On Wednesday, 22 April 2020 13:39:29 UTC+1, Jon Parker wrote:
I'm considering building a basic 12v power system for my garage comprising a car battery, a solar panel on the outside to keep it charged, and a 12v outlet on the exterior wall and probably one on the inisde also. Going to be used for running the air pump mainly when pumping up bike and car tyres, paddling pools etc.

I intend to use a 3-position switch to enable me to easily switch the battery positive to the solar charger, or over to the 12v outlets, and to prevent the outlet from receiving power when the battery is connected to the charger. I'll need some sort of weatherproof box for exterior wall outlet of course. I expect I'llneed to put a suitable fuse in the positive line, around 6 inches from the terminal, 10 or 20 amp should be fine I think.

Does that all make sense? Any suggestions?

Jon

Get rid of the switch then it'll work. Just connect it all together. Battery fuse should be chosen to suit the wiring in use. If you use 2.5mm T&E you can use 30A fusing - you could even re-use an old Wylex fuse box if you want.