House battery charging eating fuses

[pawleyk]

Hey guys,

I helped a friend we met on the road with a 1997 VW bus. It's the old style, but water cooled with the same 1.8l from the early 90's Jettas. That's probably not important, but it's a neat rig. Built in Brazil for the Mexican market..

Anyway, he had his house battery wired into his charging system, but wanted some sort of relay. I picked up a Ford starter relay and wired that in, but we burned up two of them in short order. I've done several of these simple systems and never had an issue, so was a little surprised. He picked up a constant duty relay from a golf cart place just to be sure, but he's blowing the fuse between the alternator and the relay now.

Here's how we've got it setup- 4ga cable from the alt to the relay, then from the relay to the house battery. There are 100 amp fuses between each 12v+ source (alt and house battery) and the relay. He's only blowing the one on the alternator side.

Relay is powered off of the 12v+ into the ignition coil so the batteries are connected while the van is running, disconnected when it's off.

The alt is original, but rebuilt and is rated to 90 amps..

Are we missing something here? All the connections are good. We've both checked multiple times.

Thanks!

 

[dwh]

Does the voltage regulator sense voltage at the alternator or somewhere else?

If somewhere else, then you are bypassing the regulator and feeding unregulated alternator power to the aux battery.

Quick Fix: Feed the relay from the main battery instead of directly from the alternator.

 

[dlh62c]

I picked up a Ford starter relay and wired that in, but we burned up two of them in short order.....He picked up a constant duty relay from a golf cart place just to be sure, but he's blowing the fuse between the alternator and the relay now.

Sounds like the constant duty relay is blown as well. That's three. Check for shorts to ground in the cable to the relay. Bypass it all together if you can to see if you blow another one.

 

[dlh62c]

Anyway, he had his house battery wired into his charging system, but wanted some sort of relay.

Is the hardware and wiring to support the above still in place?

Are you utilizing it?

Is it just tied up and hanging some where?

 

[RoyJ]

I'm betting the fuse if getting blown during CRANKING.

If you're relay is wired to energize under ignition, then it's probably energizing during cranking (ignition on).

During engine crank, the starter would be pulling relatively equal amperage from both the starting and house battery. The electrical path would go: house batt - fuse - alternator - starter batt (junction) - starter.

Otherwise, during engine running conditions, the charging current cannot possibly exceed 100 amps, unless you have a very high current alternator and the house batteries are fully depleted.

Perhaps try isolating the house batt during cranking, and see if it still blows?

 

[ckupq]

What else is on this battery. I'm betting its either a bad battery or marginal alternator. If its a bad battery and he's using one alternator to charge both his house battery and the bus battery, the differential could be what is killing the relay and fuse because both are trying to equal out.

FWIW you're still being a little light on the details. What is running off the "house" battery and what is running off the "bus" battery? Are there any high demand items on either that could cause a significant draw down on one?

I think you'll need an isolator over a relay to control current flow. Its probably what's killing your circuit.