David Beierl wrote:

> Lots of modern battery chargers will pulse-charge at more-or-less full
> output once the battery leaves the deep-drained condition.
>

Yep, it appears that the DC > DC deep cycle battery charger is the culprit.

After trickle charging the house battery on shore power all night, there
were no observable voltage pulses. So what I saw was the DC > DC charger
yanking on the engine battery to bring the house battery up after a few
nights of self-discharge. The charger can pull as much as 15A, and
appears to have a 900ms/100ms on/off duty cycle, which causes a 300mV
pulse every second, which in turn causes the dash and headlights to
increase in brightness every second. Mystery solved.

I spent the night at a motel a couple years ago and had to keep the
van's electric refrigerator running all night in the parking lot because
it was a warm night and I had stuff in the refrigerator that needed to
be kept cool. I left early the next morning and while driving on the
freeway in the pre-dawn darkness I noticed that the voltage at the dash
was about 12.1 volts. The headlights were none too bright, either.

After some fretting that the alternator might have quit, I realized that
what I was seeing was the result of skinny wire and the combined loads
of the headlights and the DC > DC charger working full-bore to charge
the 30% depleted house battery.

I'm going to do a little wire beefing up here and there to provide these
two heavy loads with their own dedicated 12V lines. I'll take the time
to touch up the grounds, too, even though the large voltage drops tells
me that the current paths to ground are in good condition.

--
Mike "Rocket J Squirrel" Elliott
84 Westfalia: Mellow Yellow ("The Electrical Banana")
74 Westrailia: (Ladybug Trailer company, San Juan Capistrano, Calif.)
Bend, OR
KG6RCR