Better to charge off the car, unless you have a proper regulated charger like a ctek, just to be safe to the module and the wiring. If you haven't had a flywheel then you might be lucky, old flywheels kill starters on the 1800 tdcis over time. Knackered starters spin slowly and might not turn a cold engine over fast enough to reach target rpm. Target rpm is the minimum rpm at which the ecu will fire the injectors and try and start the engine.