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I'm currently working on a project that contains a battery pack and consists of 10 nimh batteries each at 1.2v 2400mAh running in series (12v @ 2.4amps). Because I don't want to keep opening and closing my external case to get access to the battery, I'm looking into having a way to easily charge this using a power adapter via a charging circuit.

This charging circuit is where I stumble. Because I've got 10 cells, should I provide a constant current source of 240mA @ 12v (10%) which will take 10 hours to charge my pack (I can have a microcontroller to isolate the charging after 10 hours)? Because each cell might different between one and another, I don't know how I would go about providing a balanced charging current for each of these, as they are in series.

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    \$\begingroup\$ NiCd batteries were said to tolerate overcharging at the C/10 (10 hour) rate - and I haven't seen NiMH batteries flagged as very different. That makes them self-balancing to a certain extent - traditionally, simply extend the C/10 charging time to 14 hours and call it good enough. (Fast charging is different : it might be an option to fast charge to somewhere less than nominal and trickle charge at C/10 for proportionally less time). Now, modern intelligent chargers may be able to do better, but I don't know how much better... \$\endgroup\$ – Brian Drummond Mar 4 '16 at 12:27
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Your best bet is to locate a NiMH charge controller IC and build your charger using the recommended circuit for that IC.

Charging batteries is a tricky process if you intend to do it without destroying the cells. It is even trickier if you have a large number of cells in series.

Figuring out how to do it right is a job for a professional engineer, and finding all of the needed conditions has been the subject of many a research project.

The ICs will have all of this knowledge and know how "baked in." If you just need to build a charger, this is the easiest way.

If you are building the charger to learn how to make such a controller, you will need to do a lot of reading. You can probably also learn a lot just by looking at the datasheets for charge controllers.


As an example, there is the MaxDS2715.

This chip is built to control the charging of up to 10 NiMH cells. It is configurable for the number of cells, includes connections for the charge current sense resistor and for a thermistor. Proper charging requires regulated current and voltage, both of which this chip provides. Among the criteria for "full charge" detection is the temperature of the cells - this chip has the needed connections for a thermistor to monitor the battery pack temperature.

You will, of course, have to provide a powersupply that can deliver enough voltage and current to the chip.

There are numerous other chips out there. Given the keywords you can find in the description of this IC, I expect you can find others easily enough. Any large supply house (Mouser, RS, etc.) should have a good selection.

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toodles! Of course, it will be better to control charging current by a MCU with ADC which feeding potential from a serial connected resistor (to control a flowing current). And, if it possible, the temperature of batteries too. As a bonus in this case U can use start charging currents above 240mA and the charging proress will be running faster.

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