Charging NiMH battery pack

[The Magic Rabbit]

I have a battery pack made with 8 AA cells, NiMH chemistry, 2000mAh stated capacity, average 1900mAh capacity when tested.

My usual charging method is to remove them, and use a smart charger which charges individually, 4 at a time.

I use the pack to power lighting in my shed, since there's no easy way to get power out there. Sometimes, however, I run a power lead outside to use power tools. I replace the batteries with a 12V power supply.

That would be an ideal opportunity for a top-up charge.

Is there a simple circuit I could use to charge the batteries? I'm thinking perhaps a slow trickle charge for a few hours?

My idea is an LM350 with a 6.8 ohm resistor to put a 184mA constant current through the cells? Time could be anything from 1 to 6 hours.

Would this damage the cells? Is there a risk I'd overcharge them?

Many thanks.

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[IanB]

You can charge AA cells at a 0.1C rate for 12 to 24 hours without doing any harm to them. If your pack has a tested capacity of 1900 mAh then a charging current of about 190 mA will be fine. Just charge all the cells in series at that current for as long as you have power available.

[IanB]

If you are able to keep a close watch on the battery you can charge it at a higher rate like 400-500 mA. However, you must monitor the pack temperature and stop charging as soon as it starts to get warm. If any cell feels "hot" to the touch it is time to stop.

[The Magic Rabbit]

Define "warm"? I have a microprocessor in the lighting that is really just a cheap way to give me PWM dimming for the lights (LED). It would be easy enough to add a temperature monitor - TC74 or similar.

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[The Magic Rabbit]

The cells are already balanced to within +/-100mAh, so should charge evenly.

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[IanB]

Define "warm"? I have a microprocessor in the lighting that is really just a cheap way to give me PWM dimming for the lights (LED). It would be easy enough to add a temperature monitor - TC74 or similar.

The best way is to look at the rate of change of temperature. When it starts rising steeply the battery pack is charged. But if you didn't want to get that complicated, just stop charging if the temperature gets above, say, 35°C or 90°F.

Attached is a plot I made of charging an AA cell. You can see how the temperature starts to shoot up once it is fully charged. The plot is for a fairly high charging current, but the temperature will have a similar profile when charging at lower rates. The temperature goes up because once the battery can no longer absorb any charge the supplied power is dissipated as heat rather than being stored inside the cell.

[The Magic Rabbit]

So I could pump a steady 1A (ish) into the cells, cut the charge at 35°C, and that would be charged to probably 80% or 90%, without damaging the cells?

Presumably I'd be best putting a bit of copper around the chamber like a belt, to conduct heat to my sensor, rather than relying on just one spot?

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[james_s]

It can be done, although 1A is harder than I like to push AA cells, even with a good intelligent charger. For a DIY approach like that I prefer the 0.1C trickle charge method, it needs nothing more than a suitable resistor and a power source.

[IanB]

So I could pump a steady 1A (ish) into the cells, cut the charge at 35°C, and that would be charged to probably 80% or 90%, without damaging the cells?

Presumably I'd be best putting a bit of copper around the chamber like a belt, to conduct heat to my sensor, rather than relying on just one spot?

You could do this. You would ideally want to embed the temperature sensor in the middle of the battery pack rather than fixing it to the outside.

Whether you can charge at a 1 A rate depends a little on the quality of the cells. With Eneloops I wouldn't worry too much, but with cheaper cells you might want to limit the rate to 750 mA or so. It's best to experiment and adjust things according to what you find.

[The Magic Rabbit]

They'll always be Eneloops, unless somebody recommends something better. The cheap cells I've had have never lasted very long, but these Eneloops perform as well now as they did when I first got them about 4 or so years ago. :-)

The holder doesn't really allow for embedding a sensor inside the pack unfortunately.

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[james_s]

Eneloop cells have all been excellent in my experience, even the earliest ones I bought are still going strong and still meet their original specs. I have some Fujitsu cells too that I understand are made by the same Japanese factory that made Eneloop cells for Sanyo. Main thing I look for is "Made in Japan".