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AN2390

Theory of operation

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1.2

NiMH battery charging

The NiMH batteries uses a constant current algorithm as shown in Figure 3.

Figure 3.

Variation of NiMH battery voltage

The NiMH batteries use constant current charging. The end of charging can be detected by

using the following methods.

1.2.1

Negative delta V method

As shown in Figure 3, the voltage of the NiMH batteries drops a little at the end of charging.

So at the time, when the voltage slope versus time becomes negative, charging is stopped

and trickle charging is started.

1.2.2

Zero delta voltage method

This method is a variant of the Negative delta V method. Actually in case of NiMH, there is a

very slight drop in voltage (5-10 mV/ Cell) at the end of charging which is very difficult to

detect using a 10-bit ADC. Also there is a chance of detecting the wrong end of charging

due to noise. So instead of the negative delta voltage, we use a dV = 0 condition for a

certain time duration. This gives very good approximations for detecting the end of charging.

For this reason, this method is used in the evaluation board example instead of the negative

delta method described in Section 1.2.1.

1.2.3

Max. temperature detection method

In this case if temperature rises above a threshold, charging is stopped and trickle charging

is started.

In this demo the Zero Delta Voltage method is used as the primary technique for terminating

the charging. Time Out, Max Voltage and Max Temperature are used as the secondary or

back up methods for ending the charging.

−∆V

-10 mV/cell

Charging time (t)

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