>I'm needing some battery backup in my next project. I'm planning on using a
>3v NiMH cell. I'm hoping on a simple trickle charge system. Do I need
>anything more complicated than a current limiting resistor? If not, what
>resistance do we assume the battery will have under charge?

If you're not in a hurry, and you don't have to worry about the
current used by the circuit that it's in, then using a resistor is
possible.

You figure the difference between the voltage of the charger and
the voltage of the battery.   That difference is going to be
the voltage across the resistor.    Now, if you know the charging
current that you want, you can figure out the resistor value using
Ohm's law.

When the battery is dead, the difference voltage will be higher
than when it's charged.   This difference voltage does not change
very much if your charger's voltage is substantially higher than
that of the battery.   You can see this if you try it with
different numbers.

Stay tuned as I believe others will have something to say about
the wisdom of leaving NiMH batteries on charge and so forth.  I
haven't used them much.   I believe they are a lot like NiCd.
It is the Li-ion that are different and you can't use tricks
like this on them.

Barry

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