If my USB charger for AA/AAA NiMH batteries by BTY charges a NiMH battery, the current (ampèrage) of charge only depends on the USB input voltage (usually 5V) and the battery manufacturer (input current at 5V USB varies between 0.12A and 0.17A).

It keeps drawing the same power forever, even if the NiMH battery is full. The only difference is that the NiMH battery gets warm if charged completely.

I measured it with an USB multimeter (KW203).

That BTY charger has no charging controller.

Why does the charger keep drawing the same power forever?

  • \$\begingroup\$ Because it never stops charging. \$\endgroup\$ May 12, 2018 at 11:16
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    \$\begingroup\$ @Andyaka How does this not conform? It is about charging a NiMH battery. I expect a brief explaination for any mistakes I did. \$\endgroup\$
    – neverMind9
    May 12, 2018 at 13:51
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    \$\begingroup\$ Ask the manufacturer. It would also be a good idea to measure the actual charge current going into the battery. Most likely Ignacio is correct that it simply continues to charge the battery forever. \$\endgroup\$
    – user57037
    May 12, 2018 at 17:15
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    \$\begingroup\$ NiMH (and also NiCd) is very tolerant of over-charge. But it will still shorten their service life. If you CC charge a NiMH battery indefinitely at C/10, I don't think it will ever explode. \$\endgroup\$
    – user57037
    May 12, 2018 at 19:42
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    \$\begingroup\$ It probably has a voltage regulator in it and wastes most of the power as heat even when it's not charging, or its a crap chinese charger. Either way without a schematic it's all speculation and a bad question \$\endgroup\$
    – Voltage Spike
    May 14, 2018 at 17:52

2 Answers 2


That BTY charger has no charging controller. Why does the charger keep drawing the same power forever?

Because it contains nothing more (expensive) than a resistor. That is the cheapest way to produce these chargers.

At currents below C/10, you won't damage the battery too badly when overcharging. But this also means that a ~2Ah AA battery will be full tomorrow instead of today.

  • \$\begingroup\$ These are known as dumb chargers. \$\endgroup\$
    – neverMind9
    May 13, 2018 at 23:23

If the battery current continues when the battery is charged then the battery will usually be damaged.

In most cases I'd destroy such a charger.

There is a (very) SMALL chance that the charger absorbs energy when the battery if full.
I have never seen this done.

AA NiMH batteries under about 1800 mAh used to be able to be trickle charged at <= C/1- (eg 150 mA for a 1500 mAh cell.)
Cells over 1800 mAh had the trickle charge recombination chemicals removed to allow higher capacity. Trickle charging at ANY rate will destroy them.
Modern NiMH AA at under 1800 mAh capacity MAY use the old construction methods. Or may not.


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