Good idea on the fork lift batteries.
A person should be able to step down the voltage.

In fact you can step it down to run 12 volt appliances of a 48 volt system. So it might be cheaper in the long run to convert it to 12 before you run it through your system. That would give you a back up power in cae of problems with your system.
http://www.powerstream.com/dc48-12.htm