i can send you schematic diagram of battery charger(by schematic editor later)this time only with hand made..This circuit consist of LM324(quad op/amp),
moc3021(optocoupler which drive a triac),BTA16(triac),7809(9 VOLT REGULATOR),and some passive components like res and caps.
Main transformer should be 12~15 volt ac with 15 AMP current,one can adjust charging current with trimpot VR100K(104).
Shunt voltage can obtain from negtive wire(almost 50 to 100mv)are sufficiant.