A current limited voltage source can be used. For example, to desulfate a 12.6 volt, 100 amp lead-acid battery a 30 volt source with a 100 ohm, 10 watt resistor can be used. The battery terminal voltage will drop as the battery desulfates.
If you don't have a 100 ohm resistor two or three 12 volt automobile lamps can connected in series. As the battery desulfates the lamps will increase in brightness thereby giving a visual indication of the process.
