Battery charging question

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What's the best way to charge batteries (Nicad or Nimh)? Series or parallel? I'm guessing parallel, but as I have a rechargeable light which charges the battery in series is it worth taking them out of the light to charge them with an external charger in parallel?
Cheers
Steve
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It must depend on the voltage of the charger. 2 batteries in series would need twice the voltage they would need if in parallel.
Rob Graham
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Oldskoolskater wrote:

Do you mean batteries, or cells?
If you want to e.g. charge a pack of identical cells in more or less identical state of charge, in a hurry, you need a delta peak charger. And they need to be charged in series really.

No, not really. Its fine to charge more or less identical cells in series at the same current. It sonds like its a trickle charge anyway. Eventually the cells that charge first just get warm and dump the excess charge as heat, the others stay on charging till they are full too.

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That's one way to shorten their service life by a factor of several...
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Dave Plowman snipped-for-privacy@argonet.co.uk London SW 12
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Dave Plowman wrote:

Not under trickle charge it ain't.

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have their problems.
If you charge in parallel there's no way to guarantee that they share the charging current properly, thus the more discharged one will (at least initially) get most of the current and that may be too much. Also it may well be that one cell is old/faulty and won't charge to the full correct voltage so the other (good) cell doesn't get fully charged.
If you charge in series then cells with less capacity (older, more misused) will get overcharged and/or better cells will be undercharged.
When charging battery packs you usually can't avoid charging in series and sets of cells in packs are going to be pretty similar so series charging is acceptable if not ideal.
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Well, of course, any pack will be charged in series, so I doubt it makes much difference.
The ideal way would be to charge each cell individually so it can be measured individually as well. But provided the cells aren't run completely flat this shouldn't matter in practice.
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On 2 Dec 2003 23:57:15 -0800, snipped-for-privacy@mailinator.com (Oldskoolskater) wrote:

Individually; failing that - in series so long as the size, age and discharge state is similar. Parallel charging won't work with a constant current charger (which all NiMH and NiCd chargers are) and will damage cells.
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Peter Parry.
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If you have a power supply (not battery charger) available then provide individual current limit resistors and charge them in parallel - a reasonable charge current is one tenth of capacity.
Dave S
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wrote:

Utterly wrong. How would you ensure equality of current between the cells?
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Because each battery would have its own current limit resistor from a constant voltage source.......
Dave S
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wrote:

Unless you limited your charger to an intrinsically safe charge rate of about C/20 this wouldn't work. Alkaline batteries require constant current not constant voltage charging. You could certainly put constant current sources in each battery feed, but this is independent not parallel charging.
Simply putting a resistor from a voltage source would lead to a variable charge rate. If the resistor was sized for the initial charge rate it would be too large for the later charge rate, if sized for the later it would be too small for the initial. The former would have to be the solution chosen but the results would be unsatisfactory.
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Peter Parry.
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And that is *exactly* how most cheap drill chargers work - or did last time I looked. And since they're meant to give a four hour charge - result quickly knackered batteries.
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Dave Plowman snipped-for-privacy@argonet.co.uk London SW 12
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Dave Plowman wrote:

I don't think so. At least mine appears to be a gebuine delta peak charger.

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The Natural Philosopher wrote:

I am replying to this message as TNP seems to know a lot about charging batteries.
I have a good quality nicad charger (a Robbe Lader 6+2 if it makes any difference) that I am loath to replace. I was wondering if it was possible to add a delta V switch to the output that could disconnect the cells when they are charged. Does anyone know of a circuit diagram on the web that I could use to add this functionality?
Regards,
Andy.
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ma005724 wrote:

You sure it isn't a delta peak already?
Ther are chips available that do all teh stuff you need.
HOWEVER a trip to the local model shop will net you a charger that will generally do from 4-7 cells at usually about 3A, with built in delta peak, for next to bugger all - about 25 quid from memory.
e.g. http://www.ripmax.com/FS_Access_Chargers.html and have a look see.
Ideal for sub C nicads,

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The Natural Philosopher wrote:

Positive.
Do you have any suggestions for specific chips that I can search for?
I know I could buy a new charger but I fancy trying to cobble something together just for the hell of it.
Regards,
Andy.
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ma005724 wrote:

http://www.national.com/ds/LM/LM3420.pdf
Does NiMh, NiCd and even Li-ion.

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ma005724 wrote:

There's the MAX712 IC used here: http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1666
and the esteemed Mr Parry has this: http://www.wppltd.demon.co.uk/WPP/Batteries/Charging_Methods/charging_methods.html
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Toby.

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NICd/NiMh charger chips that I got just for the asking.
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