>keep the current at 220mA.  Sure, you could put a 21ohm resistor in series

Remember that Ohm's Law deals with the voltage dropped through the resistor,
not with either the supply voltage or the voltage at the load. For get 220mA
through 21 ohms, you need 4.62 volts more than your battery pack, or about
10 volts. It sounds to me like you were not supplying it with enough
voltage. Also consider that the charging voltage will average about 1.35
volts per cell. Yet another thing to consider is that a brand spanking new
nicad battery should be charged to double its capacity, so give it 20 hours
at 220 mA.

Cheers, Bob