Zach wrote:
[snipped]

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

    You mean 2200mAh, I hope!

>I wired them up in series (making 4.8V). I read on the
>side of the battery "Charge for 15 hours at 220mA."

    at 220mA you are basically charging at C/10.
    You can fast charge Ni-Cd batteries at much
    faster rate, typically at 1C , i.e. 2200mA for
    one hour.

> 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 with the pack, that is if
> the battery pack had no internal resistance.  But
> the amount of current drawn by the pack changes
> depending on how charged it is.
> Am I worrying to much about details?

    Typically, commercial battery chargers uses
    step-down DC converter with current sense
    feedback.
    Check out the industry standard TL494/TL594.
    Motorola site have very good documentation
    on this device.
    If you want to do it cheap, just pump in a
    constant voltage of 1.6V per cell through a
    diode.
    The Ni-Cd is a very tough battery although it
    does suffers from "memory effect" and is not
    environmentally friendly, i.e. the Cd is
    poisonous, the reason for the shift to NiMH.

> Well, here is why I ask.  I charged this pack up over
> 15 hours, and then connected my pic.  The pic ran
> for about 5 minutes and then stopped.
> Voltage on the batteries was 4.1V and falling.
> I measured the current drawn by the pic circuit.
> It was only pulling about 60mA.  Why didn't the
> batteries last MUCH longer?!?

    It is either the battery is not properly charged
    or you have a severe case of "memory-effect".
    To get rid of these "memory-effect", which is
    basically hysteresis, you need to discharge the
    Ni-Cd and recharge. Repeat if necessary.

> I am either not charging the pack correctly or I have
> a bad cell.  If I have a bad cell, is there any easy
> way to test this? They were supposed to be new
> batteries, they were expensive ($7 bucks each).

Regards,
Peter Tiang