Abstract:
Method and apparatus for increasing the amount of power that may be extracted from a plurality of parallel connected battery banks by periodically effecting a switching between the banks for operative coupling to the power supply of an electronic or electrical device. The apparatus comprises at least two battery banks, a switching means, a microcontroller and a battery powered load. Upon each expiration of a predetermined time interval, the microcontroller monitors the voltage of the battery banks to determine which of the banks has the highest voltage. The microcontroller then positions the switch so that the bank with the highest voltage is coupled to the power supply of the load. In another embodiment, switching occurs upon a determination that the active bank has reached a specified power output.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This is a continuation of application Ser. No. 08/012,696 filed on Feb. 3, 1993, now abandoned which is a continuation of application Ser. No. 07/656,265 filed Feb. 14, 1991, now abandoned, for Rechargeable System with Separate Battery Circuits in a Common Module. 
    
    
     TECHNICAL FIELD 
     The invention relates generally to battery powered electrical and electronic systems and specifically to the efficient power management of battery capacity within such systems. 
     BACKGROUND OF THE INVENTION 
     An increasing number of electrical and electronic systems are being designed to be powered by batteries. Prime examples of such systems include portable computers such as laptops and notebooks. Portable computers are rapidly developing the capability to include most of the peripheral devices and high speed circuitry previously implemented only on conventional desktop personal computers. As the number of peripheral devices supported by portable computers increases, so does the amount of power consumed by the computer. As a result, it has become increasingly difficult to provide users with the maximum functionality and still maintain a reasonable battery life. 
     In early stages of the development of portable computers, it was possible to accomodate increased power demands by chaining several large capacity batteries together in series. With the demand for smaller, more light-weight computers, it is necessary to use a larger number of smaller batteries. The smaller battery cells are chained together in series to achieve the required watt/hour power rating to run the computer. Generally, these batteries are a rechargeable type, such as nickel cadmium (NiCd) cells, as they are capable of greater longevity and therefore are considered more economical than regular alkaline batteries. 
     However, the number of cells that can optimally be chained together in series is limited because as the number of cells increases, it becomes more difficult to detect and prevent cell damage due to a condition referred to as &#34;reverse charging.&#34; It is recognized that even closely matched cells connected in series will never have exactly the same power capacity, with the result being the lower capacity cell (or cells) becomes discharged more quickly than the higher capacity cells. Once discharged completely, current from the other cells flow through the discharged cell in the reverse direction. Such reverse charging of a cell causes it to overheat and, eventually, causes permanent damage to the cell. Reverse charging of a cell can lead to leakage of electrolyte and thus damage to surrounding components. 
     Since the number of cells that can be chained together in series is limited, typically two or more small chains of cells connected series are configured as separate banks which are then connected in parallel. Problems associated with connecting multiple rechargeable battery banks in parallel are well recognized. For example, the banks must be electrically isolated from one another to prevent one bank from discharging another one, should one bank experience a short circuit. The typical way of connecting multiple banks is by installing diodes in the lines connecting the banks so that current can exit each bank, but not be allowed to enter another bank in parallel. This solution is not entirely satisfactory because each diode dissipates a considerable amount of power, so less power is available to be supplied to the computer. 
     Another problem involved in battery powered systems is that in practical applications, such as a portable computer environment, the batteries are discharged at a rate which is greater than the optimal rate. Due to limits on ion mobility, the chemical reaction taking place in the cell has insufficient time to equalize throughout the cell. As a result, less energy can be extracted from each cell than would be the case if the cells were discharged more slowly or were allowed to be inactive for a period of time to allow the batteries to equalize. As used herein, the term &#34;inactive&#34; means not being charged or discharged. 
     A solution is needed which both minimizes power losses associated with existing battery arrangements and further which effectively manages the rate of battery discharge among multiple banks in order to extend total battery run time. 
     SUMMARY OF THE INVENTION 
     The foregoing problems are solved and a technical advance is achieved by method and apparatus for extending total battery life by intermittently switching the operative coupling of a power load between alternative banks of batteries connected in parallel to the load. In a departure from the art, the battery banks are isolated from one another by a controlled switching arrangement, rather than by diodes, thereby eliminating the dissipation of power by the diodes. In a further departure from the art, a microcontroller periodically monitors the banks so that a decision to switch from one bank to another is based on elapse of a predetermined time interval. Upon elapse of each interval, the bank possessing the higher voltage among the banks being compared is coupled to the load. 
     In an illustrative embodiment, a battery power management control system of the present invention comprises at least two banks of batteries connected in parallel via a relay switch to a load. In one implementation, the load comprises various components of a portable computer. A microcontroller controls actuation of the relay switch and thereby determines which of the banks is connected to the load through a power supply. The banks are connected to the microcontroller via operational amplifiers (op-amps), which scale and filter a voltage signal of each bank, and an analog-to-digital (A/D) converter, which interfaces with the microcontroller. Upon each elapse of a predetermined time interval, the microcontroller reads the voltage of the banks and determines which of the banks has the highest voltage potential. The microcontroller then positions the relay switch so that the bank having the highest voltage is coupled to the power supply. Consequently, each bank remains inactive for a period of time sufficient to allow the batteries to chemically equalize, thereby enabling more energy to be drawn from the banks than would otherwise be possible. 
