Nickel Metal Hydride ( NiMH ) Charger

Posted on Feb 15, 2013

This circuit will charge a 12v battery (10 NiMH cells) @ approx 220mA. It is a very simple circuit but requires correct setting up. The circuit is designed around a 12vDC, 500mA plug pack. This type of plug pack delivers about 17.5v to 18.5v DC on no-load and about 13v when the current is 500mA. Our circuit delivers 220mA to a set of AA cells and the output of the plug pack is approx 16v (when 220mA flows) and the voltage across a set of 10 NiMH AA cells is approx 13.5v during the charging process. This gives approx 4.5v for the `head voltage,` and this creates the situation where the plug pack will deliver a current to the battery.

Nickel Metal Hydride ( NiMH ) Charger
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Nickel Metal Hydride ( NiMH ) Charger - image 1
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Do not use a 300mA plug pack. It will get too hot in this application. Some 12v DC 500mA plug packs are quite small, others are much larger. What is the difference? Plug packs (actually the transformer part of the plug pack) have a hidden rating called REGULATION and for this application you must use the biggest and heaviest plug pack you can buy. REGULATION is the comparison of the voltage under load with the voltage on no-load. When the voltage under load is only slightly les than the no-load voltage, the transformer is said to have Good Regulation. If we take a small 12v DC 500mA plug pack, the no-load voltage may be 17.5v, but the fully loaded voltage may drop to 11.9v. A larger 12v 500mA plug pack will have a no-load voltage of 17.5v, but the fully loaded voltage will drop to 13.2v. When a smaller current is drawn from the plug pack, the output voltage is slightly higher. That's why we can use a 12v DC plug pack for this application. We need about 13.6v to charge 10 NiMH cells and a large 500mA plug pack will deliver more than this voltage when the current is 220mA. The small plug pack will not work at all. But we can't connect a plug pack to the 12v battery without making an important test. You must place a multi-meter (set to 500mA or higher) in series with one lead when the plug pack is connected to the 12v battery. The diagram below shows you how to do this. It will take up to 24 hours to charge...

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