Bruce:

Did you measure the voltage with the charger connected, or after you removed
it?
A standard 12v lead-acid automotive battery has a nominal voltage of 12.6
volts. Even after a float charge, once the charging current is removed, the
battery will return to about 12.6 volts fairly rapidly. Thus, the voltage
you measured would be considered normal for a charged battery.

While the trickle charger is connected to the battery, any current in excess
of that needed to fully charge the battery is converted to heat. You will
only read the charging voltage when the charger is actually connected and
operating. about 10ma of the current is going into the loads you've already
measured, with the rest going to keeping the battery at top charge.

Unless you are measuring the voltage with the charger connected, you
probably don't need to have one with more current.

--Rick



"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.