Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Electrical signal parameter measurement system
Patent
1997-12-29
2000-05-09
Hoff, Marc S.
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Electrical signal parameter measurement system
702 66, 702 71, 324427, 340636, G01R 3136
Patent
active
060616393
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a method and a device to predict the resulting capacity of a battery.
STATE OF THE ART
The possibility to estimate the remaining energy content in a battery being discharged, is desirable, as the information may be used to determine the remaining operational time of the battery before it must be recharged or replaced.
There are several different ways of achieving this, and in the patent specification EP 0420530 the estimate of the time for the discharge of the battery is based on a discharge curve being divided into one linear and one exponential part, and on a measured plateau voltage. This, combined with a real time supervision of the dynamic variables of the battery enables a continuously improved estimate of the remaining discharge time of the battery to be made. Unfortunately, the system is highly dependent on ideal discharge profiles, from which many batteries deviate.
Another method of operation, according to WO 91/08494 is a system that supervises and measures several parameters during discharge, and calculates, with a high sampling frequency, on the basis of algorithms stored in the form of tables, the remaining capacity and the discharge time. On the occasions when the tables do not comprise the values sought for, interpolation is used.
In a third method, according to U.S. Pat. No. 5,371,682, the remaining discharge time and capacity are calculated by means of a curve showing a discharge profile of the battery voltage as a function of the battery capacity spent.
SUMMARY OF THE INVENTION
The state of the art solutions for predicting the remaining discharge time of a battery requires much memory space and computer capacity, using complicated calculations based on curves stored in memory, which furthermore do not always correspond to the discharge characteristics of the battery. The calculations take a long time and the result is not always correct.
The present invention takes into account the discharge parameters determining the shape of the discharge curves for several different discharge currents. The method is compact and thus fast without the need to lose the desired precision. By normalizing the measured values, a table is created, which only needs to hold ten discrete voltage levels and three different discharge currents. By selecting the currents in an appropriate way the table may be used as a starting point for retrieving the remaining capacity in a battery at an arbitrary discharge current, which must be between the highest and the lowest current in the table.
In order to generate a normalized table, three discharges are made with different loads, representing three selected discharge currents. A measurement is made for each discharge, measuring the voltage and the current as a function of time, down to a predetermined final voltage. The measured voltage is divided by the number of battery cells found in the battery, which means that the voltage will be an average value of the individual voltages of all battery cells. In addition, the battery capacity spent is calculated by multiplying the time by the discharge current. When the measurements and the calculations are finished, the voltages are normalized, to enable a correct interpolation between the columns of the table. The equations to normalize the voltages comprise, among other things, the internal resistance of the battery. The last calculation of the capacity spent (at the final voltage) is used to calculate the remaining capacity of the battery, and the remaining capacity is then normalized by division by the nominal 10 h capacity. The currents are normalized in the same way.
Through interpolation of the calculated values for the normalized voltage, and the normalized remaining capacity for each discharge current, the value to be entered into the table is calculated. The currents are entered into the table without being interpolated, as they vary very little.
In order to determine a remaining capacity for an arbitrary discharge current, it is important that the normalized tabl
REFERENCES:
patent: 4952862 (1990-08-01), Biagetti et al.
patent: 5371682 (1994-12-01), Levine et al.
patent: 5394089 (1995-02-01), Clegg
patent: 5847566 (1998-12-01), Marritt et al.
PCT International Search Report re: PCT/SE97/00723 dated Aug. 12, 1997.
Hoff Marc S.
Telefonaktiebolaget LM Ericsson
Vo Hien
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