Thanks Andrew.
I wonder how long an A23 12V battery (the small ones) would run a dew strap (they are apparently merely 8 x 1.5V button batteries in series)
. Some claim their 9V straps run a year on one battery.
Failing all this, I may just buy a small 12V and run it off that. I got 4 years out of 1 jump start pack that uses a similar battery, so 4 years is pretty decent odds. I just want the compactness of the battery alone.