I thought of a better question to ask/answer.
Which tools do you keep backups of in your van?
I have backup screwdrivers and Allen keys. Everything else I reckon I can get by without for a couple of jobs, but no way am I going into battle without either of those things.
I guess a drill is nearly essential too and I actually have an old one that needs repairing. Once I’ve done that, it’s going into the tool box.
I also keep a fully charged 18V battery in the glove box. It’s strictly for emergencies only and it’s got me out of jail once or twice when I had to drill and realised I’d been slack with keeping the rest of them charged.