state-of-charge metering system also computes the miles conventionally available in electronic circuits, the same
remaining at the current operating speed and state-of- relationships and mathematics can be used to examine
charge of the battery, and displays this information to the performance of such a circuit. Similarly, the resis
the vehicle operator. tances encountered in a storage battery circuit are usu
A state-of-charge metering system made according to 5 ally very small and vary with the size of the battery, the
the invention thus provides a system for measuring and temperature of the battery and the chemistry of the
displaying the instantaneous charge remaining in a stor- battery. These resistance values and variations can,
age battery. Such a metering system presents unique however, be determined, and a battery equivalent cir
advantages over prior methods for measuring the state- cuit model can be developed. A detailed examination of
of-charge of the storage batteries employed in EV's. 10 the storage battery model demonstrates a rational physi
The state-of-charge metering system continuously pro- cal model of the battery, as well as how this model can
vides to the operator of the EV an indication of the accurately reproduce actual battery performance data,
amount of charge remaining in the vehicle battery with- Storage Battery Model
out the need for the operator to exit the vehicle and use A lead-acid storage battery stores electrical charge in complicated instrumentation such as hydrometers and 15 an electrochemical state akin to an electrolytic capacivoltmeters. An indication of the remaining charge al- tor. Although a storage battery is primarily an electrolows the operator to determine when recharging of the chemical system, a very useful and easily manipulated vehicle battery is required, thus avoiding loss of opera- model can be expressed using only electronic compotion of the EV at a distance from available recharging nents. Referring to FIG. 1, what is shown is a simplified facilities. 20 circuit model equivalent of a storage battery, generally
„„„„ ,TM„TMTM_„,.T . „T„T„„ designated at 10. The circuit model 10 includes as com
BRIEF DESCRIPTION OF THE DRAWINGS pQn^ts a paralld capadtof n md resistor u m serfes
FIG. 1 is a circuit model equivalent of the electrical configuration with a second large capacitor 16 .
parameters of a storage battery; It will be appreciated by those skilled in the art that
FIG. 2 is a graph of the voltage and resistance values 25 the model depicted in FIG. 1 applies not only to lead
of ten 12-volt batteries connected in series configura- acid storage batteries, but in general to any storage
tion; battery such as those employed with heart pacemakers,
FIG. 3 is a graph of the internal resistance of a stor- portable computers and other consumer, medical or
age battery measured against the open circuit voltage of industrial devices. Consequently, the state-of-charge
the battery; 30 metering system of this invention can be employed to
FIG. 4 is a computer program which simulates the model any storage battery without departing from the
actual performance of a real storage battery; spirit and scope of the invention.
FIG. 5 includes two discharge voltage curves for a The physical capacity necessary for the large capaci12-volt, 100 Ah (20h), SLI battery at 25 ° C, where tor 16 is extremely big in comparison to the values FIG. 5(a) shows measured low current discharge data, 35 currently available from standard electronic compoand FIG. 5(b) shows measured high current discharge nents. In fact, for most of the battery applications mendata, tioned above, the required value for the large capacitor
FIG. 6 includes two computer-generated plots of the 16 is on the order of thousands of farads. It must be of
discharge voltage from the computer program shown in such high magnitude to be capable of delivering hun
FIG. 4, where FIG. 6(a) shows simulated low current 40 dreds of amperes of current for sustained periods of
discharge data, and FIG. 6(6) shows simulated high time. For example, the typical current that a standard
current discharge data; internal combustion vehicle battery delivers is 600 cold
FIG. 7 is an analog circuit model of a compelled cranking amps (CCA),
agreement system used to model the state-of-charge of a Of course, the current that a battery can deliver is
storage battery; 45 limited by the internal resistance of the battery. This
FIG. 8 is a preferred computer program employing a resistance is represented by the resistor 14 depicted in
finite difference routine for use with an actual battery the electrical model 10 of FIG. 1. Although as such in
and a battery metering system made according to the FIG. 1, the capacitance and resistance of an actual stor
invention; age battery do not always operate in a linear manner.
FIG. 9 is a graph of the output generated from the 50 Thus, the resistor 14 must behave in a non-linear manpreferred computer program shown in FIG. 8; and ner to accurately model the internal resistance of a
FIG. 10 is a block diagram of a presently preferred storage battery,
digital computer system of the state-of-charge metering The internal resistance of a storage battery has been
system made according to the invention. found to vary with the state-of-charge of the battery.
r>FTATT Fn nFsrRTPTTON Of THF 55 Battery tests indicate tbis resistance is very small
PRE?fOTSP^SSST Embsd^nts WheD thC ... iS ... thUS PermittmS larSe cur'
PRESENTLY PREFERRED EMBODIMENTS rents tQ be delivered to a load attached to the battery.
Theoretically, a storage battery may be modeled by a However, as the state-of-charge, or more accurately the
network of resistors and capacitors, which represent the specific gravity of the electrolyte contained within the
electrical characteristics of the battery. Although the 60 battery charges, the resistance value changes. This
capacity of a storage battery needs to be expressed in change in resistance translates to a limited useful operat
terms of thousands of farads instead of the microfarads ing range of the battery as shown in Table 2.
SOC 40 A 80 A 120 A 160 A 200 A 240 A 280 A 320 A 360 A 400 A Volts Res.