It really depends on what charger you are using, how many batteries it can charge and at what rate,
For example if you are looking at buying a dual Battery charger that charges at a constant 500mah for each battery (half an Amp per hour) for each battery you need an adapter that can cope with a constant 1 Amp draw at the minimum, but that means the adapter will be running at its maximum capacity all the time and could get quite hot. A charger that can charge 2 cells at 1 Amp will need a 2 Amp adapter minimum.
If you use an adapter that can't provide the current that the charger needs your batteries will charge, but it will be very slow - and you will be absolutely pushing the adapter into the red zone.
Many phones don't have large capacity internal batteries and they don't charge at a fast rate - the adapter and lead may be designed to charge at something like 500mah max. If you look at the adapter it should say what current (Amps) it can cope with.
Personally if I bought a charger that needs 1 Amp I would like an adapter that can cope with 2 Amps, for a 2 Amp charger I would prefer an adapter that can cope with 3 Amps. The charger will only try to pull the current it needs to charge the batteries so it doesn't matter if you plug the USB lead into a plug adapter that can cope with a higher current, but if you plug it into a low Amp adapter designed for a phone it may overheat, and in the worst case go up in flames.
If in doubt - don't risk it, buy the adapter rated for the charger and use them together and throw away any adapters you may have for old electrical gear you don't use. You may be pissing away a few £, but at least you won't burn your house down.
