A very simple answer is enough voltage to yeild the desired amount of current to flow into the battery. The voltage applied will always have to be slighly above the battery voltage at any given moment in order for the battery to charge. The open circuit voltage of a simple charger may be very high relative to the desired battery voltage but because of it's design only a limited current will flow. The resulting voltage settles where it settles based on internal resistance and other things.
There are limits to the rate at which you can charge a battery. Use of a charger based on voltage alone might ruin the battery or worse, create a dangerous situation. Some batteries are made to be charged at high rates, others aren't. Sorry to be so unclear but it's not a simple question.
If you aren't concerned about the time charging takes you might construct a simple current limited charger with a 10 or 12 vdc power supply capable of delivering 50 to 100 milliamps. See the datasheet for an LM317 - it shows a simple current regulator that you could set up for 50 or 75 ma. It might take 24 hrs to charge the battery but you stay way away from overheating and other problems. There are numerous ways to charge batteries at faster rates - some very complex but that preserve battery life - some simpler at the expense of time or some battery life. Many of these are described at websites- you might search prior posts under battery chargers. Some of these are really sophisticated but thanks to modest cost ICs are within the range of a person with modest electronics skills.
As a general rule, from what I've seen - a simple, low-cost charger can be used if time is not an issue. As the desire to reduce the time becomes a bigger factor then more elaborate schemes are applied to increase the charge rates in a way that minimizes danger -possibly at some expense in battery life. My son used these batteries in so called fast chargers. They did work and the batteries got hot but didn't explode. I think that it was more "cut and try" rather than good design on the part of the charger manufacturer. I'm also convinced that battery life was much less than it could have been but then like everything else in life you trade on thing for another.
You could "cut and try" using the scheme I described, increasing the current. That IC (LM317)is good for an amp or more. I can't tell you how hot is too hot for the battery or at what point it might let go. An amp gets the charge time down to a few hours or less. I've no way to determine if you apply 12 volts if the battery will charge at 1 amp though I'd think it would.