Not long ago and in another thread many of you gave me great advice on
how to make a car battery float charger. I wanted to just connect a
properly sized wall wart, but everyone recommended voltage regulation.
So I connected a voltage regulator (13.6V) to a 500mA wall wart. The
wall wart has an open-circuit voltage of 18V and is rated 500mA at 12 V.

Further background, I built this charger to prevent my having to start a
friends car once a week while they're on extended vacation.

Now two weeks later I check the battery. Its voltage is 12.7V. The
charger circuit measures 13.7V. And I measured the drain, from the
alarm and radio, it is 10mA.

The ambient temperature on average is about 40F.

What went wrong? Why is the battery only 12.7V instead of 13.7?

Lacking a better solution from you guys it seems we need more power,
ugh, ugh. 2A ought to do it.

Spec's say that car batteries (at room temperature) are best regulated
at 13.3V. For 32 degrees F 14.2V is better.

Yet the failure analysis remains incomplete. Where did we go wrong?

Thanks for your help.