Look at 'Starfire's' early posts on here, lots about home brewed charging systems using car / truck alternators.
The problem with electric is storing it, that's where the £200 petrol genny comes into it's own for occasional use, it's self regulating and provided you turn off the fuel and run the carb dry (so it doesn't gum up as the fuel replenishes the level in the float chamber) the're great.
In you shoes I'd go down the car alternator route, it's simple, the alt's come with there own regulators ect. and are easily replaced.
It's a bit of a pain having a battery to look after, a run very week or two would be the way ahead, but that and a small inverter is all you'll need.
And it'll probably work out cheaper.
Cheers Stef