BATTERY CHARGE MAINTENANCE SYSTEM AND METHOD

Abstract: An electrical system of a vehicle has a starting battery and a reserve battery. The charge of the starting battery is maintained by a charge maintenance device under control of a controller which couples energy from the reserve battery to the starting battery during periods when the vehicle is not being used or during periods of operation where the starting battery voltage requires additional charge.

Description: BATTERY CHARGE MAINTENANCE SYSTEM AND METHOD Technical Field The present invention relates generally to battery charging systems and more particularly to a system for maintaining the charge of one or more batteries in a dual-battery system. Background of the Invention Automobiles and other combustion engine powered vehicles typically include an electric starting motor for starting the combustion engine for operation. In this regard, the starting motor is coupled to a starting circuit which generally receives electrical energy from an on-board electric storage battery. The starting circuit selectively couples electrical energy from the battery to the starting motor that operates to cycle the engine to initiate sustained operation. In common vehicle applications, the battery also provides electrical energy to a variety of electric power consuming devices such as engine control electronics, lighting systems, and vehicle accessories. Traditional batteries for these applications, often referred to as starting, lighting and ignition (SLI) batteries, are multi-cell, lead-acid batteries. That is, the batteries are constructed from lead plates pasted with active material and arranged into stacks. The stacks are inserted into partitioned cell compartments of a battery container, electrically connected, and flooded with dilute acid electrolyte. SLI batteries of this construction are more than adequate for providing the relatively high power demand required of engine starting as well as the relatively low power demand to maintain electrical accessories during both vehicle operation and periods of non-operation. However, because of the seemingly disparate functions the SLI battery is required to perform, short duration high-power output and long duration low-power output, the battery design can not be optimized for performing either of these tasks. An additional drawback of these batteries is relatively low specific energy (kilo-watt hours/grams (kWh/g)) as compared to other battery constructions owing to the weight of the lead plates and the liquid electrolyte. There has been suggested a battery system for vehicle use which includes two batteries. A first battery in the system, a starting battery, is optimized for engine starting, that is, designed specifically for short duration, high-power output. A second battery in the system, a reserve battery, is optimized for operating and maintaining non-starting electrical loads. An advantage of such a system is that the starting battery may be made smaller and lighter yet capable of provide a high power output for a short period of time. In addition, the reserve battery may be made smaller and lighter yet capable of satisfying the relatively low power requirements of the vehicle accessories. In combination, the two battery system may require less space and weigh less than a single traditional SLI battery. A limitation of such a system lies with the starting battery. Small, light weight batteries capable of high operational discharge rates typically have high self-discharge rates. That is, left unattended, the starting battery will self-discharge to a level where it is incapable of providing sufficient electrical energy for starting the engine. During vehicle operation, the starting battery is charged using the vehicle electrical system. Therefore, where the vehicle is regularly used starting battery self-discharge is not a concern. However, if the vehicle is left unused for an extended period of time or the vehicle is used for very short trips during which time the vehicle electrical system does not sufficiently charge the starting battery, the starting battery may become discharged leaving the operator stranded. Therefore, a dual-battery system for vehicle starting and operation that provides the advantages of reduced size and weight and yet overcomes the disadvantages of starting battery self-discharge is highly desirable. Summary of the Invention The present invention constitutes a battery which has a first plurality of cells that are electrically coupled to form a first battery portion. A second plurality of cells are electrically coupled to form a second battery portion. A charge maintenance device couples the first battery portion with the second battery portion. The charge maintenance device applies energy from the first battery portion to the second battery portion for maintaining the charge status of