CA2008431C - Battery monitoring system - Google Patents

Abstract

A battery monitoring system 10 supplies the user of a battery power system with information indicating the capacity remaining in the battery 9 during discharge. During discharge the system 10 monitors various parameters and calculates on a rapid sampling basis derived parameters from algorithms stored in memory in tabulated form. The recharging efficiency is also evaluated, stored and updated on each charge/discharge cycle so that ageing of the battery 9 is monitored.

Description

BATTERY MONI'rOR WHICH INDICATES REMAINING CAPACITY BY
CONTINUOUSLY MONITORING INSTANTANEOUS POWER CONSUMPTION
RELATIVE 'TO EXPECTED HYPERBOLIC DISCHARGE RATES
This invention relates to a battery monitoring system.
In the use of battery operated equipment, such as battery powered electric: vehicles and standby power systems, it is desirable to monitor the battery in order to provide from time to time a prediction informing the user of the remaining capacity in the battery.
It is an. objects of the present invention to provide a battery monitoring system.
According to t:he present invention a battery monitoring system comprises current measuring means for evaluating the level of instantaneous current drawn from a battery when supplying a load voltage measuring means for evaluating the level of instantaneous battery voltage when the battery is supplying the load; means for selecting a final battery voltage level at: which the capacity of the battery is to be considered exhausted; and arithmetic means for continuously receiving measurement values outputted form said means and for computing continuously on a rapid sampling basis a measure of remaining battery life; wherein said arithmetic means comprises: a first: arithmetic unit for computing the instantaneous power level delivered by the battery and for evaluating continuously on a rapid sampling basis according to a first predetermined algorithm the total discharge duration (T) available from. the battery when continuing to supply power at the measured instantaneous power level based upon the selected final battery voltage level; a second arithmetic unit for evaluating continuously on a rapid sampling basis according to a second predetermined algorithm a measure of elapsed discharge time (t) from the measured instantaneous battery voltage and battery current; a third arithmetic unit for

2 evaluating continuously on a rapid sampling basis the remaining life of the battery if it continues to supply power at the measured instantaneous power level, by subtracting the measure evaluated by the ;second arithmetic unit from that evaluated by the first arithmetic un_Lt; and wherein (i) said first predetermined algorithm comprises a hyperbolic equation of the form T = a + b(i f )- 2 + c(i f )- 2 where a,b,c, are c:o-eff~~_cients which are a function of the final battery voltage level and which are held in sets in a storage table to ~>e outputted according to the selected final battery voltage level; and (i) said second predetermined algorithm comprises a cubic equation of the form V = A+ Bt + Ct2+ Dt:3 where A,B,C,D are co-eff=icients which are a function of instantaneous battery current (ip), and which are held in sets in a storage table to be outputted according to the measured value of instantar..eous battery current(ip), and V is the measured value of instantaneous battery voltage.
An emboo.iment of the present invention will now be described with reference to the accompanying drawing, in which:
Fig. 1 schematically illustrates a battery monitoring system according to the present: invention; and Fig. 2 illustrates in graphical form details useful for an understanding of the Fiq. 1 system.
A battery monitoring system 10 in accordance with the present invention is illustrated in Fig. 1 and monitors the condition of battery 9 which is connected to supply a load 8.
System 10 comprises a first evaluating arrangement 11 for evaluating the level of instantaneous current drawn from the battery 9, and a second evaluating arrangement 12 for evaluating the level of instantaneous battery voltage. The outputs of arrangements 11 and 12 are fed to a third A

2~G~~~~~1

- 3 -arrangement 1;3 whic:E~ computes the level of instantaneous power delivered by the battery. A fourth arrangement 14, in this embodiment ~~ontaining a preset value, determines a final voltage level for the battery 9 when the capacity of the battery is exhausted and the outputs of arrangements 13 and 14 are delivered to a fifth arrangement 15 which evaluates the estimated final battery current. The output of arrangement. 15, 'together with the output of arrangement 14, is delivez-ed to a sixth arrangement 16 for calculating IO according to a fir st predetermined algorithm (as will be explained) the: total discharge duration of the battery 9.
The outputs of =he first and second arrangements 11, 12, in addition to :~eing fed to the third arrangement 13 are fed to a :event.': arrangement 17 which is arranged to 1~ evaluate according v~ a second predetermined algorithm (as will be explained) _e accumulated discharge duration and the outputs of: the :_=xth and seventh arrangements I6, 17, are fed to an eightl: arrangement 18 which evaluates the remaining discharge Duration available from the battery 9.
20 The algorithms which are used in the system 10 are formulated on the basis of the output from first arrangement 11 being in a norma:iised form, in part this simply being a percentage of the ten hour discharge rate but in part being modified by variations in temperature o.~. the battery 9 with 25 respect to a F~redet~er:nined temperature (usually 15°C) .
Accordingly, a temperature sensor 21 is provided for sensing battery temperature and the output of sensor 21 operates an 2~CL~43~.

