Abstract:
A portable electronic system which has two electrically separate battery banks in a single removable battery pack. A power-management controller switches between the two banks in service--even when neither of the banks is exhausted--to provide optimal active and resting times for the batteries.

Description:
RELATED APPLICATION DATA 
     Ser. No. 07/656,265, filed Feb. 14, 1991 is the parent application which is now abandoned; Ser. No. 08/119,904, filed Sep. 10, 1993 is a continuation of Ser. No. 07/656,265 which is now abandoned; Ser. No. 08/443,610 is the present application which is a continuation application of Ser. No. 08/119,904. 
    
    
     PARTIAL WAIVER OF COPYRIGHT 
     All of the material in this patent application is subject to copyright protection under the copyright laws of the United States and of other countries. As of the first effective filing date of the present application, this material is protected as unpublished material. 
     However, permission to copy this material is hereby granted to the extent that the owner of the copyright and maskwork rights has no objection to the facsimile reproduction by anyone of the patent document or patent disclosure, as it appears in the United States Patent and Trademark Office patent file or records, but otherwise reserves all copyright and maskwork rights whatsoever. 
     CROSS-REFERENCE TO OTHER APPLICATIONS 
     The following applications of common assignee contain at least some drawings in common with the present application, and are believed to have effective filing dates identical with that of the present application, and are all hereby incorporated by reference: 
     Ser. No. 655,889, filed Feb. 14, 1991, entitled &#34;Portable Computer with BIOS-Independent Power Management&#34;; 
     Ser. No. 656,647, filed Feb. 14, 1991, entitled &#34;Portable Computer System with Adaptive Power-Control Parameters&#34;; 
     Ser. No. 655,619, filed Feb. 14, 1991, entitled &#34;Portable Computer with Dual-Purpose Standby Switch&#34;; 
     Ser. No. 656,262, filed Feb. 14, 1991, entitled &#34;Method for Detecting Low Battery State Without Precise Calibration&#34;. 
     BACKGROUND AND SUMMARY OF THE INVENTION 
     The present invention relates to portable battery-powered electronic systems. 
     Constraints of Available Rechargeable Battery Technology 
     Portable computers, like many other portable electronic or electrical systems, often use rechargeable batteries for their power source. Users often prefer rechargeable batteries for convenience and for long system life. 
     The specific rechargeable battery type most often used is the Ni--Cd battery. As of 1990, no other rechargeable battery technology can match the power-to-weight ratio and cost-per-joule of Ni--Cd batteries. Even if affordable new battery technologies are introduced (as may be expected), such improvements will be driven by battery manufacturers, not by computer system designers. Moreover, many of the same battery-management requirements will apply to other battery technologies as well as to Ni--Cd batteries. 
     Ni--Cd batteries have some significant nonlinearities which make optimal control difficult. Other battery types may share some or all of the following characteristics; but the following discussion will be primarily directed toward Ni--Cd batteries, because of their dominance of the rechargeable battery market. 
     One non-linear effect is the well-known &#34;memory&#34; effect. If a Ni--Cd battery is repeatedly only partially discharged before recharging, the microstructure of the battery will gradually adapt, so that the battery&#39;s full capacity is no longer available. 
     Another non-linear effect is that total amount of energy which can be withdrawn in a discharge cycle is somewhat dependent on the rate of discharge. 
     A further non-linear effect is the use of &#34;trickle charge&#34; currents. A battery which is already fully charged can be maintained at maximum readiness by applying a very small current to the battery. This phenomenon is very well known in lead acid batteries and also applies to Ni--Cd batteries. 
     Bank-Switching in a System with Two Battery Banks 
     The chemistry of a Ni--Cd battery can become imbalanced at the battery&#39;s peak rate of discharge, with resulting net loss of stored charge. The present invention permits this problem to be avoided. 
     The present invention provides improved management of the discharge cycle of Ni--Cd batteries, by drawing current alternately from two banks of batteries, while monitoring the voltage across and current through each. 
     The innovative teachings disclosed herein provide a portable electronic system which has two electrically separate battery banks in a single removable battery pack. A power-management controller switches between the two banks to provide optimal active and resting times for the batteries. 
     Bank-switching is preferably performed by a relay. This has been found to give low power loss during switching, and adequate robustness against transients. 
     In a contemplated alternative (but advantageous) embodiment, current monitoring is used to permit the changing characteristics of the batteries during discharge to be precisely monitored. However, in the invention as presently practiced, a simple timing function is used to govern the bank-switching. 
