It ramps up the voltage provided by the car's alternator to make sure that the caravan's battery is being fully charged.

It compensates for any wiring-induced voltage loss, for one thing - it's quite a long way from the alternator to the battery in the caravan, and there are extra connections along the way.

You should get an output voltage at the booster terminals of about 14.4V when the car is running above idle.

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Check that the green light on the booster comes on when the car engine is revved.

There's a fuse mounted on the floor that can blow.