- 4 -arrangement 2?. to establish a temperature correction factor which is fed t:o the first arrangement 11. The arrangement 11 therefore <:omprises a current sensor 11A, a temperature normalising unit 11'.9 and a scaling unit 11C.
The second arrangement 12 comprises a voltage sensor 12A, a preset store 12B containing the number of individual cells within t:he specific battery 9 and an arithmetic unit 12C :which provides t.e output for arrangement 12 so that the output is measured volts per cell. The third arrange-ment 13 evaluates the level of instantaneous power delivered by the batter~~ by multiplication of the values delivered to it by the first <~nd second arrangements 11, 12. The your ~h arrange:me.~. t 1 ~ is provided in this example in a preset manner, with ~ze final voltage level of each cell 13 and the fifth ar=an<:_.-.;ent 15 contains a calculating unit '_3~ v~hich by ciiv_sio_-_ of the power value delivered to it by arrangement: 13 a~:c the voltage measure delivered by arrangement 1~E et;alu~tes the final current and this is scaled by unit: 1~3 v~ the ten hour discharge rate.
The sixth a=ra~:~ement 16 comprises an arrangement 16A
which stores a f~rs~t set of co-efficients and which is arranged to output a single set of such co-efficients to unit 16B accoz-ding ~t~ the voltage value provided to unit 16A by arrangement :L-~. Unit 16B is a Calculation unit which evaluates a hyperbolic equation having specified co-efficients as delivered by unit 16A and a specified variable as delivered by unit 15B. The evaluation of this ~t~~~~~31

- 5 -hyperbolic equation establishes a total duration of battery discharge.
The seventh arrangement 17 comprises a unit 17A which stores a table of second co-efficients and according to the value of scaled current delivered to it by unit 11C
provides a specific set of such second co-efficients to a calculating unit 17B which is arranged to evaluate the accumulated discharge time from a predetermined cubic equation relating instantaneous voltage with accumulated discharge time.
The eighth unit 18 is arranged to subtract the values delivered to it by u:.its 16 and 17 to thereby provide a measure of the remaining time to final discharge of the battery 9. Arrange:~ent 18 may also express the remaining 1~ discharge time avai_a:ole as a percentage of the total discharge time or ma-: express the accumulated discharge tine as a percentage of the total discharge time.
The cubic equation evaluated by unit 17B is derived from graphical data =rovided by battery manufacturers expressing the relat_onship between measured battery voltage and accumula~ed discharge time for a particular .
level of instantaneous current. It has been found that the divergence between the graphical data and the cubic equation is minimal as illustrated by Fig. 2 and by storing sets of co-efficients in unit 18A for particular values of current a substantial reduction of storage space is achieved together with an increased ability to interpolate for ._. ~~C8~~3~.

- 6 -unstored set.; of co-efficients. Likewise the hyperbolic equation eva:Luated by unit 16B is derived from graphical data provided by battery manufacturers expressing the total discharge period to a particular value of final battery voltage for a series of specific values of instantaneous battery voltage and the divergence between the hyperbol:Lc equation and the graphical data is negligible. Interpolation of unstored co-efficients for the hyperbolic equation is readily effected by unit 16A.
By way of operation of the system 10 a specific example will now be evaluated for a battery of the lead acid CP17 type having t he following parameters:
Ampere hour rate 800 = 10 hour capacity Vo. of cells 164 Temperature cc~r=ection factor to per °C
dated temper a-t::= a 15 ° C
Final voltage 294 r final volti ce:L' 1. 79 '~he 'ollowinc~ measurements are taken:
Current 146 Amperes Voltage 314.8 Volts Temperature 20°C
1) Sensor l.lA measures the present current (iin) flowing through the x>attery leads as + 146 amps, the positive sign indicating treat the= battery is being dischazged.
Unit 11F~ normalises iin to rated temperature (15°C) and because t:he measured temperature in the battery room is

