Possibly derived from an isolation transformer???? 60-0-60 with the centre-tap fixed to ground??? Such transformers are used as 'site transformers' for portable power tools.
Either way it's going to be 120V AC as the main source supply therefore any standard (linear or SMPS) power supply circuit or module can be used - even an old computer PSU. But a lot depends on the battery being charged - its charging requirements etc.
More info needed.