     In the presently preferred embodiment, the microcontroller can read the voltage of both battery banks at all times. Therefore, by observing the voltage differential as the load is switched, the banks are load-tested. This can be performed as part of the POST (power-on self-test) routine. 
     A difficulty with current battery systems is detecting when the battery is nearing the end of its life. A further advantage of the disclosed innovations is that they permit the full history of each battery pack to be monitored accurately. 
     Some prior systems have had the capability to switch between two banks of batteries. See, for example, U.S. Pat. No. 4,451,742 to Aswell, which is hereby incorporated by reference. However, such prior teachings are believed to have concentrated on issues of reliability and additional redundancy. By contrast, the present invention provides an arrangement wherein two banks of batteries are used, not primarily for reasons of improved reliability, but rather to provide improved battery management (and maximization of battery life). 
     In particular, portable computers have been proposed with two separate removable banks of rechargeable batteries. In such systems, the user could remove and replace the battery packs without ceasing operation (as long as a usable battery pack remained available in either one of the battery slots). 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The present invention will be described with reference to the accompanying drawings, which show important sample embodiments of the invention and which are incorporated in the specification hereof by reference, wherein: 
     FIG. 1 shows the overall electrical organization of the portable laptop computer of the presently preferred embodiment. 
     FIG. 2 shows the analog connections used, in the presently preferred embodiment, for monitoring the state of the two battery banks. 
     FIG. 3 shows more details of the power-supply and power-control circuitry actually used, in the presently preferred embodiment. 
     FIG. 4A shows the physical structure, and FIG. 4B shows the electrical connections, of the rechargeable battery module of the presently preferred embodiment, containing two electrically separate banks of 7 batteries each. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The numerous innovative teachings of the present application will be described with particular reference to the presently preferred embodiment. However, it should be understood that this class of embodiments provides only a few examples of the many advantageous uses of the innovative teachings herein. In general, statements made in the specification of the present application do not necessarily delimit any of the various claimed inventions. Moreover, some statements may apply to some inventive features but not to others. 
     Overall Organization 
     The presently preferred embodiment provides a 80C286 or 386SX laptop computer, with hard disk and LCD display, powered by rechargeable Ni--Cd batteries and a nonrechargeable lithium battery. 
     Electrical Organization 
     FIG. 1 shows the overall electrical organization of the portable laptop computer of the presently preferred embodiment. 
     The presently preferred embodiment is actually planned for production in two versions, one using a CMOS version of the 80286 processor and one using a CMOS version of a 386SX processor. Of course, these two processors are extremely similar to each other, and the differences between them have little relevance to the power of management architecture features described. Disclosed innovations can be applied not only to other Intel 8086-derived processors, such as the 80386 and 80486, but can also be applied to other processor families which may, in the future, find use in low-power portable computer systems. 
     The presently preferred embodiment relates to systems used in the ISA architecture. (Such systems are also referred to as systems which use the &#34;AT bus.&#34;) However, it is alternatively possible to adapt at least some of the disclosed teachings to other architectures, such as EISA bus systems or to other buses which may find use in the future. 
     In the presently preferred embodiment, an HT21 chip, from Headland Technologies, is used to provide a variety of peripheral support functions to the main microprocessor. These include bus management, memory management, interrupt control, and DMA control. Serial port management and keyboard interface are provided by an 82C186 combination chip from VTI. Of course, other implementations of support logic and glue logic can be used if desired, within this well-known architecture. 
     The presently preferred system embodiment is a family of two highly similar notebook computers, varying primarily in the processors used. Both have an external closed size of about 8.5×11×2 inches. One version is based on an Intel 80C286 microprocessor running at 12.5 MHz, and the other version is based on an Intel 386SX processor running at 20 MHz. Both notebooks contain similar I/O devices, including, in the presently preferred embodiment: 
     a Sharp VGA fiat panel display; 
     Conners Peripherals 222 or 242 20 or 40 MB 2.5&#34; hard disk; 
     Epson 3.5&#34; floppy disk drive; 
     WD 90C20 VGA controller chip; 
     VTI 82C106 I/O combo chip; 
     1 MB on board VSOP memory; 
     2 expansion memory slots; and 
     Power management microcontroller, with the Microcontroller Interface chip (&#34;MILES&#34;) gate array assembly. 
