On 22/04/2020 13:44, Andy Burns wrote:
Jon Parker wrote:

Does that all make sense?
Even assuming you've got a 'free' 12V battery, what you spend on 3-way
switch, 2x sockets and a weatherproof enclosure will probably cost more
than occasionally running the car and plugging the air pump into the
cigar lighter ...

You are assuming the because the OP has a garage that he also has a car.