Electricity: measuring and testing – Electrolyte properties – Using a battery testing device
Reexamination Certificate
1998-06-02
2001-02-20
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Electrolyte properties
Using a battery testing device
C324S426000, C324S430000, C340S636210, C320S134000
Reexamination Certificate
active
06191590
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a device and a process for monitoring the capacity of a battery, in particular in key transmitters or remote controls such as in automotive applications, comprising measuring means for the terminal voltage and means for calculating the internal resistance of the battery as well as further means to determine the state of charge of the battery from the measured values.
A device of the type mentioned in the introduction is known from DE-OS 44 18 194. It is a system for determining the capacity of a chargeable storage battery of an automobile, in particular for determining the residual capacity (state of charge) of such a battery. From this printed publication it is known that the residual battery capacity is a function of electrolyte density, and that the electrolyte density can be correlated to the internal resistance of the battery and the discharge current of the battery. In this, the internal resistance of the battery is calculated by acquiring the terminal voltage in an uninterrupted state in which the connections to all electricity-consuming devices are interrupted, and a terminal voltage different from this in a selectively-connected state in which a connection is established to only one electricity-consuming device, as well as from the discharge current in this connected state. Acquisition of the residual capacity of the battery subsequently takes place at the temperature condition valid at the time, in connection with a table which shows the context between battery residual capacity, discharge current and internal resistance.
This process is expensive, because calculations in several steps must be carried out to do this, and in addition because switching off the battery from all electricity-consuming devices or selectively from individual electricity-consuming devices requires considerable switching action.
Apart from this, from DE-PS 34 07 409 a testing process for batteries is known in which the internal resistance of the battery is measured and from the comparison with a direct-current source of the same type as the device under test, of a determined state of charge, the state of charge of the device under test is determined. This process is expensive, because a second reference battery must always be at hand.
Finally, from DE-OS-38 18 034 a measuring device is known which directly indicates the internal resistance of batteries. Measurement is via a frequency-band-limited amplifier stage by pulsing of the battery as a test object with an alternating-current source.
On the other hand, from practical applications, processes are known which carry out measurements of a battery by monitoring the terminal voltage at the battery, because the voltage of a typical lithium cell at a constant load towards the end of its life progressively decreases. If however, a lithium cell is only occasionally subjected to a load, as is for example the case with a key transmitter, then the cell voltage always approaches the equilibrium value. In such a case, a method for estimating battery capacity which only refers to battery voltage is not suitable because, as a result of the temporary “recovery” of the battery, a fully-charged battery is simulated.
A further disadvantage of pure voltage measurement results from the strong temperature dependability of the voltage progression which is described by the Nernst equation:
&phgr;=&phgr;
0
+(
R*T/z*F
)*1
n
(
MWG
)
Here, the electrode potential &phgr; is given by the respective standard electrode potential &phgr;
0
and a further term into which the absolute temperature enters in a linear way. The voltage collapse observed towards the end of the service life thus also depends on the temperature and can only partially be used for assessing the capacity.
It is the object of the invention to develop a device or a process which make it possible to provide reliable information concerning the available cell capacity (state of charge), whereby in particular the capacity of a cell, even in cases of only occasional exposure to loads, is to be determined correctly. In addition, the measuring result should be as far as possible independent of temperature.
SUMMARY OF THE INVENTION
According to the invention, this object is solved in that parallel to the terminal voltage, a microprocessor-controlled arithmetic unit is connected parallel to the terminal voltage, with an ohmic resistance and a capacitance chargeable or dischargeable via the arithmetic unit being able to be connected to the said arithmetic unit, and with measuring and evaluation means for acquiring the voltage dropping at the capacitance being allocated to the arithmetic unit, in such a way that in a first operating state of the arithmetic unit, with non-connected resistance, a first point in time of reaching a first threshold value (resulting from the discharge time applicable to the capacitance in the first operating state) is registered; that in a second operating state of the arithmetic unit, with connected resistance, a second point in time of reaching a second threshold value is registered and that the time difference between the points in time of reaching the two threshold values on the characteristic of the voltage dropping at the capacitance is calculable and storable in the arithmetic unit as a measured value proportional to the internal resistance.
According to the process aspect of the invention, this object is met in that to determine a measured value proportional to the internal resistance by means of an arithmetic unit connected in parallel to the terminal voltage with a connectable ohmic resistance and an external capacitance, the voltage dropping a the capacitance is evaluated in such a way that in a first operating state of the arithmetic unit, with non-connected resistance, a first point in time of reaching a first threshold value (resulting from the discharge time applicable to the capacitance in the first operating state) is registered; that in a second operating state of the arithmetic unit, with connected resistance, a second point in time of reaching a second threshold value is registered and that the time difference between the points in time of reaching the two threshold values on the characteristic of the voltage dropping at the capacitance is calculable and storable in the arithmetic unit as a measured value proportional to the internal resistance.
One advantage of the solution according to the invention consists of the device or the process being independent of whether the battery is subject to constant load or only occasional load, because the internal resistance (cell resistance) determined according to the invention is independent of the type and manner of the load experienced. A further advantage of the invention is the large degree of temperature-independence of the internal resistance of the battery. Finally, by determining the internal resistance of the battery by means of a reference resistance, costs can be kept lower than is the case with determining the internal resistance by means of a reference voltage or a reference current.
According to a preferred embodiment of the invention, the charge/discharge circuit of the device according to the invention comprises a parallel connection of a resistor and a capacitor, with discharge of the capacitor taking place by way of this resistor.
A further preferred embodiment of the invention provides for a further resistor being connected in series to the charging device by means of which resistor the current is limited during charging of the charging device.
To further illustrate the invention, below an embodiment is described by means of drawings, in which like reference numerals designate the same elements:
REFERENCES:
patent: 4388618 (1983-06-01), Finger
patent: 4740754 (1988-04-01), Finger
patent: 4947123 (1990-08-01), Minezawa
patent: 4968941 (1990-11-01), Rogers
patent: 5179340 (1993-01-01), Rogers
patent: 5404106 (1995-04-01), Matsuda
patent: 5496658 (1996-03-01), Hein et al.
patent: 5656919 (1997-08-01), Proctor et al.
patent: 5686815 (1997-11-01
Faust Benedikt
Klütz Siegfried
Delphi Automotive Systems Deutschland GmbH
Hollington Jermele M.
Metjahic Safet
Proskauer Rose LLP
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