     In addition, the following I/O connectors are available on the back panel for external devices: 
     25 pin D connector for parallel/floppy disk; 
     9 pin D connector for serial; 
     15 pin D connector for external CRT; 
     6 pin DIN connector for mouse; and 
     6 pin DIN connector for external keyboard. 
     Also available, through slide off panels, are an expansion connector for an optional modem, an 80387SX numeric coprocessor socket, and the 2 expansion memory connectors. 
     Rechargeable Battery Module 
     FIG. 4A shows the physical structure, and FIG. 4B shows the electrical connections, of the rechargeable battery module of the presently preferred embodiment, containing two electrically separate banks of 7 batteries each. Note that a fuse is included in the middle of each bank of batteries. 
     In the presently preferred embodiment, the battery module is configured as two banks of 7 Ni--Cd batteries in series. Thus, each bank provides a rated voltage of 8.4 Volts. 
     The battery sizes are selected, in the presently preferred embodiment, to provide a charge capacity of 1700 mA-hr for each bank; but of course the battery sizings could be changed if needed. 
     Connections for Power Supply and Management 
     FIG. 2 shows the analog connections used, in the presently preferred embodiment, for monitoring the state of the two battery banks. FIG. 3 shows further details of the power-supply and power-control circuitry in the preferred embodiment. 
     The power system for any battery powered computer varies quite a bit from a standard desktop computer. The power system used in the presently preferred embodiment is unusual, even for battery powered systems. Power is available from several sources; the main battery, a reserve battery, and an AC to DC adapter which is external to the system. Power from the main battery and the AC to DC converter must be regulated to 5 volts through the DC to DC converter 210. This is located on the System Power Module (SPM, not shown), which is located along the back right hand wall of the case. The ON/OFF switch is also located on the SPM, and protrudes through the plastic case on the right side of the unit. Since the external AC to DC adapter and main battery are on a common node on the input to the DC to DC converter, the battery banks are protected from overcurrent from the AC adapter by diodes D1A and D1B. Diodes D1 can dissipate up to 900 mW at their 2 Amp nominal current draw. A significant dissipation results even when a low voltage drop Schottky Barrier diode is used. In the presently preferred embodiment, this power loss is reduced by shorting diodes D1 with a pair of FETs whenever there is no external power being supplied. (This circuit arrangement, with FET pair Q3&#39; and Q4&#39;, may be seen in FIG. 3.) 
     The battery management circuitry, in the presently preferred embodiment, is centered around a National Semiconductor COP888CF microcontroller (shown as U5 in FIG. 2). This device has 8 analog inputs to an analog to digital converter, 2 timer outputs that can be set up as pulse generators, several digital I/O lines and internal program ROM. The microcontroller monitors both banks of batteries 220A and 220B for both the current through, and the voltage of, each string of 7 cells. (In the presently preferred embodiment, each battery bank includes seven KR-1700AE Ni--Cd cells, as shown in FIGS. 4A and 4B.) The microcontroller software applies a very short duty cycle pulse for a period of time to check that the battery is accepting the charge current properly, and is not shorted. Then, when the battery voltage reaches about 7 volts, the pulse width is increased until about 800 mA are being applied. 
     The microcontroller has a feature called the &#34;Watchdog timer output&#34;. Through a fairly safe scheme, this output will generate a pulse on line WPOUT if the microcontroller is not executing its program properly. If this occurs, U6 latches the condition, which tristates U7A and U7B. A pullup on the output causes the gate of Q1A and Q2B to be pulled to ground, shutting the charger&#39;s current path off to the battery. This provides a safe condition during reset, and a safe condition in case the microcontroller should fail. 
     The charge current is regulated through a pulse width modulation scheme in which Q1A and Q1B are switched at a frequency of about 10 kHz. The width of the pulse determines how much average current is allowed to flow through the battery. L1A and L1B are toroidal core inductors that prevent excessive amounts of current to be sourced from the ac adapter. The two diodes D2A and D2B provide negative current to flow through L1A and L1B after Q1A and Q1B are turned off, and the field induced in L1 collapses. Transistors Q3A and Q2A are turned on to allow the gate of Q1A to be pulled to the 21 volt level of the dc to dc converter. Op amp U2A is used as a differential amplifier across RIA (0.01 ohms) to obtain a signal I A  which measures the current from battery bank 220A. The output of U2A is filtered and scaled by U3A, and is read by the microcontroller U5 through one of its analog to digital converter inputs. The pulse width is adjusted by the microcontroller U5 to maintain a constant current of about 800 milliAmps. U3A is also used to filter and prescale the battery voltage measurements, to produce an analog signal V A  to U5. Signals V B  and I D  are similarly generated to measure the voltage and current of the other main battery bank 220B. 