second battery portion. In the preferred embodiment of the invention, the charge maintenance device comprises a charge pump circuit. For example the charge maintenance device has a circuit with a resistor coupled in series with a diode and a capacitor in parallel to the resistor and diode. A transistor is connected intermediate the resistor and diode and is controlled by a control element that is operable to disable the charge maintenance device responsive to the state-of- charge of at least one of the first and the second battery portions. Brief Description of the Drawings Figure 1 is a schematic diagram of an dual-battery system for vehicle starting and operation in accordance with a preferred embodiment of the present invention; Figure 2 is a diagram of a charge control circuit in accordance with a preferred embodiment of the present invention; Figure 3 is a diagram of a charge control circuit in accordance with a preferred embodiment of the present invention; Figure 4 is a diagram of a charge control circuit in accordance with a preferred embodiment of the present invention; and Figure 5 is a diagram of a dual-battery system for vehicle starting in accordance with an alternate preferred embodiment of the present invention. Detailed Description of the Preferred Embodiments The present invention is described in terms of several preferred embodiments adapted for use in a dual-battery based vehicle electrical system. The batteries in the system provide electrical energy for various vehicle operation functions and receive charging from the vehicle electrical system. It will be appreciated that the scope of the invention is not limited vehicle applications or dual-battery systems. For example, the invention may find application in any multiple battery system. The invention may also find application in a vehicle starting system in which a starting battery is replaced with a starting capacitor for providing starting energy. Referring to Figure 1 , a vehicle electrical system 1 0 includes battery system 1 2 having a starting battery 1 4 connected to provide electrical energy through switch 1 6 to engine starting motor 1 8. Starting motor 1 8 is mechanically coupled to the engine of the vehicle (not shown) for starting the engine, as is well known in the art. Starting battery 1 4 is preferably a high-rate battery that also is coupled through charge maintenance device 22 to reserve battery 20 and to the remainder of electrical system 1 0. Reserve battery 20 is preferably of the absorptive glass mat (AGM) type construction having high reserve capacity. That is, reserve battery 20 is adapted to provide a relative low-rate discharge for an extended period of time. Reserve battery 20 is connected to electrical system 1 0, and particularly to vehicle loads 26, for providing electrical energy during normal vehicle operation and during idle periods. Each of starting battery 1 4 and reserve battery 20 are coupled to alternator 24. Alternator 24 is mechanically coupled to the engine in a manner that is well know in the art and during periods of vehicle operation provides electrical energy for charging starting battery 1 4 and reserve battery 20. Alternator 24 also provides electrical energy to vehicle loads 26 during normal operation. The output of alternator 24 is controlled through field voltage regulation or other suitable means under operation of engine controller 32, as is known in the art. In accordance with a preferred embodiment of the present invention, charge maintenance device 22 is provided and under control of controller 30 couples energy from reserve battery 20 to starting battery 1 4 for maintaining the charge status of starting battery 22. For example, energy may be channeled to starting battery 14 during periods when the vehicle is not being used or during periods of operation where the starting battery voltage requires additional charge. Since a relatively small power draw from reserve battery 20 may be used to maintain starting battery 14 at a substantially full state-of- charge without adversely effecting the charge status of reserve battery 20, the self-discharge characteristic of starting battery 1 4 may be overcome. Referring to Figure 2 a preferred embodiment of charge maintenance device 22 is a circuit 200 providing a low level, milli-amp level, current pulse from reserve battery 20 to starting battery 1 4. Circuit 200 includes NAND gates 202, 21 2 and 21 4, resistors 204 and 208, diode 206 and capacitor 21 0 operatively coupled to form a pulse generator. Reserve battery 20 is coupled through switch 238 to a first input of NAND gate 202. The second input of NAND gate 202 and the output of NAND gate 202 are connected to resistors 204 and 208, diode 206 and capacitor 21 0 and collectively form an oscillator. That is, when switch 236 is closed, NAND gate 202 produces a periodic pulse train. In the preferred