  18  by providing bubbles  40  of gas which mix with the electrolyte as they float up to the surface  19  of the electrolyte  18 , after exiting the openings  38  of the fluid conduit  28 . The physical movement of the air bubbles ( 40 ) mixes the acid electrolyte in a lead-acid cell and prevents stratification. Moreover, bubbles  40  of relatively dry air absorb moisture from the water in the electrolyte  18  and carry it out of the cell  10  as water vapor. Since it takes energy to provide the latent heat of evaporation of the water, that energy is removed from the cell, which reduces the temperature of the remaining electrolyte. This is an evaporative cooling effect. Preferably the air is as dry as possible so, especially in a stationary air supply, a desiccant or other dryer may be used to dry the air before letting it enter the cells  10 . Direct air cooling, i.e., chilling of the air, has some effect but it is slight compared with evaporative cooling due to the low specific heat of air.  
         [0027]     Instead of minimizing water evaporation from the electrolyte as was previously believed desirable, the present invention desirably causes evaporation. That is, it ignores the need to conserve water but rather encourages its evaporation to maximize cooling. To compensate for rapid water loss, a single-point watering system is used with the battery  54  to replace the evaporated water quickly and efficiently. Such systems are well known in the art, an example of one being described in U.S. patent application Ser. No. 10/694,276 filed Oct. 27, 2003 and which is hereby incorporated by reference herein.  
         [0028]     The present invention, contrary to what is done conventionally, such as in conventional air mixing, may be set to provide cooling air continuously, day and night, indefinitely, if necessary. The charging process can add significant amounts of heat to the electrolyte  18  which may require the cooling process to run long after the charging cycle is over to remove the heat added. The effect on cooling of the battery is roughly proportional to the on time of the air flow so the present invention cools the battery very effectively. The on time can total 24 hours per day, and run during all cycles, i.e., charging, discharging, and idle or rest times (an example of rest being weekends when the vehicle may not be in use). A battery that is out of service, such as one removed from the vehicle and waiting for disposal, or a battery that is out of service for a significant length of time, need not be cooled. Although the air flow may be continuous at all times, in an alternative embodiment, it may be controlled by at least a simple thermostat  82  configured to control the pumps  70 , switching on the air flow when the battery temperature rises above a set point and switching it off when the temperature drops below another set point. The thermostat could, for example, be connected to an electronic controller that controls the pumps. Other simple controls may consist of fixed or adjustable on/off timers or other more sophisticated controls as would be obvious to those familiar with batteries. For example the run time for the cooling air could be about 4 hours per day or more. As another possibility, the cooling system may be switched on and off periodically in any variety of ways. For example, an advanced predictive algorithm the controller may sense immediately when a high-rate charge is taking place and switch on the air flow as the heat is being generated and before a high battery temperature has been reached.  
         [0029]     For lead-acid cells, the air flow may also be switched on independently of temperature for the additional function of de-stratifying the electrolyte  18  during charge even if the battery  54  is cool. For example, 2 minutes every 30 minutes during charge will suffice.  
         [0030]     The cooling effect of the air flow is believed largely a function of evaporation of the water in the electrolyte (although some minor direct heat transfer will take place) so humid air will reduce the cooling rate substantially. For example, air having 100% humidity (saturated air) will provide no evaporation. The preferred application, therefore, is in locations where the air is not saturated, and preferably dry and relatively cool. However, the air may be dried with a desiccant or other type of air dryer inserted in the path of the air flow before being admitted into the cells  10  to maximize its evaporative effect.  
         [0031]     For a given volume of air delivered to the cell  10 , evaporation is increased as the size of the bubbles  40  is reduced. Thus a porous diffuser at the end of the fluid conduit  28  to generate microscopic bubbles  40  may be desirable.  
         [0032]     Another consideration is the amount of cooling air flow used per cell  10 . Once again, conventional air-mix systems seek to avoid water loss and, therefore, use relatively low air flow rates, e.g., 100 ml to 200 ml per minute per cell. The volume of air flow in the present invention may be anywhere from 100 ml to 2,000 ml per minute per cell  10 , although these rates are not absolutes, but preferred examples.  
         [0033]     Where the air comes from and how it is delivered is not relevant. It may come from a stationary source or a source on the truck or the battery. Any gas if available may be substituted for air, for example, nitrogen in cylinders. If the air comes from a stationary source, and the battery is on the truck, the cooling will obviously be limited to the periods when the charger is connected to the battery. Due to the nature of the rapid charge process, that may only be a small fraction of a 24 hour day and will limit the degree of cooling. However, with sufficient air flow in a dry condition, that may be an acceptable solution. A preferred embodiment, however, is that the air is supplied from a source on the truck  78  or the battery  54  itself, where it may be used to cool the battery at all times, day or night, during charge and during discharge, as required. If the air source is on the truck or battery, the control system for the air flow should preferably be on the truck or battery. A benefit of placing the air source on the truck or battery is that if a high rate of air-flow is used during discharge, the capacity of the battery can be enhanced due to the rapid movement of the electrolyte. As one example, a high flow rate of air may be released into the battery when the he controller senses a heavy load on the battery.  
         [0034]     An added benefit of the present cooling system is that the use of large amounts of air flow into a cell  10  dilutes any hydrogen gas inside the cell gas space, reducing it explosive intensity, and thereby providing a further safety feature for the present invention.  
         [0035]     A field test was run using the present invention on a large fork truck that was powered by twin batteries (1000 ampere hour cells). One battery was equipped with a continuously running evaporative cooling system according to the present invention and the other battery was unmodified. For the cooling system, air was provided at about 200 cc/min per cell, 24 hours a day. The air was provided by a DC pump drawing air from an air conditioned space. After two weeks of continuous operation in a rapid-charge environment, in which each battery was subject to precisely the same conditions, the evaporative cooled battery had an average temperature 15 deg F. (8.3 deg C.) lower than the unmodified battery.  
         [0036]     Thus it is appreciated that the present invention can lower the average temperature of electrolyte  18  over a given time period, for example 24 hours, as compared to a similar cell or battery that does not incorporate the present invention. Average temperatures can be at least 5, 10 even 15 or more degrees F. lower than that of an electrolyte in the absence of the present cooling system over a given time period, such as a 24 hour time period that can include one or more charges of the battery (charging can take place numerous times in a given day, and each charge can be for a different amount of time, e.g., 20 minutes, one hour, two hours or six hours).  
         [0037]     It is understood that the above-identified arrangements are merely illustrative of the many possible specific embodiments which represent applications of the present invention. Numerous and varied other arrangements can readily be devised in accordance with the principles of the invention without departing from the spirit and scope of the invention.