> What is a gel cell, and I can I charge it with a dynamo from a bike?

A sealed lead battery. The 6V 3Ah I buy is around NZ$22 (US$9).
One thing you need to keep a careful eye on is charging voltage
and limiting the discharge voltage. Go below a certain voltage and
not charge it PDQ and you'll permanently stuff it. Maxim will have
details about battery mangement ICs, which I'd recommend for
getting the best lifetime. I have a smart charger for gel cells so
haven't needed to look into it and can't supply details, sorry. As
for using the dynamo, I've never measured the output of one but
I guess it must be a few volts. If it isn't enough to directly charge
a battery you may need to build a voltage doubler and then regulate
it. It's all sounding very complicated but it isn't really. The money
you'll save over disposable batteries and the piece of mind gained
from having a second source of power will be worth it

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