How large of a battery pack do you have that you want to charge it at 50 amps?
You can buy a 45amp 48v charger for electric fork lift batteries for about $1,000 used
250CII by HOBART BROTHERS CO - Buy or Repair at PLCCenter - PLCCenter.com
And they weigh around 150 pounds.
I always figured that charging my batteries was something that I wanted to do carefully because my pack in my car cost $900 and I want to get as many years out of it as I can.
If you want a proper cheap 48v charger, check out golf cart chargers, they come in either 36v or 48v.