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Recharge - Cells or Battery

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TaDa

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I want to create a bit of kit that has 3 rechargeable D cells in series in it.
I don't want to have take the batteries out to recharge - I'd prefer to add the recharge circuitry to the device and then just plug in a wall wart when required.

Now my question is, should I try to make the system recharge each cell on its own individual circuit or would it be ok (or even preferable) to recharge the whole 4.5 battery?

I suspect the purist answer is to charge each cell individually and that the pragmatic solution is to charge them as a battery. So I guess my real question is would the purist solution be "worth it"?
 
If you want maximum lifetime (number of charge cycles) from the cells it's best to charge them individually so that their individual capacities can be allowed for. Even if the cells are initially matched for capacity they may not stay matched as they age. If you can tolerate a less-than-max lifetime then charging them as a stack would be much simpler.
 
You can charge them in series without a lifetime penalty if you stop the charge when the cell with the lowest capacity is full. But to reliably do that you would need a temp sensor on each cell to determine which one is getting hot after it reaches full charge.

Since you normally only operate the battery until the lowest capacity cell is discharged, there's no advantage to charging them in parallel from a capacity consideration.
 
Thanks.

Are individual temperature sensors a viable option? Seems likely to then require a much more clever circuit. Mind you I guess knowing when to stop charging a single cell is an art.

Once prototyped up I might be constrained on space so I fear it'll be the 'whole battery' solution. Thanks again!
 
You could go with a specialized IC such as this, which is designed to charge two to four NiMH cells in series. It only has one temp sensor (as an option) but it should do a satisfactory job of charging your cells.

Edit: The -ΔV charge option detects when the charge voltage slightly drops, indicating one of the cells has reached full charge, and stops the charge. This minimizes overcharging of the lowest-capacity cell.
 
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Check some of the recent issues of Circuit Cellar. One of the author's describes, although not fully, a fail safe charging/load balancing system,
 
Thats a pretty cool chip!

Shame its so pricey - Cost/Value are all part of the pragmatism I guess.

Very tempting though!

I see there are parallel charging chips too (DS2711, 12 and 14)
Shame they seem only for AA and AAA

Seems you can pay your money and take your choice.

Cheers
 
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