From VanagonWiki

[edit] Battery "Isolator" or Battery "Separator"

(Courtesy someone too modest to request a byline)

The second (house or auxilliary) battery is there to run our conveniences while camping, but must be recharged when we are driving the van. So it must be connected to the engine's alternator to receive the recharging voltage. But, if the batteries are connected permanently, then two things would happen:

• When the auxiliary battery was run down, the main battery would be depleted also.
• When starting the van, if the two batteries were connected by a less-than-jumper-cable-size wire, then smoke and flame would occur when the starter started demanding hundreds of amps from the connected power sources.

So we need something that connects the batteries when the alternator is producing a battery-charging voltage, and disconnects the batteries when it is not. The device that I used is called a Battery Separator or Battery Isolator. Semantics could keep us up all night, but what is important is that a device exists that can keep the two batteries separated (or one battery isolated) while not driving, but join them together when driving^[1].

I used the Sure Power Model 1315 battery separator. It has features beyond simply keeping the auxiliary battery isolated while stopped. It can be used to help the main battery start the van if the main battery is dead. I did not implement this feature because I carry jumper cables and will utilize the second battery if needed.

In this application the 1315 uses three connectors. One goes to the main battery, so it can measure the charging voltage and connect the batteries when charging is occurring. The second goes to the auxiliary battery to receive the charging voltage. The third connection is to ground.

In the picture you'll see that the 1315 device is mounted to the battery compartment lid. This seems to be rather handy, since there is not a lot of room to mount it any other way, short of cramming it down into the space beside the battery. Sure Power technical support assured me there is not a problem with mounting the device in an inverted position. Anybody mounting the device in this position should make sure the wires' insulation is pushed well inside the insulated part of the fork or ring lug connectors. This helps keep the copper from flexing so much when the cover is closed.

Notes on the Sure Power 1315:

1. The main battery and aux battery are kept separated until the alternator is producing a voltage high enough to be considered a "charging voltage". To the 1315 this is around 13 to 13.5 volts. When this voltage is reached the 1315 energizes its internal relay to join the two batteries. Now both batteries see the charging voltage. When the 1315 sees less than about 12.5 volts, the 1315 de-energizes the relay. In the original VW configuration the alternator puts out a signal (blue wire) that says "I'm charging the battery now", and that energized the relay that joined the primary and secondary batteries.
2. Sure Power says to use 8 gauge wire for a short run from the main battery. For sure the existing VW wire is too small. Under a significant charging load the 1315's relay can be heard clicking in and dropping out as the voltage drop across the skinny wire reduces the voltage down to the de-energize voltage. It clicks back in when the load is dropped and the voltage goes back above 13 volts.
3. On installation, when the ground is connected to the 1315, it will energize briefly, then drop out after about five seconds. This is normal and is mentioned in the installation instructions.