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Randall, RVIA Certification
Category: RV
Satisfied Customers: 10273
Experience:  RVDA/RVIA Master Certified Technician with many years experience.
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I have an Onan 5500 carbureted genset in my 5th wheel. It

Customer Question

I have an Onan 5500 carbureted genset in my 5th wheel. It has only 100 or so hours on it and worked flawlessly until now. The coach has two batteries installed, one that runs the coach 12v systems which is charged by either shore power or generator power through the converter and all is working fine, the battery maintains 12.8 ish volts.
The other battery, bought new in 10/15 and tested yesterday at 12.8 volts with 560 cca seems not to be receiving a charge is totally dedicated to the genset. I believe the genset needs a12v supply to keep it running, I guess to power the controller logic. As the genset runs over time the battery charge depletes until the unit quits.
My question is does the Onan have an alternator, voltage regulator or how does the battery get its charge? With the unit running a test meter across the leads shows 12v only, I was expecting 13.2 +. What can I do to troubleshoot this issue and fix it? Will the genset have to be removed?
Submitted: 2 months ago.
Category: RV
Expert:  Randall replied 2 months ago.

BOTH batteries need to be getting a charge from the converter to stay charged. Fix that and you should be fine. These batteries would need to be connected in parallel. Positive of one to the positive of the other and the same with the negative leads OR have charging leads come off the converter to BOTH batteries. Hope that clears that up for you. Please get back to me later if you need more as all follow ups are included in our fee.

Customer: replied 2 months ago.
Read my question again the genset battery is totally isolated from the rest of the coach and has been that way since 2013. This battery is charged from the genset itself. I believe through a voltage regulator.
Expert:  Randall replied 2 months ago.

I read your question and fully understand what it says. I can tell you that it is NOT being charged up separately from the generator. That set does not put out 12 VDC for charging. If you would like another opinion on that let me know.

Customer: replied 2 months ago.
I downloaded a service manual that talks about an optional Regulator Controller. What is that for? Again this battery is not and never has been connected to the coach 12v system that way if the clack battery dies then you can start the generator to charge it. The coach battery does charge through the converter which is working fine.
Expert:  Randall replied 2 months ago.

There may be an aftermarket product that does this for some applications, but I have not heard of any used in RV situations. It would be easy enough to add one if you have a 120 VAC outlet close to the genset or add one. That might be more practical than adding charge lines depending on the distance between batteries. A trickle charger would work or even a small solar panel.

Customer: replied 2 months ago.
It's not aftermarket its in the Onan Service Manual.It's clear you don't know the answer please refund my $37
Expert:  Randall replied 2 months ago.

I was referring to the regulator you mentioned...........not the manual. I do know this set an know it won't charge a battery. No problem on the refund. Just take it into a shop and have them fix this for you.

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