embodiment, the precise frequency of the pulse train is not important to operation of circuit 200, but it is preferably set at about 5 - 30 kilohertz (kHz) . The pulse train is buffered and amplified through NAND gates 21 2 and 21 4 and coupled via a gate resistor network including resistors 21 6 and 21 8 to the gate of transistor 220 for periodic on/off cycling. Preferably, transistor 220 is a field effect transistor (FET), but it should be understood that any suitable switching device may be used without departing from the fair scope of the invention. With transistor 220 in the on position, current is allowed to flow through inductor 226, which is coupled to reserve battery 20 at "B", transistor 220 and resistor 224. When transistor 220 is switched off, a voltage buildup within inductor 226 is dissipated through current limit resistor 234 into starting battery 14 coupled at "C" for providing a charge maintenance current. Diode 228 provides reverse current flow protection, and resistor 230 and zener diode 236 provide a voltage dumping path for protecting transistor 220 from excessive voltage buildup. Zener diode 236 is preferably a 1 5 volt device for clamping inductor 226 voltage at between 1 5 and 1 6 volts. Controller 30 acts to open and close switch 238 for activating and deactivating circuit 200. It is possible to allow circuit 200 to operate continuously without adverse affect to either starting battery 1 4 or reserve battery 20. However, to maximize the standby capability of the preferred system, circuit 200 is activated when starting battery 1 4 voltage falls below a threshold. For example, controller 30 may be adapted to sense starter battery 1 4 voltage and when it falls below approximately 1 2.75 volts to close switch 238 to activate circuit 200. Once activated, controller 30 initiates a timer, and circuit 200 is allowed to operate for between 6 - 24 hours depending capacity of starting battery 14 and the ability of circuit 200 to provide charge current to starting battery 1 4. At the conclusion of the time period, switch 238 is opened deactivating circuit 200. Controller 30 might also be adapted to sense when starting battery 1 4 voltage exceeds a threshold value for deactivating circuit 200, or controller 30 may continuously activate circuit 200 in response to various operating conditions, for example, environmental conditions such as extreme ambient cold. Controller 30 is shown in Figure 1 separate from engine controller 32. It will be appreciated that the function of controller 30 may be implemented within engine controller 32 or another on-board vehicle controller without departing from the fair scope of the invention. It will be further appreciated that controller 30 may be adapted to monitor in a sophisticated manner the state-of-charge of starting battery 1 4 for initiating circuit 200 operation. An example of an apparatus and method for state-of-charge monitoring is shown and described in United States Patent No. 5,321 .627. Also, though not shown in Figure 2, a direct current path is provided for charging starting battery 1 4 directly from alternator 24 output during vehicle operation. Referring now to Figure 3, an alternate implementation of charge maintenance device 22 is a circuit 300 shown for use in a system where starting battery 14 and reserve battery are of different voltages. It is contemplated within the scope of the present invention that the starting battery, because of its high-rate characteristics, may be specified at, for example, 1 0 volts as compared to a traditional 1 2 volt starting battery. Reserve battery 20 is maintained in the exemplary embodiment as a 1 2 volt battery. Circuit 300 includes a current limit resistor 304 and blocking diode 306. Since reserve battery 20 is of a sufficiently higher potential than starting battery 14, a current flow is initiated through resistor 304 for maintaining the charge of starting battery 14. Circuit 300 further includes a current path existing of diodes 31 0, 31 2 and 31 4. The inherent voltage drop across diodes 31 0-31 4 substantially inhibits a current flow through this path when the vehicle is not operating, i.e., alternator 24 is not producing energy. However, when the vehicle is operating and the alternator is producing electricity at approximately 1 4 volts, current does flow through diodes 31 0-31 4 for rapid charging of starting battery 1 4. As can be further seen in Figure 3, relay 302 provides for selective coupling of starter battery 1 4 and reserve battery 20. Relay 302 is closed upon controller 308 energizing coil 31 0 (controller 308 is shown in Figure 3, but it should be understood that its function can be incorporated into controller 30) . Controller 308 is coupled to sense both starting battery 1 4 voltage and reserve battery 20 voltage. When closed, starting battery 1 4 and reserve battery 20 are coupled in