Was it a name-brand Battery Tender?
There are two different kinds of battery chargers:
1) constant current, and
2) constant voltage
The really cheap ones ($10 to $20) - which can come
in handy - are [b]constant current[/b]. I suspect you had an
AC/DC adapter that converted 120VAC to 12VDC at an
amp or two of output. Those are great if you manually
monitor the voltage with a voltmeter and pull them off
when you hit 15V or whatever. However if you leave
them on too long, they can cook the battery with excessive
voltage.
The expensive ones ($40) are [b]constant voltage[/b], like
the Battery Tender. As the volts go up, the amps go
down, so the battery doesn't get cooked. You can
leave them on permanently. I have vastly simplified
what goes on, but you get the idea. It has a regulator
like on your engine, to control the voltage. If you
record it, the charging voltage might look something
like this:
[img width=500 height=419]
http://edelweiss.smugmug.com/Cars/Porsc ... ithm-L.jpg[/img]
Constant current is open loop, constant voltage is
closed loop. Closed loop control is almost all the
time superior. Except when it's not smart enough
and you get a oscillation going, and you have to
drop back to open loop to get anything productive
done.
I have bought, no shit, at least 20 Battery Tender
Jr's in the past few years. They are perfect for
almost any vehicle battery that isn't in use.
Hope this helps. A very long time ago, I was an
electrical engineer, but really, no one gives a shit
about stuff that happened so many decades ago.
It's funny though, kids are amazed when you show
them this:
[img width=500 height=167]
[/img]
This was pretty cool. 13 microseconds through a Cortex A-5 SOC:
[img width=500 height=109]
[/img]
Mark 1 is the end of a digitized automotive SENT going into the SOC.
Mark 2 is the start of a SPI transaction going out of the SOC to
a D-to-A voltage converter.
Line 5 (green) is the DAC transitioning to the analog voltage
specified in the digitized SENT waveform that I decoded with
a FSM in the A-5.
That is fucking cool, even for a TC big belly from Tower C.