AN2818

Theory of operation

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Theory of operation

1.1

NiMH battery charging

The NiMH batteries follow a constant current-charging algorithm.

Figure 1 shows the NiMH battery voltage, temperature and current profile during charging.

There are three main phases: the pre-charging phase, the charging phase and the

saturation mode (shown in Figure 1).

The end of the NiMH battery charging is generally detected using the methods described in

the following paragraphs. After the end of charging is detected, the charger switches to

saturation mode during which a trickle-charging current is provided to the battery to

compensate for the self-leakage.

1.1.1

Negative delta V method

As shown in Figure 1, the voltage of the NiMH battery drops a little at the end of the charge

time. As a consequence, when the voltage curve versus time becomes negative, charging is

stopped and trickle-charging is started.

1.1.2

Zero delta voltage method

This method is a variant of the negative delta V method. With a NiMH battery, there is a very

slight voltage drop (5-10 mV/cell) at the end of the charge time which is very difficult to

detect with a microcontroller’s 10-bit ADC. There is also a risk of detecting the wrong end of

charging due to noise. Therefore, instead of the negative delta voltage, it is possible to use a

dV = 0 condition for a certain time duration, which gives a very good approximation for

detecting the end of charging. For this reason, this is the method used in the present

demonstration board instead of the negative delta method or the other ones described in the

next paragraphs.

Figure 1.

NiMH battery charging profile

AM01039v1