parallel and in this arrangement, reserve battery 20 is made available to provide additional energy for starting the vehicle. Conditions such as low starting battery 1 4 voltage or cold ambient conditions may be sensed by controller 308 for closing relay 302. Figure 4 shows a circuit similar to that shown in Figure 3 for use in a system where starting battery 14 and reserve battery are of different voltages, again reserve battery 20 being at least about 2 volts greater than starting battery 14. Circuit 400 includes a current limit resistor 404 and blocking diode 406 for providing a current flow path to starting battery 14 for maintaining the charge of starting battery 1 4. Circuit 400 further includes switching transistor 41 8 operating under the control of controller 408 (controller 408 is shown separate from controller 30, but its functions may easily be incorporated into controller 30) which allows controller 408 to open the current path between reserve battery 20 and starting battery 14. Controller 408 will preferably operate as described above for selectively opening and closing the current path in response to starting battery 1 4 voltage, a sensed state-of-charge or other operating condition. Circuit 400 also includes a current path existing of diodes 41 0, 41 2 and 41 4 for direct charging of starting battery 1 4 as described, and relay 402 and coil for selective coupling of the starting and reserve batteries. With reference now to Figure 5 still another embodiment of charge maintenance device 20 is shown as circuit 500 for use with a modified starting battery which is indicated for clarity as 1 4' . Starting battery 1 4' is partitioned into two battery portions 51 4 and 51 6. Each battery portion consists of a group of cells of starting battery 14' less than the total number of cells. A voltage tap 520 is provided for selectively interconnecting each of battery portions 51 4 and 51 6 with charge maintenance device 22. The number of cells in each of battery portions 51 4 and 51 6 may vary, but the number of cells is limited such that the voltage potential of each of battery portions 514 and 51 6 is sufficiently below the voltage potential of reserve battery 20. In this regarding, starting battery 14' may have a total voltage potential exceeding that of reserve battery 20. For example, starting battery 1 4' may be a 1 4, 1 6 or greater volt battery, yet each battery portion 51 4 and 51 6 is maintained at or below approximately 1 0 volts (and shown in the this example as 6 volts each.) Hence, the charge of starting battery 1 4' may be maintained by reserve battery 20 notwithstanding starting battery 1 4' having a greater voltage. With continued reference then to Figure 5, circuit 500 includes a first current path, indicated by arrow "A" and a second current path indicated by arrow "B" . Circuit 500 further includes relay 502 which has at least two, and preferably three, operating positions controlled by selectively energizing coil 51 6 by controller 508 (controller 508 is shown separate from controller 30, but its functions may easily be incorporated into controller 30). In a first position, relay 502 couples reserve battery 20 to battery portion 51 6 along current path "A" consisting of blocking diode 506, controller 508, limit resistor 504 and tap 520. In a second position, relay 502 coupled reserve battery 20 to battery portion 51 4 along current path "B" consisting of tap 520, limit resistor 504 and controller 508. In a third position, relay 502 uncouples reserve battery 20 from starting battery 1 4' . While not shown, it should be appreciated that additional switching may be added to circuit 500 for coupling starting battery 1 4' and reserve battery 20 in parallel as previously described. In accordance with the preferred implementations of the present invention, controller 508 selectively couples battery portions 514 and 51 6 to reserve battery 20 for maintaining the charge of each portion. Controller may operate in response to a voltage of the battery portion falling below a threshold, a sensed state-of-charge condition, a timed schedule or other conditions. As will be appreciated any number of charging strategies may be employed without departing from the fair scope of the invention. In one preferred implementation, when one or the other of battery portions 51 4 and 51 6 voltage falls below a threshold, that portion is coupled to reserve battery 20 for a time period and then the other portion is coupled for a time period. In this manner, the total voltage potential of starting battery 1 4' is maintained. The present invention has been described in terms of several preferred embodiments of batteries and battery systems adapted for use in vehicles. Its scope, however, is not limited to the examples presented herein, and one of ordinary skill in the art will readily appreciate its broad application.

