Wiggle, thanks for the response and the link. I think yours was the initial one that I saw.

I've got a pack with 8 18650s now that I have wired 4S2P for 14.8 volts, but I could change that over easily enough. That would give me ~20Ah capacity, so I should be able to find a level that will run 12 hours on that. I don't need 12 hours all the time, just every now and then.

Steve