1. Yes, you need to determine the battey amp hour capacity. The battery may have a name plate that will give you this, if not remove one of the inner cell connectors near the the positive battery cable (the red one) you will find a number like 18-125-13. Take a way 1 from the last two digits & divideby 2 then multiply the result by the middle set of numbers this will = the AHC of battery at a 6 hour rate (in Europe & Asia & probably the rest of the world - use a 5 hour rate). In my example 13 minus 1 divided by 2 = 6 times 125 = 750 AHC.

2. Charger should have an Amp Capacity + or - 20% max of teh battery AHC no more or less - personnaly I like to size the charger by plus or minus 10%. The Amp Capacity of the charger will determine the Charger out put. Most 36 volt charger will have an out but of 130 amp or greater.

3. The next thing you need to know is if the charger is Single Phase or Three phase. A Single phase charge can be wired into a 3 phase power supply but a 3 phase chager cannot be wired into a single phase power supply - with out cahnging the charges transformers = too much $$

4. Finally, you need to know if the battery is a gel or maintenance free type battery - the require a SCR type charge (do no confuse that with a GNB SCR FLK charger - all that means for a few hunder bucks you can convert teh charger to full SCR otherwise they operate liek a ferro reonate charger) as a different charge profile for these batteries is required.

I'll stop right here - you question is a good one but right sizing a charger to the battery is very important. If you get a charger too big you will basically microwave the battery to an early death. I had a customer that the local "lean to" type forklift dealer sold him. He was "cheaper" than my proposal. He got a truck with a 595 AHC, 48 v battery & a 1200 Amp charger. I have since sold him 1 new battery & late last year when the battery died after 2 years in a low usage (1 - 2 hours per day) operation sold him a newer truck and a right sized battery charger package. Pay them now - then pay me more later - luv it.