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Dual Battery Relay Install

Relay Install Instructions
From: Harold Rust


Hi Volks, How to install the battery cut out relay. Second part. I already posted a list of: Items you might need for this installation. I posted this on Friday, January 29, 1999 at 10:49 am under the title "Re: Flash Alert! Dual Battery Relay".

To be on the safe side, you can purchase a relay for 2 batteries from the VW dealer. It would be the most proven to use with the trigger wire. VW uses such a relay in some vanagons. It costs $62 This stronger relay was not factory installed in the westy camper with the 3 way refrigerator.

This is how I would install the aux. batt. charging circuit, if I would use a relay. Of course, there are alternative options. Note: Do not use a relay, that would overload the two trigger wires. The relay should be just the right size for the application. Not too small and not too big. It should be able to handle 30 to 40 amps continuously, and not overheat.

This is how the new battery-cut-out-relay should work. 1) Engine running: Both batteries are connected. Aux. batt. and main batt. are charging. 2) Engine turned off: The aux. batt. is disconnected from the main battery. 3) Ignition turned on: The two batteries are still disconnected. 4) Starting the engine: The two batteries are disconnected.

Nr. 4 is important, since the system discussed is only designed to charge the aux. battery, and not for starting the motor. VW installed a relay trigger circuit in the camper. (Trigger circuit means, it switches relays on and off). One can use this circuit for the new batt.-cut-out-relay. This will ensure, the batteries are not connected, while the engine is started. It is located at terminals 85 and 86 on the existing refrigerator relay, under the front seat in the westy camper. The existing refrigerator-12-volt-heater-relay stays as is. We only tap into the two small wires, (terminals 85 and 86) to also control the new batt.-cut-out-relay. To alter the fridge relay could drain the battery very fast, if the engine is not running.

Summary: You have to route a new #8 wire from the + on the main battery to a breaker, then to a batt.-cut-out-relay, and then to the + terminal on the auxiliary battery. The new batt.-cut-out-relay is triggered by the two small wires, on terminals 85 and 86 on the factory installed refrigerator cut out relay.

The installation: 1) Take out the front seats, and gain access to the front batt. boxes. One on each side. Test fit the aux. batt. in its box. If you use a Optima, the terminals should be to the back. You should have enough space on the right side of the aux. batt., to install the new cut-out-relay, and the breaker or fuse. Also mark where to fasten the batt. ground strap to the frame, and mark a place, to drill a hole for the #8 wire. Note the fuel lines, AC hoses etc under the batt. boxes. Take your time.

2) Take aux batt. out and drill the hole for the #8 wire. Remember to use a grommet where it passes through the sheet metal. Mount the cut-out-relay and the fuse on the right side in the aux. batt. box. I suggest you install a piece of sheet plastic between the new relay/breaker, and the battery box, so the wires later don't rub against the metal. Use the sheet metal screws for fastening. Install the ground strap to the frame in the batt. box.

3)Cut two pieces of # 18 ga wire, and crimp one female connector to each and connect them to the trigger circuit on the batt.-cut-out-relay. Note: The new relay should have 4 terminals. The smaller 2 terminals should be for the trigger. (one of them might be marked as ground, do NOT ground) Find the 2 small wires that go to terminals 85 and 86 on the refrigerator-cut-out-relay. Look in the Bentley for the colors of these wires. In the 90 westy they are 1) blue 2) red/black Use two quick-splice-connectors to connect them to the trigger terminals on the new cut-out relay.

4)Use the ring terminals to connect the #8 wire. As the wire enters the aux. batt. box, first connect it to one terminal on the 30 amp breaker. Cut a short section, to connect the other terminal on the breaker to one of the big terminals on the new batt.-cut-out-relay. Now connect a long enough section of #8 to the remaining big terminal on the batt.-cut-out-relay. This end goes to the + terminal on the aux. battery. But for now, wrap the end in some plastic, so it does not make ground contact. Make sure the connections on the new relay and breaker don't make any ground contact, and are safe. Use tape, shrink tubing etc. to protect them. Route the #8 wire under the car to the main battery box. Use cable ties to secure it, and make sure there are no sharp objects near the wire. Preferably use a conduit, to give it some added protection. Give the wire lots of slack.

5) Find or make a opening in the main batt. box, to enter the other end of the # 8 wire. Again use a grommet. As an option, you can install a second 30 or 40 amp breaker with the # 8 wire next to the main battery. To reinstall the main battery: Important. Make sure the loose end in the aux. batt. box does not make ground contact. (wrap with insulating tape, or a plastic bag). Now reinstall the main battery, and connect the new #8 wire to the + terminal on the main batt. Leave the ground off, till you are finished on the other side.

6)Install the auxiliary battery. Unwrap the #8 wire, the one that comes from the cut-out-relay, and connect it to the + terminal on the aux. battery. Now connect the ground strap to the - (minus) terminal on the auxiliary battery.

Go to the other side of the car, and connect the ground on the main battery.

To test the operation of the relay: Disconnect the new #8 wire at the main battery + terminal. With the engine off, there should be no current and no ground. Reconnect the #8 wire. On the aux. battery: With the engine running, the voltage should be higher. (battery is charging) Engine off, voltage drops.

Note: The auxiliary battery compartment in the westfalia camper is not vented. One should use a sealed battery to prevent the build up of fumes. A deep cycle battery will last longer and give better service.

Note: Disclaimer: You should double-check everything with a professional auto electrician. I might have made a mistake. In my own westy, I installed a battery isolator, so I have no long-term experience with the relay setup. But the relay installation is fairly simple and straightforward, and VW installed such a system in some vanagons. Maybe some other list members can come up with alternative setups. It would give westy owners more choices for their installation.

Good luck. Harald 90 westy




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