| Re: When 12 volts is not 9.6 volts
dgk wrote:
> I looked a bit closer at my rechargables, which are, it turns out, 1.2
> volts, not 1.5 like regular batteries. All rechargables that I looked
> at are 1.2. So, 8 * 1.2 = 9.6, not 12. Other stuff, like CD players,
> work fine on the rechargeables, but the cold cathodes have a bit of a
> problem. Obviously, 10 batteries instead of 8 should do the trick, but
> I don't have, and can't find, a case that holds 10 batteries.
You already have a case that holds 8, so buy a holder for two
additional cells and connect the two battery holders in series so they
deliver a full 12 VDC. However, you should note that rechargeable
cells can be damaged if you discharge them to the point where they're
getting reverse-charged. With 10 cells in series you might have a
situation where one of the cells may have a little less capacity than
the others and therefore it goes dead first. Even if the voltage of
that cells drops to 0, you could still be getting 9 x 1.2 = 10.8 V from
the other cells which may be enough to keep your device operating. But
driving that current through the discharged cell will probably damage
it.
So when using that many cells in series you need to be careful that all
the cells are well-balanced so they discharge at close to the same
time. Keep cells purchased together as a set and always fully charge
them at the same time. Also try to avoid running them to the point
where they're close to fully discharged.
>
> So, why do some things work fine with 1.2, and others not? And why
> aren't rechargeables 1.5?
As already mentioned this boils down to the basic chemistry involved in
each type of cell. The lead-sulfuric acid cells used in car batteries
produce a bit over 2 V/cell and therefore you get 12 V from 6 cells in
series. Rechargeable lithium ion cells produce about 3.7 V/cell while
NiCd and NiMH cells are about 1.25 V/cell. The latter is close enough
for many devices designed to run on the nominal 1.5 V/cell produced by
alkaline cells.
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