     Q3 is turned on by the microcontroller pulse through U7 and U8. Turning on Q3 pulls the base of Q2 low, which causes it to conduct, allowing 21 volts to be supplied to the gate of the power FET, Q1. U1 inverts the signal from the microcontroller, turning Q4 off whenever Q3 is on. When the signal from the microcontroller goes inactive (high), the base of Q4 is driven high, causing Q4 to conduct and drain the gate capacitor of Q1 through a small resistor (about 220 ohms) to ground. This allows for a fast turn off and turn on time for the FET (Q1). Keeping Q1 in its non-linear region keeps it from becoming heated, so that no heat sink is needed for these FETs. 
     FIG. 3. is a schematic block diagram illustrating a battery switching system 300 incorporating features of the present invention. The system 300 is configured for charging two separate battery banks &#34;A&#34; and &#34;B&#34; in a battery pack 302. It is understood that the battery banks A and B each comprise one or more battery cells 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;4/3A&#34; nickel-cadmium (NiCd) cells. 
     A +12 V voltage source 304 is connected through a fuse 305 to charger circuits 306 and 307. Charger circuit 306 is connected to battery bank A and charger circuit 307 is connected to battery bank B. The negative terminals of battery banks A and B are grounded at 28. 
     During operation, if AC power is applied to AC-DC adapter 304, DC power is coupled from AC-DC adapter 304 to DC-DC converter 316 through diode 308, fuse 311, and switch 318. As a result of the voltage produced by AC-DC adapter 304, the base of transistor 320 is pulled high, thus causing transistor 320 to turn on. Transistors 322 and 324 are therefore turned off. Since transistors 322 and 324 are turned off, power from battery pack 302 is not coupled through relay 310 and transistors 322 and 324 to DC-DC converter 316. Power for the computer system is thus sourced from the AD-DC adapter 304, and since power is applied to chargers 307 and 308, battery banks A and B may be charged. 
     When AC power is removed from AC-DC adapter 304, the base of transistor 320 goes low, thus turning off transistor 320. Accordingly, the gates of transistors 322 and 324 are pulled high, thus turning transistors 322 and 324 on. Since transistors 322 and 324 are turned-on, one of the banks A or B of battery pack 302 provides power to DC-DC converter 316 through relay 310 and transistors 322 and 324. As noted from FIG. 3, relay 310 is configured such that a contact switch 310a is normally closed while a contact switch 310b is normally open. The microcontroller U5 of FIG. 2 controls the energization and the de-energization of relay 316 such that the power supplied to DC-DC converter 310 alternatively switches back and forth from battery bank A and battery bank B approximately every four minutes. As a result, maximum charge from battery pack 302 may be attained. 
     Power-Management Program 
     The following high-level pseudo-code shows the program structure which is a actually used, in the presently preferred embodiment, for power management. 
     POWER ON 
     Perform basic integrity check 
     Check power switch 
     If switch on go to NORMAL START 
     If switch off go to NORMAL CHARGE MODE 
     NORMAL START 
     Turn on POWER ON led 
     Initialize port direction and interrupt registers 
     Initialize timers 
     Test for AC Available 
     Begin Normal Operation MAIN Loop 
     MAIN--(Normal Operation) 
     Monitor--Battery Voltage Standby Switch AC Available System ON switch and blink Charge LED if on Activity Lines (Reset timeouts when active) If Dynamic adjustment enabled reset HD and floppy timeouts during keyboard activity. 
     TIMER INTERRUPT--5.12 msec. 
     Service Watch Dog Timer Register 
     Store current battery voltage 
     Compare with past for rapid drop detection 
     Compare with minimum absolute level 
     Compare with warning level 
     Test alternate battery before activating alarm and switch batteries if indicated 
     Decrement Seconds Timer 
     Each Second: 
     Decrement timeout counters 
     Battery Change 
     Hard Disk 
     System Sleep 
     Backlight Timeout 
     If Beeper active 
     Decrement pause counter and call BEEP 
     Test and debounce standby switch 
     BATTERY DETECT INTERRUPT 
     Switch to reserve battery 
     Start 2 minute timeout for system power off 
     Turn off LCD to reduce power consumption 
     If Floppy and HD not active, put system in standby mode 
     Accumulate reserve battery use time (After 1 minute of reserve on time, or one month of operation, the reserve charger will be enabled during the next battery charge cycle. After 2 minutes of use the reserve battery will be charged from the main battery if no AC is available.) 
