"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)."

Almost. I am using a smart charger but it takes 12VDC from the
alternator as input, not 120VAC from an inverter, though that would
work, too. See <http://www.powerstream.com/DCC.htm>

--
Jack "Rocket j Squirrel" Elliott
1984 Westfalia, auto trans,
Bend, Ore.

On 09/20/2012 03:43 PM, David Beierl wrote:
> 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