Electricity: battery or capacitor charging or discharging – Serially connected batteries or cells – With discharge of cells or batteries
Patent
1993-09-16
1997-04-08
Nappi, Robert
Electricity: battery or capacitor charging or discharging
Serially connected batteries or cells
With discharge of cells or batteries
320 22, 320 2, H01M 1044
Patent
active
056191189
DESCRIPTION:
BRIEF SUMMARY
The invention concerns a method of charging a rechargeable battery, wherein an electrical source of energy is connected to the battery. One or more characteristic parameters of the charging process are currently measured and optionally calculated during charging, and these are compared with reference values. The invention also concerns an apparatus for performing the method.
When charging a rechargeable battery, such as for example an NiCd battery, an electrical voltage greater than the terminal voltage of the battery is applied to the terminals of the battery, whereby a current will run through the battery. This current initiates a chemical process by which energy is stored in the battery.
When the battery has reached a full charge condition, the chemical process stops, and the added energy will instead be converted into heat. Since the battery is constructed as a sealed container, the pressure in the battery increases, which causes chemical destruction. This means that the capacity of the battery is reduced, and the capacity may eventually have been reduced significantly after several such chargings. For the battery to be utilized in the best possible manner it is therefore important partly that the battery will be charged fully partly that charging is interrupted before the generation of heat becomes too great. The problem is thus to interrupt charging as precisely as possible at the proper time.
Frequently, the charging period for a battery is desired to be as brief as possible, which has led to the use of greater and greater charging currents, and since this accelerates the heat generating process additionally, it is even more important to interrupt charging at the proper time.
In a typical charging sequence the voltage across the battery increases evenly as the battery is charged. As the battery approaches its full state of charge, the voltage increases more steeply to a peak marking the full state of charge. The voltage then drops again owing to the increase in temperature because the temperature coefficient of the voltage is negative. Correspondingly, the charging current typically falls to a minimum at full charge and then increases.
The art includes some methods which attempt to ensure optimum charging by cutting off charging at the proper time. However, they have been found to be lacking in precision. If charging is interrupted too late, the result will be heat generation and mechanical destruction, as mentioned, and if charging is cut-off too early, the battery will be undercharged.
One of the known methods comprises measuring the temperature of the battery and then cutting-off charging when an increase in temperature is observed. However, it is frequently too late when the increase in temperature is so great that it can be measured, and it is moreover difficult to measure the temperature sufficiently accurately, one reason being the possible variations in ambient temperature. This will, for example, be the case if a battery from an automobile telephone is moved in winter from a cold car to a charger which is present at room temperature.
Another know method comprises measuring the voltage across the battery and cutting off charging when the voltage assumes a determined level. However, this voltage often varies somewhat from battery to battery, even in case of batteries of the same type, and it is moreover temperature dependent so that it is very difficult to determine the voltage at which charging is to be cut-off.
Similarly, it is possible to measure the charging current, and the same observations apply here as well.
Many known chargers rely on fixed periods of time so that charging is simply cut-off after the elapse of this time. This, however, is a very inexpedient method because it is not known in advance whether the battery is completely or only partially discharged, and the necessary charging time depends strongly upon this. This might be solved by discharging the battery fully prior to charging; but in addition to the waste of energy involved, it takes a certain time, and there will stil
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Juul-Hansen Ebbe
Reipur John
Chartec Laboratories A/S
Lechter Michael A.
Nappi Robert
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