     Monitor BDT* line for new battery installed to terminate function 
     Test new battery and switch reserve off if voltage good 
     BATTERY CHANGE 
     Read current battery voltage 
     Read target battery voltage 
     Switch if alternate is same or higher 
     LOW-POWER-1 mode 
     BEEP for 5 seconds (2 times every second) (COP should enable speaker on low volume if user has it off) 
     Turn on LOW BATTERY LED 
     Set Low Power 1 Flag 
     LOW-POWER-2 mode 
     BEEP for 5 seconds (2 times every second) (COP should enable speaker on high volume regardless of the user setting) 
     Flash LOW BATTERY LED 
     Turn off LCD back light 
     Set CPU clock to slow speed 
     Set Low Power 2 flag 
     Enable keyboard interrupt and turn LCD backlight on with any key 
     Scan for presence of external power or new battery 
     LOW-POWER-3 mode 
     Place main CPU in standby mode if not already there 
     Output continuous beep for 3 seconds 
     Save voltage reading for future comparison 
     Turn off the power module 
     POWER ON ALARM (Standby Switch held low for 5 sec or more) 
     Exit immediately if external video active 
     LCD back light is turned off. 
     CPU clock speed goes to slow speed. 
     Flash Power On LED indicating Standby mode 
     A beep alarm is sounded if operating on batteries. (2 beeps every 4 minutes) 
     Monitor the standby switch to determine when the LCD panel is opened to exit this mode. 
     Exit standby immediately when the cover is opened. 
     STAND-BY/RESUME KEY 
     Enter and exit standby when button lifted 
     CPU clock set to slow speed 
     LCD back light is turned off 
     CPU is placed in HOLD mode for minimum power consumption 
     POWER LED is flashing (0.5 sec on 2 sec off) 
     COP pulses the HT21 refresh line to refresh memory Monitor the STAND-BY/RESUME key to exit stand-by mode 
     Exit hold for a fixed period on each timer interrupt to allow system time to be maintained. 
     Mask keyboard and mouse interrupts and have COP clear the keyboard controller buffer and restore the interrupt controller mask register before exiting standby. 
     SLEEP MODE 
     Reduce clock speed to slow 
     Turn off LCD backlight 
     Enable keyboard interrupt 
     Monitor system activity (keyboard, ports, and restore full speed if any activity detected 
     If inactive for more than 1 minute and AC is available, begin Sleep Charge Mode 
     Allow Standby Key press to exit sleep mode 
     NORMAL CHARGE MODE 
     Turn on CHARGE led 
     If Reserve Charge Flag set, start reserve charge with 2.5 hour fail safe timeout 
     Minimum duty cycle for 3 minutes 
     Monitor voltage rise and current 
     If max voltage and no current then battery open 
     If current rise with no voltage then shorted 
     If OK gradually increase current to target value of 750 ma. Start fail safe timeout of 4 hours 
     Monitor voltage until it starts to decline or holds constant for xx minutes 
     If voltage reaches the power supply maximum then monitor the charge current watching for an increase or a constant value for xx minutes to indicate end of charge 
     When end conditions are reached shut off charging current Flash CHARGE led at a low duty cycle when charged 
     Wait 0.5 hour with charge off before resuming trickle charge on batteries to allow them to cool off from charge 
     SLEEP CHARGE MODE 
     Turn on CHARGE led 
     Minimum duty cycle for 3 minutes 
     Monitor voltage rise and current 
     If max voltage and no current then battery open 
     If current rise with no voltage then shorted 
     If OK gradually increase current to target value of xxx ma. Start fail safe timeout of xx hours 
     Monitor voltage until it starts to decline or holds constant for xx minutes 
     If voltage reaches the power supply maximum then monitor the charge current watching for an increase or a constant value for xx minutes to indicate end of charge 
     Exit charge mode and start blink of CHARGE led before system exits sleep mode 
     Flash CHARGE led at a low duty cycle when charged 
     When end conditions are reached shut off charging current 
     HOST BIOS FUNCTIONS 
     POST 
     Reset COP 
     Checksum MILES SRAM and compare version number 
     If necessary reload COP program 
     Start COP 
     Transfer setup parameters to SRAM and clear CDONE to interrupt COP 
     Check for proper operation of COP 
     Set processor to compatibility speed (per SETUP) 
     Turn on LCD backlight 
     SWITCH DISPLAYS 
     Send Display 13  type command to COP (LCD/CRT) 
     WAIT FOR KEY (INT 16H function 0) 
     If no character is available issue a CPU-HOLD command to the COP 
     SETUP 
     COP returns status of standby button, etc. to setup 
     Transfer interrupt mask to be used in standby to COP 
     Transfer parameters to COP before exiting setup 
     CTL/ALT/DEL (soft boot) 
     Place COP in reset before resetting CPU 
     EXTERNAL PROGRAM INTERFACE 
     Verify power status before programming Flash Eproms 
     Enable reserve battery for Flash programming power backup 
     The Appendix shows a detailed implementation of the COP code to perform these functions, in the presently preferred embodiment. However, the foregoing listing shows the key relations of the preferred program structure. 
