"Bruce W...1" wrote:

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.

I don't know how you measured things - so I can't say for
sure - but you may not have a failure.

1) You need to measure the float charge voltage while the
charger is charging the battery. Don't know if you did that,
but 13.7 is good if you did.

2) The battery needs to be fully charged before connecting
the float charger. Don't know if it was. If the battery is
discharged and you connect your float charger and measure it,
you will see a voltage below 13.7 A discharged battery can
draw enough current to drop the output voltage of the wall
wart down below the 13.7 regulation voltage.

3) A battery removed from the float charge will show a lower
voltage than the float voltage. That is normal. So it is
possible that your charger is working properly and the battery
is being held at full charge.