[quote pid=633 author=giorgio date=1207130383]
As for wiring - Andy has pretty much got it spot on.[/quote]
My employer would be glad to know I'm pretty much not a complete retard...
Hayesey - even if it's a fully sealed battery I'd stick it in a proper battery box. Will make things a lot easier come MOT-time if the tester's a bit unsure. A lot of our prototypes run their LV battery in the boot with a plastic box - more for the electrical insulation rather than fumes.
Not too sure about regulations on fumes from batteries, obviously it'll depend on what battery you're using as to whether the fumes from it are harmful or not - for example the HV battery used in the Toyota Pious can apparently give off some stench according to the user manual, but they'd never get it homologated if it was in any way harmful. The Pious HV battery is vented in to the cabin, and not outside.
I don't know how BMW do their harnessing from the boot, but the vehicles I've worked on with LV batteries in the rear still tended to have the main fusebox under the bonnet - so had a single positive cable run to the fusebox in the front of the car. If that's the case with the BMW, then you can be confident that the cable can carry the current.
Giorgio - I think you'll find with a healthy Polo-sized battery it's the starter motor that's limiting your cranking torque & speed. A larger capacity battery won't gain you a single thing. Even with a 'charger creating extra drag and 9:1 CR 1341cc pistons you've not got an engine that's difficult to crank. The Mk3 Polo 1.4 diesel is much much higher CR, and they don't need massive batteries.