     In another embodiment, the microcontroller of the system periodically reads both the voltage and current of the battery banks, and computes an instantaneous power output of each bank. The power output values are accumulated and used to compute a total power output of the then active bank, since it was last inactive. When the total power output reaches a predetermined threshold, the relay switch uncouples the bank and couples the next bank. This enables an even discharge among the banks which may be experiencing differing discharge rates when active, due to the changing demands of the load. 
     A technical advantage achieved with the invention is improved battery longevity between charges for systems with power demands requiring a high battery discharge rate. 
     Another technical advantage achieved is a battery system with a flat voltage discharge characteristic, resulting in improved power efficiency and system reliability. 
     A further technical advantage achieved is versatility in implementation of the control system of the invention. The system is readily configured and easily modified for any arrangement of batteries and loads. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic block diagram of a parallel battery bank charging and discharging control system embodying features of the present invention; and 
     FIG. 2 is a flowchart illustrating switch control logic implemented in a microcontroller of the system of FIG. 1. A copy of the source code illustrated by FIG. 2 is provided in Appendix A, attached hereto. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, reference numeral 10 designates a parallel battery bank charging and discharging control system embodying features of the present invention. The system 10 includes battery banks A and B, a relay switch 12, a power supply 14 connected to a load 16, and a microcontroller 18. It is understood that the load 16 may consist of any electrical or electronic device or circuit designed to run on batteries, for example a portable computer. As will be described, the system 10 operates under the control of the microcontroller 18 to efficiently supply power to the load 16 by alternating between use of banks A and B. 
     Battery banks A and B each comprise one or more battery cells (not shown) connected in series. For computer applications, the cells are often packaged in a self-contained, removable pack which comprises each of the banks A and B. In one embodiment, for example, each bank A and B includes a chain of seven (7) size &#34;4/3 A&#34; Nickel-Cadmium (NiCd) cells. 
     A line 20 connects a negative terminal of each of the banks A and B to electrical ground (GND). Lines 22 and 24 respectively connect a positive terminal of each of the banks A and B to the relay switch 12 and to respective operational amplifiers (op-amps) 26 and 28. Lines 30 and 32 respectively connect the op-amps 26 and 28 to an analog-to-digital (A/D) converter 18a. The A/D converter 18a preferably comprises part of the circuitry of the microcontroller 18. 
     The relay switch 12 is a double pole, double throw switch having a normally-closed (NC) lead 12a and a normally-open (NO) lead 12b, both connected via a line 34 to the power supply 14. Leads 12c and 12d of the switch 12 are respectively connectable by movable wiper arms 12e and 12f to the leads 12a and 12b. The leads 12c and 12d are respectively connected to the lines 22 and 24. As shown in FIG. 1, the switch 12 is in a first position in which the battery bank A is electrically connected via the lines 22 and 34 to the power supply 14. An actuator 12g of the switch 12 is provided for changing the state of the switch to a second position to electrically disconnect battery bank A from the power supply 14 and instead electrically connect battery bank B to the power supply 14, via lines 24 and 34. The actuator 12g is connected to a voltage source Vcc and to the microcontroller 18 via a control line 36. The microcontroller 18 controls the state of the switch 12 via the control line 36 in a manner subsequently described in detail. 
     In the preferred embodiment, the microcontroller 18 is commercially available from National Semiconductor as part number COP888CF. The architecture and design of the microcontroller 18 will depend on the particular requirements of the system 10. While not shown, it is understood that a microprocessor, logic devices, memory and other components are included in the microcontroller 18, as necessary. It is understood that the operation of the microcontroller 18 may be implemented by microcode instructions (firmware) stored in the microcontroller 18. The means of construction and programming of the microcontroller 18 will be apparent to one skilled in the art from the disclosure herein and the general state of the art. 
     The power supply 14 is connected to the load 16 via lines 38 and 40. Lines 38 and 40 are respectively connected to a voltage source Vcc and ground (GND). 
     Lines 42 and 44 are connected between the banks A and B, respectively, and the A/D converter 18a and are provided for furnishing amperage information regarding each bank to the microcontroller 18, for reasons subsequently described. 
     While not shown, it is understood that a battery charging circuit may be suitably incorporated in the system 10 for recharging the battery banks A and B. The switch 12 isolates the banks A and B from one another during charging. It is further understood that, although only two banks A and B are illustrated in FIG. 1, the system 10 circuitry can be extended to any number of additional banks. 
