At 05:03 PM 9/20/2012, Rocket J Squirrel wrote:
>I have a little "smart charger" in the van which trickles the house
>battery when I connect the side inlet to household AC power. But I
>unplug the extension cord before starting the van, and turn off the van
>before plugging in the extension cord.

Oh.  I thought you were talking about running down the road using an
inverter to power a smart charger, and manually switching to it after
current monitoring to the house battery showed that bulk charge from
the engine alternator had diminished below the output of the
AC-powered charger (which would then go through its own
bulk-finish-float sequence).

If you're just concerned about paralleling the alternator and your
AC-powered charger when the engine starts and automagically parallels
your two batteries, I think you can probably stop worrying.  The AC
charger's bulk cutoff voltage will be higher than the alternator
regulator setting, so it will continue charging until you unplug
it.  If the terminal voltage of the combined batteries rises above
the alternator regulator setting, the alternator will reduce its
field current or shut off entirely.*

* A while back I did a little bench testing of one of the regulator
units which indicated (to my surprise) that the regulator simply
switches the field current on and off rather than modulating
it.  This keeps the regulator cool but generates a lot of hash on the
B+ line (and I suspect is not ideal for battery longevity either).

However, to be certain about this, ask the charger mfr. about it.  If
the charger has already shifted into float mode then the alternator
might try to back-feed it, which it might or might not care about.

If it's an issue, I suggest using a fancier (spdt/dpdt) relay on your
paralleling arrangement.  Common terminal to the house battery, NC to
the AC charger, NO to the starter battery.  Bypass diode between NC
and common, in case unhooking the powered charger generates a spike
(many chargers specify to connect the battery before plugging in the charger).

Yrs,
d