In reply to Phil Anderson:
I bought a cheaper amazon one (35ish)... Haven't tested it on the car, but it's stopped charging my phone effectively: charges for 2 mins, stops charging, will charge the phone if I reinsert the charger every 2 mins, but is a bit of a ballache.
Wouldn't bet on it being effective anymore after 6 months occasional use charging the phone while camping.
So, maybe... but might be worth investing more money if you're likely to need it.