To charge a battery 6V 4Ah, how much voltage do I have to supply to it so as to recharge it at a good rate? Is 12V alright, or 9V? Plus,I need to know where I can find a transformer for nearly 24Ah, or even greater if possible, since I need to build an inverter which will have backup for atleast 3 hours or so... The best transformer I found was that one in a UPS, which was for 7Ah!