Nigel P.A. Smith wrote: it was a University maintenance vehicle for some time, and it has a 120 volt charger, but only 72 volts in batteries, can I ad a couple of batteries, say to 96 or 120 volts, without changing the controller?
Having had a quick look at the ebay listing, no you're at the limit of the controller and would have to upgrade. For a car of that size, I'd say a minimum of 96 volt and my personal preference would be more like 120 upwards. It does depend on the torque per amp and rpm per volt characteristics of the motor. Since the quoted rated load rpm is quite low, you may have to go quite high in voltage to get a really decent torque spread.
I'd also be a little suspicious of the charger. It say's it's universal, so what current does it put out and what voltage were the batteries actually being charged to?