     Further Modifications and Variations 
     It will be recognized by those skilled in the art that the innovative concepts disclosed in the present application can be applied in a wide variety of contexts. Moreover, the preferred implementation can be modified in a tremendous variety of ways. Accordingly, it should be understood that the modifications and variations suggested below and above are merely illustrative. These examples may help to show some of the scope of the inventive concepts, but these examples do not nearly exhaust the full scope of variations in the disclosed novel concepts. 
     The presently preferred embodiments, as discussed above, use Intel microprocessor for the CPU. However, of course, the disclosed innovations can also be applied to systems using other non-Intel microprocessors of comparable architecture. The disclosed innovations can also be applied to other systems using other types of CPU, such as 680×0, SPARC, MIPS, or others. It is contemplated that, in the future, the disclosed innovations can also be applied to systems using a multiprocessor CPU. 
     The presently preferred embodiment relates to systems used in the ISA architecture. However, it is alternatively possible to adapt at least some of the disclosed teachings to other bus architectures, including not only the EISA bus architecture (which is an extension of ISA), but also the many other buses which are now in use or which may find use in the future. 
     The presently preferred embodiment uses Headland Technology and VTI chips for CPU support functions. However, of course, a wide variety of other chips are available to perform these functions, and many substitutions are possible. In particular, some microprocessors have been proposed with added support functions on-chip. For another example, compact modules incorporating a microprocessor with peripheral support functions are also available. A huge variety of such substitutions can be made, while still obtaining the benefits of the disclosed inventions. 
     Of course, many I/O and storage peripherals can be added into a laptop system. The disclosed innovations are generally applicable to such systems, regardless of what peripherals have or have not been added. Thus, for example, a laptop which contains a large bank of NVSRAM, or which is connected to an Ethernet adapter, or which includes speech recognition or synthesis, would still present many power-management issues similar to those discussed above. 
     For example, the principal disclosed embodiment, as presently practiced, does not include any available expansion slots for the user to add cards into. However, it is contemplated that addition of an expansion bus might be advantageous, and particularly so in combination with the microcontroller power-management architecture described above. 
     For another example, the principal disclosed embodiment, as presently practiced, never stops the system clock. In the 286 embodiment, the clock is slowed to 250 kHz, and in the SX embodiment the clock is slowed to 2 MHz. A fully static chip set, which would permit the system clock to be stopped would be even more advantageous; but, in the presently preferred embodiment, the HT21 chip and the SX chip are not compatible with fully static operation. Nevertheless, this is an obviously desirable modification, as the appropriate chipsets become available. 
     For another example, the principal disclosed embodiment, as presently practiced, uses Ni--Cd rechargeable batteries, and a small lithium cell as a non-rechargeable backup battery; but at least some of the disclosed innovative teachings can be practiced with other rechargeable battery technologies (such as NiH cells), if such technologies become commercially practicable, and/or can be practiced with nonrechargeable batteries in place of the NiCds of the preferred embodiment, and/or can be practiced with nonrechargeable batteries other than lithium cells. 
     The specific relay used in the presently preferred embodiment consumes only 80 mW when active, and thus does not greatly degrade the power budget. In alternative embodiments, a bistable relay could be used for bank-switching, to avoid the on-state power consumption of the relay. Such embodiments would need to include some very reliable way for software to know which bank is currently connected. 
     As will be recognized by those skilled in the art, the innovative concepts described in the present application can be modified and varied over a tremendous range of applications, and accordingly the scope of patented subject matter is not limited by any of the specific exemplary teachings given. ##SPC1##