Just recently the starting system on my generator began to act up… Sometimes it would spin just as it should… Other times I would just get a click and maybe a half a turn on the crank… I've been investigating the electrical system and found that the solenoid type battery isolator in my coach typically doesn't last very long… A few years or so…I'm thinking the isolator that's in there now is the original from the manufacturer... My question is… On the dash of my motorhome is an auxiliary start switch which I assume ties the two batteries together when activated to assistant starting the coach engine… Do I need a special isolator to keep this feature intact or will any three post battery isolator suffice...so long as the amperage rating is correct... I'm planning on cleaning all the battery connections and running a separate ground to the generator from the coach frame… Hopefully that will keep the generator starter happy…

You should look in your battery control center.On my rig, the charge relay and the auxiliary start are the same relay.The switch on the dash should be connected to the same terminal that comes from the circuit board in the battery control center. The other side of the switch should be hot from the battery control center.I went through a couple relays before I was able to seal a new one up. I got the current one from NAPA many years back. The small terminals corrode inside the relay causing it not to energize. Make sure you get a good continuous duty relay of at least 100 amp rating.I used standard silicone between the two parts of the metal can to keep moisture out.Your generator issue is most likely a corroded connection somewhere in the circuit. It could be a ground connection or the positive. Use an Ohm meter to check each connection. Use a voltmeter to check for voltage loss.

KnowNuthin wrote:Just recently the starting system on my generator began to act up… Sometimes it would spin just as it should… Other times I would just get a click and maybe a half a turn on the crank… I've been investigating the electrical system and found that the solenoid type battery isolator in my coach typically doesn't last very long… A few years or so…I'm thinking the isolator that's in there now is the original from the manufacturer... My question is… On the dash of my motorhome is an auxiliary start switch which I assume ties the two batteries together when activated to assistant starting the coach engine… Do I need a special isolator to keep this feature intact or will any three post battery isolator suffice...so long as the amperage rating is correct... I'm planning on cleaning all the battery connections and running a separate ground to the generator from the coach frame… Hopefully that will keep the generator starter happy…The relay (solenoid) based isolator is necessary for the aux/start function to work properly. Removing (or not replacing) that relay will result in NO aux/start function and NO charging of the batteries from alternate sources. (i.e. no house bat charge from alternator and no chassis bat charge from converter.)There are other reasons that you don't want the diode based isolator, (three post isolator)also. The main reason they are no longer used in large motorhome applications is the inherent voltage loss through the diodes. Your batteries may never receive a proper full charge.I'd stick with the solenoid based isolator. JMHO

In many factory installations Two functions are requested of the "boost" relay In this application it needs to be a 100% Duty Cycle rated unit with proper current rating. The cheap ones die quickly get a good one. Be for warned the coil current of many of the higher current rated units have excessive as in 2amps or more coli current demands. 1 - The ignition key activates the "boost" relay which connects the alternator to the house battery bank enabling the engine to charge the house batteries 2- The dash mounted momentary contact "Boost" switch lets the house batteries help start the engine if the chassis battery is low

This is not entirely accurate. Many RV manufacturers continue to use "diode" based isolator, but yes, they do have a significant voltage drop. The "diode" style isolators are being obsoleted by MOSFET-based isolators where the MOSFETS (semiconductors) behave just like large, high current diodes, but have an insignificant voltage drop my magnitudes of efficiency and some high end motor coach manufactures are migrating to this never technology. There are several companies that offer MOSFET based isolator, but the best I could find was Perfect Switch in California. No affiliation but a super-please customer, FWIWOnaQuest wrote:KnowNuthin wrote:Just recently the starting system on my generator began to act up… Sometimes it would spin just as it should… Other times I would just get a click and maybe a half a turn on the crank… I've been investigating the electrical system and found that the solenoid type battery isolator in my coach typically doesn't last very long… A few years or so…I'm thinking the isolator that's in there now is the original from the manufacturer... My question is… On the dash of my motorhome is an auxiliary start switch which I assume ties the two batteries together when activated to assistant starting the coach engine… Do I need a special isolator to keep this feature intact or will any three post battery isolator suffice...so long as the amperage rating is correct... I'm planning on cleaning all the battery connections and running a separate ground to the generator from the coach frame… Hopefully that will keep the generator starter happy…There are other reasons that you don't want the diode based isolator, (three post isolator)also. The main reason they are no longer used in large motorhome applications is the inherent voltage loss through the diodes. Your batteries may never receive a proper full charge.I'd stick with the solenoid based isolator. JMHO

Battery isolator | Good Sam Community

13 Replies

OnaQuest
Explorer
Aug 26, 2015
KnowNuthin wrote:
Just recently the starting system on my generator began to act up… Sometimes it would spin just as it should… Other times I would just get a click and maybe a half a turn on the crank…

I've been investigating the electrical system and found that the solenoid type battery isolator in my coach typically doesn't last very long… A few years or so…I'm thinking the isolator that's in there now is the original from the manufacturer...

My question is… On the dash of my motorhome is an auxiliary start switch which I assume ties the two batteries together when activated to assistant starting the coach engine… Do I need a special isolator to keep this feature intact or will any three post battery isolator suffice...so long as the amperage rating is correct...

I'm planning on cleaning all the battery connections and running a separate ground to the generator from the coach frame… Hopefully that will keep the generator starter happy…

The relay (solenoid) based isolator is necessary for the aux/start function to work properly. Removing (or not replacing) that relay will result in NO aux/start function and NO charging of the batteries from alternate sources. (i.e. no house bat charge from alternator and no chassis bat charge from converter.)

There are other reasons that you don't want the diode based isolator, (three post isolator)also. The main reason they are no longer used in large motorhome applications is the inherent voltage loss through the diodes. Your batteries may never receive a proper full charge.

I'd stick with the solenoid based isolator. JMHO
enblethen
Nomad
Aug 25, 2015
You should look in your battery control center.
On my rig, the charge relay and the auxiliary start are the same relay.
The switch on the dash should be connected to the same terminal that comes from the circuit board in the battery control center. The other side of the switch should be hot from the battery control center.
I went through a couple relays before I was able to seal a new one up. I got the current one from NAPA many years back. The small terminals corrode inside the relay causing it not to energize. Make sure you get a good continuous duty relay of at least 100 amp rating.
I used standard silicone between the two parts of the metal can to keep moisture out.
Your generator issue is most likely a corroded connection somewhere in the circuit. It could be a ground connection or the positive. Use an Ohm meter to check each connection. Use a voltmeter to check for voltage loss.
MrWizard
Moderator
Aug 25, 2015
moved from technology corner

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Battery isolator

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