Yes, I didn't make that clear. I know from the readings with the engine running or it plugged to AC with the battery switch in "Use" that I'm reading the charge voltage.
I couldn't see how it could be a problem with anything but the battery, except the first time it went down we were driving down the highway with the battery in "Use." But maybe there's a dead short in the battery.
I've left it charging, I'll disconnect it tomorrow and see if it holds, which, I expect, it won't.
And I'll pull the warranty papers and see what I can find.
Many thanks