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\$ – Ignacio Vazquez-Abrams May 12 '18 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 '18 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\$ – mkeith May 12 '18 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\$ – mkeith May 12 '18 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\$ – laptop2d May 14 '18 at 17:52

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 '18 at 23:23

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