LED, batteries, and current

[Fazerider]

[quote author=limax2 link=topic=1113.msg7511#msg7511 date=1323720755]If you linked the two 12v batteries together in series you would have the equivalent of a 24v battery as you say. However as I see it the draw from each battery would be 450ma. (I'm in disagreement with rustyrider here).

Why?  You've got 2 batteries connected together in series to give 24V and a load which is drawing 450mA.  If you were to draw 450mA from each battery, you would have 900mA of current flowing.  The lamp draws 450mA so where is the other 450mA going?

If you used a pair of 12V batteries in parallel, the draw at 12V would be 900mA, that would be 450mA per battery but by increasing the voltage you decrease the current required.
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If you connect batteries in series the voltages add, the current is the same through each ... so both are supplying 450mA.
Think of the batteries as pumps pushing water uphill: in parallel a pair would give twice the current, in series they could pump the same current as one pump... but twice as high (they're pumping the same current because it's the same water).


As for the original problem of which is better in this instance, it makes little real difference. A low voltage cell in a battery will get worse as the battery discharges (in fact it will become reverse charged), but it's in series with the other cells anyway so if that battery is doing anything useful the sick cell will get worse whether the batteries are in series or parallel.
I'd have a slight preference for series connection, at least that way the healthier battery isn't wasting power by trying to charge the sicker one... though it would make charging more fiddly if you only have a 12V charger.