Electricity: measuring and testing – Electrolyte properties – Using a battery testing device
Reexamination Certificate
2002-08-05
2004-09-07
Le, N. (Department: 2858)
Electricity: measuring and testing
Electrolyte properties
Using a battery testing device
C324S426000, C320S162000
Reexamination Certificate
active
06788069
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and a device for calculating the parameters of a rechargeable direct current generator, or accumulator battery, for supplying power to an electric motor for a motor vehicle.
BACKGROUND OF THE INVENTION
It relates in particular, but not exclusively, to a method and a device for calculating the parameters of a power battery for a hybrid type vehicle.
Let it be noted here that a hybrid motor vehicle comprises an internal combustion engine to drive the vehicle on the one hand and on the other hand, an electric motor powered by a battery. The internal combustion engine serves not only to propel the vehicle but also to turn an alternator which charges the power battery via a current rectifier. The decision to control the movement of the vehicle, either by the internal combustion engine, or by the electric motor, or by both at the same time, is usually made automatically according to the state of charge of the battery and the discharge power of the battery, among other things. Starting and stopping battery charging are also carried out automatically by means of equipment for detecting total discharge and overcharge.
Adding together the electricity supplied and the electricity received (in ampere-hours) in order to determine the state of charge of a power battery is known. This sum can also be used to determine the power battery's state of ageing.
The battery's state of charge is expressed as a % of the maximum charge. For states of charge between approximately 30% and 80%, the voltage supplied by the battery has a definite value. However, for states of charge lower than approximately 30%, the voltage supplied decreases with the state of charge and, similarly, for states of charge above approximately 80%, the voltage increases with the state of charge.
Overcharge detection consists of comparing the battery output voltage at zero current with a first threshold value and cutting off the charging and/or triggering an overcharge warning when the voltage exceeds this first threshold value.
In the same way, detection of total discharge of the battery is carried out by comparing the battery's voltage at zero current with a second threshold value, charging and/or a discharge warning being triggered when the battery voltage drops below this second threshold value.
The invention is the result of the observation that the known method for calculating the charging capacity and discharge power and the methods for detecting states of overcharge and total discharge are not accurate enough for correct management of the battery and propulsion of the vehicle. For example, inaccurate calculation of the battery's discharge power makes it impossible to determine with certainty whether the electric motor will be able, together with the internal combustion engine, to supply enough torque to provide the driving force required by the driver. For another example, inaccurate information about the overcharge or the state of total discharge may cause random triggering of warnings.
SUMMARY OF THE INVENTION
To overcome these disadvantages, the invention provides for determining the internal resistance of the battery and making use of this internal resistance to determine at least one of the following parameters: charging capacity calculation; discharge power calculation; overcharge detection and total discharge detection.
Thus, the battery's charging capacity and discharge power can be determined by equation (1).
P
=
UI
=
U
E
±
U
R
(
1
)
In this formula, U is the voltage at the battery terminals, E is the electromotive force (discharging), or back electromotive force (charging), R is the battery's internal resistance and I is the measured intensity of the output current (discharging) or input current (charging). In addition, in this formula the + sign refers to charging and the − sign to discharging.
Th voltage U can be measured directly. On the other hand, parameter E is calculated from correlation table, loaded in memory in the factory, showing the relationship between value E and the state of charge (in %), calculated by adding together the electricity received or supplied, and the temperature of the battery, if necessary. The resistance R of the battery is measured and, preferably, corrected, as explained hereinafter.
The single or double entry correlation table is established empirically, for example, using measurements made on a large number of batteries of the same type and same specifications as the one installed in the vehicle.
To determine the threshold U
max
for overcharge detection, the relationship shown in equation (2) can be used:
U
max
=E
90%
+R
90%
I.
(2)
In this formula, E
90%
and R
90%
are the battery's back electromotive force and its internal resistance for a state of charge of the order of 90%, respectively. Of course, the value of 90% used for the state of charge is an example of a state of charge for which the back electromotive force is higher than the back electromotive force for a normal state of charge (usually between 30% and 80% as mentioned above).
The expressions U
max
, I and, generally speaking, R
90%
, can be measured. On the other hand, the back electromotive force E
90%
is obtained from a memory as explained above. The expression R
90%
can also be obtained from a table as explained later.
Similarly, the second threshold for total discharge detection is determined using the formula shown in equation (3)
U
min
=E
30%
−R
30%
I.
(3)
In this formula, E
30%
is the battery's electromotive force for a state of charge of the order of 30%, i.e. for a state of charge lower than normal and R
30%
is the battery's internal resistance for the same state of charge.
The value E
30%
is determined with the help of a memory containing a table of the relationship between the state of charge and the electromotive force. This correlation table, which preferably also includes the temperature as an entry value, is loaded in the memory in the factory and is the result of empirical determinations as in the case of the back electromotive force.
Note that comparison of the internal resistance measurement with a normal value can also be used to diagnose the ageing of the battery. It is a known fact that the internal resistance of a battery increases significantly in the last third of its life.
The invention applies in particular to NiMH and NiCD type batteries.
According to another feature of the invention, which can be used independently of the various features explained above, to measure the internal resistance of the power battery, an auxiliary battery, usually having a lower voltage than the power battery and sometimes called a “slave” battery, is used and the power battery is charged via this “slave” battery by supplying to the slave battery two electric currents of intensities I
2
and I
1
respectively and measuring the voltages U
2
and U
1
with these two currents at the terminals of the power battery. The internal resistance, R, then has the value given by equation (4).
R
=
U2
-
U1
I2
-
I1
(
4
)
In this way, the battery's electromotive force and back electromotive force are not involved in measuring the resistance.
To enable the power battery to be charged via the slave battery and also to enable the slave battery to be charged via the power battery, a two-way DC/DC converter must be provided between the two batteries. A “two-way” converter means a converter which converts the high voltage of the power battery into a low voltage equal to that of the slave battery and, vice versa, which converts the low voltage of the slave battery into a high voltage equal to that of the power battery.
Alternatively, the battery's internal resistance can be measured by using the current supplied by the alternator and the rectifier connected to it. But in that case, it is necessary to use a voltage or current stabilizer for accurate measurement (because of the inevitable variations in the voltage output from the
Le N.
Nguyen Vincent Q.
Peugeot Citroen Automobiles SA
RatnerPrestia
LandOfFree
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