Best method for charging a homebrewed battery pack

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#1
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Hi,

I've got a little robot, it will runs on 12 or 16 NiMH batteries organized in groups of 6, 6 and 4. All batteries in one group are of the same type, however, not all groups are of the same type (they are NiMH, but not the same capacity, manufacturer etc.). Charging them means taking them all out and using a normal charger. They are all connected in series, like this:


--+--(Batteries1-4)--+--(Batteries5-10)--+--(Batteries11-16)--+--

Where each '+' symbol indicates a connection, that is connected to a central charging connector (which I implemented for the case of an event that I'd get bored of charging them the old fashioned way, which has just occured).

(thanks for reading so far ;-) )

The question: From what I've read so far ( http://www.powerstream.com/NiMH.htm and some else) charging them should be relatively simple. However, can I charge six (of the same type) serialy connected batteries in the same way, I'd charge just one (a constant current source, approximately described in the text, that will stop at U=1.78*nubmer of cells)?

Oh, and I've found this little bugger:
http://www.maxim-ic.com/quick_vi...
But I guess the same job could be done with an LM317 current source and an AVR.

Thanks,

David

There are pointy haired bald people.
Time flies when you have a bad prescaler selected.

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Since you can program, you can make the charger as smart as the programmer. Brainstorming on whether its easier to charge 16 cells in parallel and monitor 16 voltages, or 16 cells in series and monitor 16 currents, the 1st way seems easier than sticking current sense Rs in between all the cells and monitoring the current with floating differential amps across the current sense Rs. Conveniently, a mega1280 has a 16 channel a/d. Hmmm... the current in the series string should be the same everywhere... Maybe you need to monitor the voltage between every cell? I'm not sure which way is better

Imagecraft compiler user

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bobgardner wrote:
Brainstorming on whether its easier to charge 16 cells in parallel and monitor 16 voltages, or 16 cells in series and monitor 16 currents

Well Bob, if the cells are in charging in parallel, the voltage will be the same across each individual cell and the current through each cell will be somewhat different. The exception to this would be if each cell employed it's own current source.

But if the cells are charging in series, the current in each cell will be the same, and the differential voltage across each will be somewhat different.

Are you just having a spells of dyslexia :cry: or, are you getting old enough that CRS (Can't Remember Shit!) disease is finally beginning to set in. Soon, you'll find yourself pushing a walker... :lol:

You can avoid reality, for a while.  But you can't avoid the consequences of reality! - C.W. Livingston

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I had this picture in my mind of 16 batteries being charged by 16 resistors to a big lab supply, and 16 battery voltages back to a micro. I guess the micro could tell the operator 'Disconnect cell number 7. Its done" or something

Imagecraft compiler user

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The best way would be to implement a custom charge controller for exactly your configuration. You could design a charge controller which charges each pack separately, using a switch/relay setup to connect it to the first, second and third pack in sequence. The cells within a pack should be of the same type.

If the middle pack is the largest in capacity you could even charge them all in series and switch the charge supply from the end to a middle point when the corresponding packs are full.

A--Pack1--B--Packl2--C--Pack3--D

1) Start charging by applying the constant current to A and D.
2) When Pack1 is full, disconnect A and connect to B (Charging from B and D)
3) When Pack3 is full, disconnect C and connect to C (Charging from A or B and C)
4) When Pack2 is full - stop charging

You are missing temperature probes in your packs. These are important to detect the correct point to stop charging.

The charge energy you supply is going into chemistry until the cell is full. When it is full the energy is converted to heat instead as chemistry is exhausted. You can measure heat two ways: As temperature increase :-) and as flatting out/decreasing of the cell voltage. The latter depends on the chemistry NiCd decreases, NiMh flattens only.

There is an AVR appnote about battery charging using AVR's (AVR450).

Markus

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