Claims: CLAIMS We claim: 1 . A battery comprising a first plurality of cells electrically coupled to form a first battery portion and a second plurality of cells electrically coupled to form a second battery portion; a charge maintenance device coupling the first battery portion with the second battery portion. 2. The battery of claim 1 wherein the charge maintenance device comprises a charge pump circuit. 3. The battery of claim 1 wherein the charge maintenance device comprises a circuit having a resistor coupled in series with a diode and a capacitor in parallel to the resistor and diode. 4. The battery of claim 3 wherein the charge maintenance device comprises a circuit having a transistor positioned intermediate the resistor and diode, the transistor being coupled to a control element. 5. The battery of claim 4 wherein the control element is operable to disable the charge maintenance device responsive to the state-of-charge of at least one of the first battery portion and the second battery portion. 6. The battery of claim 1 wherein the charge maintenance device couples a charging current between the first battery portion and the second battery portion. 7. The battery of claim 6 wherein the charge maintenance device is responsive to the state-of-charge of at least one of the first battery portion and the second battery portion . 8. The battery of claim 6 wherein the charge maintenance device comprises a control element for selectively activating and deactivating the charge maintenance device. 9. The battery of claim 1 wherein the first battery portion is optimized for high-rate discharge. 10. The battery of claim 1 wherein the first battery portion is optimized for reserve capacity. 1 1 . The battery of claim 1 wherein the charge maintenance device comprises a switch for selectively coupling the first battery portion and the second battery portion in parallel. 1 2. The battery of claim 1 wherein the charge maintenance device is operable to detect a voltage potential of the first battery portion below a threshold and to selectively couple the second battery portion to the first battery portion upon detection of the voltage potential below the threshold. 1 3. The battery of claim 1 wherein the charge maintenance device is operable to detect a voltage potential of the first battery portion below a threshold and to selectively couple the second battery portion to the first battery portion upon detection of the voltage potential below the threshold for a predetermined time period. 1 4. A battery system comprising: a first battery; a second battery; a charge maintenance device operatively coupled between the first battery and the second battery. 1 5. The battery system of claim 1 4 wherein the first battery comprises a plurality of interconnected electro-chemical cells. 1 6. The battery system of claim 1 4 wherein the first battery is optimized for one of: high rate discharge and reserve capacity. 1 7. The battery system of claim 1 4 wherein the charge maintenance device comprises a charge pump circuit. 1 8. The battery system of claim 1 4 wherein the charge maintenance device comprises a circuit having a resistor coupled in series with a diode and a capacitor in parallel to the resistor and diode. 1 9. The battery system of claim 1 8 wherein the charge maintenance device comprises a circuit having a transistor positioned intermediate the resistor and diode, the transistor being coupled to a control element. 20. The battery system of claim 1 9 further comprising a control element, wherein the control element is operable to disable the charge maintenance device responsive to the state-of-charge of at least one of the first battery portion and the second battery portion 21 . The battery system of claim 14 wherein the charge maintenance device couples a charging current between the first battery portion and the second battery portion. 22. The battery system of claim 21 wherein the charge maintenance device is responsive to the state-of-charge of at least one of the first battery portion and the second battery portion. 23. The battery system of claim 21 wherein the charge maintenance device comprises a control element for selectively activating and deactivating the charge maintenance device. 24. The battery system of claim.1 4 wherein the charge maintenance device comprises a switch for selectively coupling the first battery and the second battery in parallel. 25. The battery system of claim 1 4 wherein the charge maintenance device is operable to detect a voltage potential of the first battery below a threshold and to selectively couple the second battery to the first battery upon detection of the voltage potential below the threshold. 26. The battery system of claim 1 4 wherein the charge maintenance device is operable to detect a voltage potential of the first battery below a threshold and to selectively couple the second battery to the first battery upon detection of the voltage potential below the threshold for a predetermined time period. 27. A vehicle electrical system comprising operatively coupled: a first battery; a second battery; a charge maintenance device: an electrical energy source powered from the vehicle. 28. The vehicle electrical system of claim 27 wherein the first battery is optimized for one of: high rate discharge and reserve capacity. 29. The vehicle electrical system of claim 27 wherein the charge maintenance device comprises a charge pump circuit. 30. The vehicle electrical system of claim 27 wherein the charge maintenance device comprises a circuit having a resistor coupled in series with a diode and a capacitor in parallel to the resistor and diode. 