Its really a matter of wattage, the charger handles the required voltage. The charger has to be at a higher voltage to 'drive' any current into the battery. For NiMh, this means not only being higher than the total pac voltage, but also overcoming the packs internal resistance (IR) that is relatively high and causes some of the heat you'll notice when these are charging. A good NiMh sub c cell will get to +/-1.5v/cell too so a 12 cell pac will see 18+v right off the charger.

As fas as a power supply is concerned, it needs about 20% more watts than the charger is outputting to enable charger to take care of business. So for the 12 cell batt, charger will put out say 20v at maybe 5amps or 100watts of power (this is an example, but won't be that far off for a real 12 cell pac charging at 5amps). You'd need a power supply capable of delivering at least 120watts, so if its a true 12v supply, it'd need 10amp minimum output. You'll find a lot of RC oriented power supplie slisted as 12v really deliver 13.5-13.8v.

Power supply is an area where more better. For the above, I would look for a 15a supply (180watts). Power supplies last longer and run cooler when they are not running at 80-90% of their rated capacity.