     In operation, the voltage signals of the banks A and B are filtered and scaled by the op-amps 26, 28, and then supplied to the A/D converter 18a for appropriate conversion prior to being input to the microcontroller 18. Upon each elapse of a predetermined time interval, the microcontroller 18 reads the voltage signals and determines which of the banks A or B has the greatest voltage capacity. It is understood that the optimal predetermined time interval is that which results in the maximum increase in battery run time. The optimal predetermined time interval between reading the condition of the banks A and B to make switching decisions is a function of the capacity of the battery banks and the characteristics of the particular load 16. For certain applications of the system 10 involving a portable computer, it has been determined that an optimal predetermined time period between banks A and B switching decisions by the microcontroller 18 is four (4) minutes. 
     The microcontroller 18 causes the switch 12 to be positioned so that the bank A or B having the highest voltage is coupled to the power supply 14. For example, the switch 12 as illustrated in FIG. 1 is initially positioned so that bank A is coupled to the power supply 14. After the predetermined time interval has elapsed, the microcontroller 18 reads the voltage signals on lines 30 and 32. If the microcontroller 18 determines that bank B has a higher voltage than bank A, it causes the switch 12 to uncouple bank A from the power supply 14 and couple bank B to the power supply 14. Conversely, if the microcontroller 18 determines that bank A the higher voltage, bank A remains coupled to the power supply 14. 
     In an alternative embodiment, banks A and B are further coupled to the microcontroller 18 by the lines 42, 44, respectively, which carry signals containing information regarding the amperage of banks A and B, respectively. In periodic intervals on the order of once every second, for example, the microcontroller 18 uses the voltage information on the lines 30, 32 and the amperage information on the lines 42, 44 to compute the instantaneous power output of the then active bank A or B. The instantaneous power output values are accumulated by the microcontroller 18 and used to calculate an approximation of the total power output by the active bank A or B since that bank was last inactive. When the accumulated value is equal to some predetermined threshold value, the relay switch 12 is switched so that the then inactive bank A or B is instead coupled to the power supply 14. 
     The switching decision is thus based on the total power output by the bank A or B during a particular period of activity, rather than simply based on the length of the period. This may be desirable when the load 16 varies because it includes devices or components which consume more power than others. For example, where the load 16 comprises components of a computer and a particular bank is supplying power to the computer during an interval of time in which the disk drive is constantly running, that particular bank will discharge at a higher rate than will a bank that is active when the disk drive is not running. 
     FIG. 2 is a flowchart illustrating control logic for operating the system 10 in accordance with the present invention. It is understood that the control logic may be implemented by microcode instructions contained within the microcontroller 18. 
     Execution begins in step 200. In step 202, a timer is initialized to a predetermined value and begins to run. In the present embodiment, it is understood the timer resides in the microcontroller 18 and is used as a counter to enable switching decisions upon elapse of a predetermined time interval. In step 204, a determination is made whether the timer has expired. If the timer has expired, execution proceeds to step 206. If the timer has not expired, execution returns to step 204 until the timer expires. 
     In step 206, variables HIGH VOLTS and HIGH BANK are initialized to zero. HIGH VOLTS represents the value of the highest voltage read from the banks and HIGH BANK represents the identity of the bank having the highest voltage. Also in step 206, a variable N is initialized to equal the number of battery banks in the system 10. In step 208, the microcontroller 18 reads the voltage of the Nth bank. In step 210, a determination is made whether the voltage read in step 208 is greater than HIGH VOLTS. If so, execution proceeds to step 212 where the voltage read in step 208 is stored in HIGH VOLTS. In step 214, N is stored in HIGH BANK. If in step 210, the voltage read in step 208 is not greater than HIGH VOLTS, control proceeds directly to step 216. 
     In step 216, N is decremented by 1. In step 218, a determination is made whether N is equal to zero. If N is equal to zero, indicating that all the banks have been read, execution proceeds to step 220 and the microcontroller 18 positions the switch 12 so that the bank identified by HIGH BANK is coupled to the power supply 12. Execution then returns to step 202 and continues as previously described. If in step 218, N is not equal to zero, indicating that at least one bank remains to be read, execution returns to step 208 and the voltage of the next bank is read and compared as previously described. 
     It is understood that the present invention can take many forms and embodiments. The embodiments shown herein are intended to illustrate rather than to limit the invention, it being appreciated that variations may be made without departing from the spirit of the scope of the invention. For example, the switch 12 may be implemented using field effect transistors (FETs), rather than a relay switch as herein described, or may be implemented in various forms of logic. Further, it will be appreciated that different elements of the system may be embodied as a single integrated chip, or any varying combination of discrete digital or analog components interconnected in a standard manner. Also, any number or type of battery banks may be connected in parallel for furnishing power to the system load. The load may comprise any device or devices requiring power. Examples include, inter alia, circuits, lighting devices and consumer electronic products such as camcorders, players and portable telephones. 
     Although illustrative embodiments of the invention have been shown and described, a wide range of modification, change and substitution is intended in the foregoing disclosure and in some instances some features of the present invention may be employed without a corresponding use of the other features. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention. ##SPC1##