31 . The vehicle electrical system of claim 30 wherein the charge maintenance device comprises a transistor positioned intermediate the resistor and diode, the transistor being coupled to a control element. 32. The vehicle electrical system of claim 27 wherein the control element is operable to disable the charge maintenance device responsive to the state-of- charge of at least one of the first battery portion and the second battery portion. 33. The vehicle electrical system of claim 32 wherein the control element comprises a vehicle system controller. 34. The vehicle electrical system of claim 27 wherein the charge maintenance device couples a charging current between the first battery portion and the second battery portion. 35. The vehicle electrical system of claim 34 wherein the charge maintenance device is responsive to the state-of-charge of at least one of the first battery portion and the second battery portion. 36. The vehicle electrical system of claim 35 wherein the charge maintenance device comprises a control element for selectively activating and deactivating the charge maintenance circuit. 37. The vehicle electrical system of claim 36 wherein the control element comprises a vehicle system controller. 38. The vehicle electrical system of claim 27 wherein the electrical energy source is adapted to be selectively coupled to charge the first and second batteries during vehicle operation. 39. The vehicle electrical system of claim 27 wherein the vehicle electrical supply system is coupled to at least one vehicle load and wherein the second battery is adapted to supply electrical energy to the vehicle load when the vehicle is not being operated. 40. The vehicle electrical system of claim 27 wherein the charge maintenance device comprises a switch for selectively coupling the first battery and the second battery in parallel. 41 . The vehicle electrical system of claim 27 wherein the charge maintenance device is operable to detect a voltage potential of the first battery below a threshold and to selectively couple the second battery to the first battery upon detection of the voltage potential below the threshold. 42. The vehicle electrical system of claim 27 wherein the charge maintenance device is operable to detect a voltage potential of the first battery below a threshold and to selectively couple the second battery to the first battery upon detection of the voltage potential below the threshold for a predetermined time period. 43. A battery system comprising: a first battery having a plurality of battery portions, each portion having a voltage potential: a second battery having a voltage potential greater than the voltage potential of the battery portions; a charge maintenance device selectively coupling the second battery to the battery portions. 44. The battery system of claim 43 wherein the first battery is optimized for one of: high rate discharge and reserve capacity. 45. The battery system of claim 43 wherein the charge maintenance device comprises switch means for selectively coupling the second battery to the battery portions through a current limit means. 46. The battery system of claim 43 further comprising a control element, wherein the control element is operable to enable the charge maintenance device responsive to the state-of-charge of at least one of the battery portions. 47. The battery system of claim 43 wherein the charge maintenance device comprises a switch for selectively coupling the battery portions in parallel. 48. The battery system of claim 43 wherein the charge maintenance device is operable to detect a voltage potential of one of the battery portions below a threshold and to selectively couple the second battery to the one of the battery portions upon detection of the voltage potential below the threshold 49. The battery system of claim 43 wherein the charge maintenance device is operable to detect a voltage potential of one of the battery portions below a threshold and to selectively couple the second battery to the one of the battery portions upon detection of the voltage potential below the threshold for a predetermined time period. 50. A method for maintaining the charge of a starting battery in a vehicle having both the starting battery and a reserve battery, the method comprising: coupling a charge current from the reserve battery to the starting battery. 51 . The method of claim 50 wherein the step of coupling the charge current from the reserve battery to the starting battery comprises: selectively coupling the charge current from the reserve battery to the starting battery in response to at least one of: a voltage of the starting battery, a state-of-charge of the starting battery, an elapsed time period and an environmental condition . 52. The method of claim 50 wherein the step of coupling the charge current from the reserve battery to the starting battery comprises: detecting a condition of the starting battery, and coupling the charge current from the reserve battery to the starting battery for a predetermined time period. 53. The method of claim 52 wherein the step of detecting a condition of the starting battery comprises detecting at least one of a voltage and a state-of- charge.

Inventors: DOUGHERTY THOMAS J, IVERSON MICHAEL E, SEGALL WILLIAM P, DOUGHERTY, THOMAS, J., IVERSON, MICHAEL, E., SEGALL, WILLIAM, P.
Applicants: JOHNSON CONTROLS TECH CO, JOHNSON CONTROLS TECHNOLOGY COMPANY
Application year: 1998
Main classification: H02J7
IPC codes: H01M10, H02J7