7 20°C the temperature correction factor provided by unit 22 is:
tempco =- 1 + (20 - 15) x (1/100) - 1.05 so that the tempez~ature--corrected discharge current (it) is 146.0 - 1.05 = 139.05 Unit 11C; calculates current in terms of the 10 hour capacity of the battery,, to give a percentage so that ip the output of arrangement 11 is:
ip = (l~>9.05 x 100) . 800 = 17.380 2) Sensor 1.2A measures the battery voltages as 314.8 volts d.c.
Unit 12E~ divides the battery voltage by the number of cells (164) to give cel7_ voltage:
vin = 31.4.8 / 164 = 1.9195 3) Arrangement 13 calculates power demand.
Power = vin x it = 1.9195 x 139.05 = 266.90 watts 4) Unit 15P, calculates final current if = power / of (vf is the preset final voltage, i.e.
1.79 established in arrangement 14).
if = 26E~.90 / 1.79 = 149.11 amps and this is scaled by unit 15B to be 18.640 of the 10 hour capacity.
5) Arrangement 16 calculates the finish time T, using final current (if) from a hyperbolic equation of the form T = a + bi f- 1 + ci f- 2 where the hyperbolic co-efficients (a,b,c) are stored in unit 16A for specified final voltage values; e.g.
a 2on8431a Final volts a b c 1.85 -111.738 6662.29 3942.7 1.83 - 95.1394 6645.34 5171.71 1.81 - 87.7079 6939.18 3580.98 1.79 - 74.9541 6913.18 3857.27 1.77 - 65.1906 6880.4 4634.92 1.75 - 56.4163 6826.9 5202.11 From the above table unit 16A selects the co-efficients for the set end point voltage of 1.79:
1.79 - 74.9541 6913.18 3857.27 and the finish time T for the specified final current of 18.640 is calculated using the hyperbolic equation:
T = -74.9541 ~- (6913 / 18.64) + (3857.27 / (18.64)2 - 307.06 6) Unit 17 calculates the accumulated discharge time t from a cubic or higher order equation of the form V = a + bt - ct2 + dt3 where the cubic ca-efficients (a, b, c, d) are stored in unit 17A for specified levels of current ip, e.g.
t 2QC~:~3~.

I

a b c d 172 1.655 -4.392 E-3 5.426 E-4 -2.127 E-4 142 1.710 -5.985 E-3 -4.174 E-4 2.164 E-5 106 1.776 -4.722 E-3 2.348 E-5 -2.609 E-6 88 1.804 -2.839 E-3 -1.082 E-5 -7.787 E-7 71 1.837 -1.240 E-3 -2.965 E-5 -1.203 E-7 60 1.852 -7.024 E-2 -3.478 E-2 6.184 E-3 36.9 1.'13 -4.214 E-2 -4.087 E-3 3.903 E-3 27 1.40 -3.234 E-2 5.836 E-3 -3.533 E-3 21.6 1.951 -1.648 E-2 6.957 E-4 -1.331 E-3 18 1.'364 -1.382 E-2 5.565 E-5 -5.936 E-4 17.38 1.'36496 -1.432 E-2 5.77 E-4 -5.8 E-4 15.5 1.'368 -1.592 E-2 2.232 E-3 -5.632 E-4 13.6 1.'372 -1.361 E-2 2.811 E-3 -5.399 E-4 1~ 12.1 1.'371 -1.277 E-2 2.092 E-3 -3.312 E-4 11 1.~a82 -1.310 E-2 2.065 E-3 -2.759 E-4 10 1.386 -9.299 E-3 1.381 E-3 -1.860 E-4 (where 10 2 etc.
E-2 rs=preser.~s ) Accordingly equation is:
'the cubic V - a ~~- b c t2 + d.t3 t We know the followingvariables V - cell volta ge - 1.9195 a - calculated coefficient - 1.96496 b - calculated coefficient - -0.01432 c - c,alculatedcoefficient - 0.000577 d - c,alculatedcoefficient - -0.00058 and unit 17B solves this equation, by iteration, for t, the ~c~c~.~~~.

result being t = lEil minutes.
7) Arrangement 18 evaluates the remaining time as the difference between ::final time and present time - 307 -~ 161 - 146 minutes so that if th.e discharge continues at the present rate the battery will last f:or a further 146 minutes.
Additionally arrangement 18 may evaluate the percent discharge remaininc; as (Remaining time / finish time) x 100 - (146 / 307) x 100 - 47.56 so that the battery ~:as 47.5 of its charge remaining.
It will be apt:==ciated that in the foregoing example, 1, =er oth the cubic nd hyperbolic equations the sets of stored co-eff:icient= and their identifier contain an identifier ec;ual in -ralue to that pr~uced by arrangement 11 and by arrangement .4 but, particularly for arrangement 11 which output: a me~~~;zre of instantaneous current which is likely continuously to change, there is no guarantee that the evaluated currEe~t value is identical to an identifier within store unit :17A. Accordingly, unit 17A is adapted to interpolate between the nearest stored identifiers and their stored co-ef:ficients to obtain the required identifier and required co-ef:~icients.
For example, :in the event that the identifier and its co-efficients are not stored in unit 17A interpolation is made between the co-efficients stored for stored identified 18.0 and those of stored identified 15.5 (see the previous Table I) by calculating the ratio that 17.38 is between 15.5 and 18.0 (760) and for each stored co-efficient evaluating the interpolated value. Thus, taking co-efficients a as the example:
a = (1.964 - 1.968) x 0.76 + 1.968 - 1.9E>5 Interpolation may be carried out on a similar basis by unit 16A in the case where the final voltage Vf is not a preset value as dictated by the battery manufactures which is normally a value specifically contained as an identifier in store unit 16A.
It will be understood that in the system 10 which has been described the system outputs on a continuous (very rapid sampling) bases the prediction of the battery capacity remaining at any point in time and this prediction can, if so desired, be used to set or operate alarm devices guarding the battery and load arrangement.
The system 10 may be modified to incorporate a self-learning regime by comparing the projected discharge of the battery 9 with the actual discharge by means of which the system 10 self-adjusts for inefficiencies of the battery and load arrangement and for battery ageing. This is achieved using "electrically erasable programmable ~~~i~~~a~~.

