| Re: When 12 volts is not 9.6 volts
On Sat, 30 Dec 2006 15:36:27 -0500, dgk wrote:
> I like to use rechargable batteries in my lights, and for the most part
> using a rechargable AA in place of a regular battery works fine. But
> with cold cathode bulbs on my bike, the little transformer thing needs
> 12 volts. So I replaced the normal 8 AAs with 8 rechargables. And it
> sort of works. But one bulb starts dimming after a half hour or so.
> Checking with normal batteries, this doesn't happen.
>
> 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.
>
> So, why do some things work fine with 1.2, and others not? And why
> aren't rechargeables 1.5?
Alkaline batteries can't deliver their rated voltage (1.5V) when connected
to a high current device like a bike light. With a bike light,
rechargeables (nicad or NiMH) may actually deliver more voltage, though
nominally rated for less (1.2V).
The "1.5V" of alkaline batteries drops quickly and steadily with use
anyway. Note how bike lights powered by alkalines start out very bright,
but dim steadily as the batteries wear down. With rechargeables, they
stay relatively bright for quite awhile before dimming significantly.
Matt O.
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