If you're going to charge a Ni-MH cell either do it at a trickle where there are no smarts required, and it doesn't matter if the cells are in series or not, or do it in two hours or less. The reason is that for 'fast' charging (not trickle charging) once the cell is full, the voltage of the cell drops once it starts to heat. The charger has to catch this drop in voltage to know its done. So called '-deltaV' termination. The slower the charge rate, the less heating, the less voltage change, and the more likely a missed termination is to occur, and the charger will either time out, or sense the increased temperature once the heat makes its way back to the temp sensor.
Personally I use a MH-C801D charger, easy, shows individual cell status, and only two options, fast or slow charge, and condition the battery or charge only. Plus it does 8 at a time, so less fiddling with changing batches.