Thanks so much for putting some brain-cycles to this. Like most things tried for the first time there are always aspects those with more experience will think of. In this case the switchover time was something I completely hadn't thought of.
How are you going to charge the battery?
In this case the battery and taking it on tour is actually a bit of an afterthought - I have a donor 12V wall wart already grafted in, so at the minute it's tied to the house and was going to be a semi-permanent. This is a bench worklight, using 4.8m of LED strip-lighting, cut to 24cm strips with brightness controlled by switching different patterns of them through TPIC6B595 shift registers via an iR remote controlled by an ATtiny85.
There are a few places where it could be used if I added battery power though. Remotely, the battery would be the primary source of power and it's charged by the vehicle, or a solar panel, or on a boat by the outboard. At home where mains is available there really isn't a need for a battery. In contrast, remotely some occasions both are present, for example for the hours when a generator is running at a campsite, or mains power is available to the site.
I think a version 2.0 designed from the start with mains charging the battery is probably the go.
This community is awesome as ever, thanks !