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

Zach,

If your charging current is changing significantly during the
charging process, then you must be using a voltage that is very
close to that of the pack with a small resistance.  You could build a
current regulator circuit to produce a constant charging current,
but there is a much simpler solution.  Start with a charging voltage
that is significantly higher than the battery pack.  Subtract the
nominal voltage of a charged cell and calculate the resistance that
is required to get 220ma at that voltage.  For example, if you are
charging the cells individually (which is generally a better
arrangement than charging them in series) and using a 12V supply,
then use a resistor that is somewhere around 49 ohms.  This will
yield your 220ma accross 10.75V (12V minus the approx 1.25V of a
cell)  What?  You don't have any 49 ohm resistors lying around?  47
or 51 will do fine. Battery charging doesn't require great
precision. If you charge the whole pack in series using that same 12V
supply, then you will need about 32 ohms to get 220ma accross 7V (12V
minus the approx. 5V of the pack).  You can use Ohm's Law to
calculate the appropriate resistance for any combination of supply
voltage and cells.  Also make sure your resistors have a high enough
wattage for the power that they will have to dissipate.  The 49 ohm
resistor in the 12V 1cell arrangement will dissipate nearly 2.4
watts.  You might also want to be sure the resistor and power supply
can survive a short circuit so that your charger doesn't die or
catch fire if you have a shorted cell or accidentally short it out.

---

                                       Peace,
                                       William Kitchen

                                       bill@iglobal.net

The future is ours to create.