No, you are pretty much on track.I'm not 100% sure how this battery management system functions exactly,it "protects" against overcharging by monitoring the cell voltages,that much is clear.They also state that the charge current should not exceed 5 amps,but don't say that it protects against this,which is of importance in our situation,maybe you can pry this out of them.Their charger is good for only 2 Amp,so the problem does not arise normally.
What happens is that if our motor generator is hooked up to the battery& controller&drive motor:the battery receives or supplies the difference between the generator current and the motor current,so let's assume we have the gen. supplying 8Amps,if the motor takes less than that say 6Amps, 2 Amps go to the battery,if the motor takes 12 Amps,the battery supplies 4 Amps and the generator 8 Amps.Of course if you shut off the motor the battery would get 8 Amps,hence my previous query.I have not said anything yet about how to manage the gas engine&generator combination.(fully automatic,manual,automatic with manual override,that's for later.
BTW I have run across another generator candidate on Ebay,it's a Bosch surplus dc motor good for about 300-400W at 50V at 4k rpm plus,as best I can determine (the specs are a bit sketchy as allways on Ebay),pretty much up our alley.There is somebody in NYC who's got a hold on a whole slew of these things apparently and has been dribbling them out. There are a few coming due in the next few days,prices vary all over the place fom $10 to $40 + $18 shipping.Not bad for $50 or so.This thing meets reqds and is pretty compact,they may nor be around forever !, he has disposed of quite a few already.