Car battery charger.
to be honest I would go for a jump start pack rather than a stand alone charger, you have to charge the pack which can take all day but they can provide the power required to start the car instantly without needing to remove it from the vehicle and then run the car to charge the battery, there portable (used to keep mine in the boot) and with certain models of jump pack you can charge them off the cars cigarette lighter socket just make sure you get the right type of jump unit (different types for petrol/diesel) and your good to go for a long time 
I have a jump start pack - but it won't take a charge, so it's resigned to the shed until I can dismantle it and check out whether it is the battery or the charging unit. 
@30FTTC06 wrote:
Just to throw a spanner in the works
Just be careful if chucking spanners around near car batteries...
