As I see it,
Power = Amps * Volts
So
650W = 650/240 = 2.7A @240V or 650/7.2V = 90A @ 7.2V (assuming you had enough racing power supplies to convert it all down).
This is massively approximated but as you can see, charging 3, 7.2V packs at 5A = 15A @ 7.2V so is well inside the max output of the generator. There are lots of other things to take into consideration so the math isn’t spot on but you get the idea. We used to run a tent of people charging and the tele off my old Honda. The other thing to take into account is that the more power you pull from it the more petrol you use.
I bought one of these last year
http://cgi.ebay.co.uk/UNUSED-PORTABL...QQcmdZViewItem but only used it twice as we started using Vermins as it was cheaper on fuel to run 2 tents worth of stuff off his and share fuel that run both tens off separate generators.
I’d consider selling it if you were interested, could bring it to Kiddie as long as my cast comes off tomorrow so I can drive. It’s like the one above but not a caged one, has a handle on top. I t is a 4 stroke one so no arseing about with 2 stroke oil. Try before you buy?