I know what you mean. I eyeballed a charger just like that yesterday that
was designed to reduce or stop charging when it saw battery as charged and
would switch on/off as needed. But what that threshold voltage spec is, < ?
>

Thinking now, thought maybe an 87-87a relay could be used but either way I
look at it, the charger and alternator both share a common  +  connection.
i.e.

alternator "off", charger +  --> 30 --> 87a --> aux. battery

alternator "on"   blue wire carries + to relay coil, switches relay to

vehicle B+  --> 30 --> 87 --> starter batter --> aux. battery

If it were me, I'd go with my original thought but use an 87-87a relay on +
wire of the charger. Have it activated by the alternator blue wire. When
powered on, it breaks circuit of charger to aux. battery.

Neil.

On Thu, Sep 20, 2012 at 2:03 PM, Rocket J Squirrel <
camping.elliott@gmail.com> 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.
>
> But there have been a couple times when I've accidentally fired up the van
> with the extension cord connected and Nothing Bad happened, but I think
> that's going to be a function of your AC-powered charger. My guess is that
> with the alternator applying power to the battery, the charger will look at
> that and say, "Uh, I dunno," but not blow up or anything.
>
>
> --
> Jack "Rocket j Squirrel" Elliott
> 1984 Westfalia, auto trans,
> Bend, Ore.
>
> On 09/20/2012 01:37 PM, neil n wrote:
>
>> I hate to ask this in the midst of truly electrically qualified people
>> but.....
>>
>> When van running, you're off shore power? IF so, alternator does it's
>> job, charger not connected.
>>
>> When van not running, you're ready to connect to shore power.
>>
>> Or is there concern that the van might be running while charger still
>> on. IF so....
>>
>> Maybe install a relay between DC charger output and house DC system.
>> Have it trigger via alternator blue wire. Or is there too much lag time?
>>
>> Neil.
>>
>> On Thu, Sep 20, 2012 at 9:51 AM, Rocket J Squirrel
>> <camping.elliott@gmail.com <mailto:camping.elliott@gmail.**com<camping.elliott@gmail.com>>>
>> wrote:
>>
>>     I'm looking at rewiring my house battery charging system.
>>
>>     I want to be able to switch between charging the battery from the
>>     alternator ..... and charging off my
>>     3-stage DC > DC deep cycle charger
>>
>>
>>
>>     To do this, I'm looking for a manual 3-position battery switch. I've
>>     already looked at West Marine's offerings and I can find either
>>     two-position (off/on) switches, or four-position (off/1/2/1+2)
>> switches.
>>     The latter could be used but it's dangerous as the 1+2 position would
>>     connect the DC > DC charger's output to its input, resulting in a
>>     collision of universes and possible smoke.
>>
>>     Two off/on switches could be used, but again, it would be possible to
>>     accidentally turn them both on, resulting in that collision of
>> universes
>>     thing.
>>
>>     Anyone know of a three position (1/2/off) battery switch?
>>
>>     --
>>     Jack "Rocket j Squirrel" Elliott
>>     1984 Westfalia, auto trans,
>>     Bend, Ore.
>>
>>
>>
>>
>> --
>> Neil n
>>
>> 65 kb image Myford Ready For Assembly http://tinyurl.com/64sx4rp
>>
>> '88 Slate Blue Westy to be named.
>>
>> '81 VanaJetta 2.0 "Jaco" http://tubaneil.googlepages.**com/<http://tubaneil.googlepages.com/>
>>
>> Vanagon VAG Gas I4/VR Swap Google Group:
>>
>> http://groups.google.com/**group/vanagons-with-vw-inline-**
>> 4-cylinder-gas-engines<http://groups.google.com/group/vanagons-with-vw-inline-4-cylinder-gas-engines>
>>
>>

--
Neil n

65 kb image Myford Ready For Assembly  http://tinyurl.com/64sx4rp

'88 Slate Blue Westy to be named.

'81 VanaJetta 2.0 "Jaco"  http://tubaneil.googlepages.com/

Vanagon VAG Gas I4/VR Swap Google Group:

http://groups.google.com/group/vanagons-with-vw-inline-4-cylinder-gas-engines