> I recently bought four new C size ni-cad batteries (1.2V 2200Ah).

I assume you mean 2200 mAH.  If you can buy 2200 AH capacity in
a C size battery package, then someone's made a breakthrough! :-)


> I wired them up in series (making 4.8V). I read on the side of
> the battery "Charge for 15 hours at 220mA."  Now here is my
> question- what is an easy way to keep the current at 220mA.
> Sure, you could put a 21ohm resistor in series...

A constant current source driven by a 10V or higher supply.

Or build a battery charger based on an active control element
such as the BQ200x series or a PIC.

Or just spend $50-100+ on a battery charger in the R/C car
section of your local (or mail order) hobby shop.  Most of
the "real" R/C cars (i.e. not the ones from Toys'R'Us) use
6 of these cells connected in series as a 7.2V pack.

You can also get basic battery charging & maintenance info
from books on R/C cars.  "Radio Control Car Action" magazine
also has some information and lots of advertisements (but is
aimed at a non-technical 16-18 year old crowd).


As an aside, the 220mA charge current is the C/10 (capacity
divided by 10) rate.  Nearly any ni-cad can be charged using
this method for 14-16 hours.

Most modern cells can also handle much higher rates up to 2C
or 3C.  Using a charge current of 4-5 Amps, you must monitor
the pack.  The Hitec CG-325 charger I have has programmable
voltages & currents.  It's a peak detection charger.  It stops
charging when the battery pack voltage peaks & then sags very
slightly.  Run from a 12V battery, it can fully charge a dead
7.2V 1800mAH pack in 20-30 minutes.

There are lots of other chargers currently available with
this kind of performance.
                                                Lee Jones

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