read only memory" (EEPROM). Each time a discharge ta:ces place, the system 10 records all the vital parameters and then integrates them with previously stored derived measurements.
For example calculations made during the discharge and recharge of the battery allow the system to calculate the effective "charging efficiency". That is to say the percenta~~e of energy that has to be returned to the battery in order to return it to the fully charged state or to a predetermined percentage thereof. To achieve this the system stores the following data for each charge/disch~~rge cycle:
Battery energ.~ level at end of discharge (EEPD).
Energy :returnee to battery during recharge (ER).
l~ Battery energ-: level at start of next discharge (ESND).
The charging effici=ncy can be expressed as:-ESND - EEPD
Charge l~fficie_ncy (CE) - X 100 ER
The result from eac:: charge/discharge cycle is integrated into a continuous sore, the "historical charge efficiency".
The system therefore "learns" the charging efficiency of the battery, load and charging unit.
Logging of previous recharge rates, times and efficiencies allow the system to indicate the predicted recharge time required to return the battery to a charge state.
In order to calculate the required recharge time the system records one further parameter:-Charge Fate (C:R) Expressed in Ampere/minutes and derives Depth of- Discharge (DD) as o Fully Charged energy level The recharge time then can be expressed as:-Charge Time (minutes) - DD X BC
CE CR
where BC is the Battery Capacity expressed in Ampere/minutes.
All the above variables are stored in electrically erasable programmable read only memory (EEPROM).

Claims (3)

CLAIMS:

1. A battery monitoring system comprising:
current measuring means for evaluating the level of instantaneous current drawn from a battery when supplying a load;
voltage measuring means for evaluating the level of instantaneous battery voltage when the battery is supplying the load;
means for selecting a final battery voltage level at which the capacity of the battery is to be considered exhausted; and arithmetic means for continuously receiving measurement values outputted from said means and for computing continuously on a rapid sampling basis a measure of remaining battery life;
wherein said arithmetic means comprises:
a first arithmetic unit for computing the instantaneous power level delivered by the battery and for evaluating continuously on a rapid sampling basis according to a first predetermined algorithm the total discharge duration (T) available from the battery when continuing to supply power at the measured instantaneous power level based upon the selected final battery voltage level;
a second arithmetic unit for evaluating continuously on a rapid sampling basis according to a second predetermined algorithm a measure of elapsed discharge time (t) from the measured instantaneous battery voltage and battery current;
a third arithmetic unit for evaluating continuously on a vapid sampling basis the remaining life of the battery if it continues to supply power at the measured instantaneous power level, by subtracting the measure evaluated by the second arithmetic unit from that evaluated by the first arithmetic unit; and wherein (i) said first predetermined algorithm comprises a hyperbolic equation of the form T = a + b(i f )- 1 + c(i f )- 2 where a,b,c, are co-efficients which are a function of the final battery voltage level anti which are held in sets in a storage table to be outputted according to the selected final battery voltage level; and (ii) said second predetermined algorithm comprises a cubic or higher order equation of the form V = A+ Bt + Ct2+ Dt3 where A,B,C,D are co-efficients which are a function of intantaneous battery current (i p), and which are held in sets i.n a storage table to be outputted according to the measured value of instantaneous battery current(i p), and V is the measured value of instantaneous battery voltage.

2. A system as claimed in claim 1, wherein a temperature sensor is provided for monitoring battery temperature, the temperature sensor output being connected to the current measuring means to establish a temperature-corrected measure of the instantaneous current drawn from the battery and which temperature-corrected current measure is outputted to the arithmetic means.

3. A system as claimed in claim 2, wherein the first arithmetic unit and the second arithmetic unit each includes interpolation means to establish co-efficients of value intermediate those stored by the respective table.