Note: Descriptions are shown in the official language in which they were submitted.
20086-2190
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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
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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
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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
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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~.
TABLE - 9 -
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~.~~~.
- 10 -
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
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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~~.
- 12 -
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
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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).
