Re: Some quick questions about cells and batteries...
- From: NoThanks <no@xxxxxxxxxx>
- Date: Tue, 28 Feb 2006 00:50:29 +0000 (UTC)
phaeton <blahbleh666@xxxxxxxxxxx> wrote:
Just some quick questions to see if I've got the concepts correctly.
Any responses, flames or pointers welcome:
1) With similar types, you can generally expect comparative battery
life to be proportional to the physical volume of material- i.e. twelve
D-cells can continuously supply 18V into a load longer than two 9V
batteries could, right?
Assuming equivalent construction and equivalent chemistries, I suppose
thats roughly correct.
2) Charging a sealed lead-acid battery is about as complicated as
connecting it to another current source and slowly pulling electrons
through it? In other words, *could* you safely charge a 12V motorcycle
or car or similar battery with an AC-to-DC wall adaptor and a simple
voltage divider circuit?
Bad idea. Very bad idea. Every battery chemistry requires its own unique
manner of charging (and sensing when a charge is finished). Lead acid
batteries should be typically charged at a constant current until they
reach a certain voltage, and then charged at constant voltage there-after.
There's a current limit and a voltage limit when charging lead acid
batteries, at no time should either be exceeded.
Two problems with what you describe. First, a wall wart with a resistor
does not make for a current source, it makes for a voltage source with
source resistance. Second, the voltage of an unregulated wallwart can
vary significantly with load. When left open circuit, a 12V wallwart
might actually be putting out 17V. Lead acid battery resistance increases
as it becomes charged, so a fully charged battery will appear nearly open
circuit, and your voltage dropping resistor will cease to work well. This
will put 17V across the battery which is quite bad for it.
3) The same can be said about NiMH or NiCd batteries.
NiMH and NiCd batteries can probably be safely charged at very low
currents (1/20th of their capacity?), but if you charge them rapidly you
better have a much more sophisticated charging scheme or you will risk
damaging the cells and/or creating a fire.
4) Do connecting cells in series add voltages but keep current the
same, connecting cells in parallel add current but keep voltages the
same?
Connecting cells in series cause the voltages to increase, but current
capability will be proportional to the worst of the cells. In theory
connecting cells in parallel should retain the same voltage but have the
sum of the current (and charge capacity). There is a big caveat here...
Putting cells in parallel is not a good idea, because if they do not
discharge equally, one cell will be trying to charge the other. If one
cell outright fails (such as a dead short or resistive short -- not
uncommon) this can lead to very unfortunate consequences.
5) Putting cells in parallel also increases the overall physical volume
of material. Thus, if you connect 10 ordinary D-cells in parallel you
still get ~1.5v but it will last roughly 10 times longer than a single
ordinary D-cell driving the same load. True, False or Maybe?
In theory, yes, if the cells are perfectly matched, you'd get 10x. In
reality, it may well be less (or if one cell fails prematurely, all may
die).
.
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