Hello All,I have a hypothetical question concerning DC electrical power in a diesel pusher RV. Our rig is a 96 Dynasty with a mechanical Cummins C8.3 (96 was supposedly the last year for the mechanical C8.3, then they went to computer controlled). Now what if the engines alternator failed. First you get a warning on your dash, most likely the "ALT" light would come on, or you "VOLT" light, then you would pull over and try to figure out what's causing the problem. So, let's say that you conclude that the alternator has failed, and here's where the hypothetical part comes in. When I run the Onan generator AC is supplied for the coaches AC needs, and some of that AC is converted into DC and fed into the DC system for charging the battery, running the lights, etc. So, would the Onan generator supply the DC voltage I needed to limp home until I could get the engine alternator repaired? The engines DC needs are relatively small as opposed to a gas engine, and I figured with daytime driving only it might actually work. Any inputs are much appreciated. - Tom

I've read where people have actually done this to get back home. You just have to use the genset to charge the chassis battery. I'm not sure how you can safely do that though--someone with better understanding of the two systems may be able to supply the procedure.

When I had my Triple E motorhome is was equipped with a Bi-Directional relay in the charging circuit. Which ever side had available charging voltage would send the power through the relay contacts to the other side.All this worked very well until the day I pulled up to the ferry terminal knowing gas was cheaper on the other side. It was a wait, and I did not open the door so the head light alarm never came on. Blah, withing an hour, the chassis battery was dead. I started the generator, and blah blah, I was so low on gas, it would not stay running. Long story was, got going and onto the ferry just in time with help from the ferry station jump start, kept the engine running for a few minutes, and all was well again.

On mine the generator supplies both battery systems.Might find a friend with a meter to verify on yours.

Yes if your house system charged by the generator also maintains the chassis battery with enough amps it will work.Two summers ago I drove several hours with the ALT light on and drooping voltmeter as I knew my solar powered trailer will push some power forward to my truck. Made for a 2.5 hour lunch stop on schedule vs. pulling over or getting stuck waiting for a tow. Still everyone was booked so I did the repair in the O'Reily's parking lot in Gillette WY.

Absolutely it will work. You do need to verify that your engine batteries are getting the voltage in that situation,. Some rv's have a feature to not allow both banks together if the engine is running and the generator is running. If the alternator is out. It still may try to disable the combining function so you would need to verify they are getting the charge. A simple jumper around the solenoid that controls that function or one car jumper cable will work but not as safely. You can either find out how to activate that solenoid mentioned or just take the wires off and bolt together or stack them up on one post if there is room. Be sure to take pictures first.

Hypothetical DC electrical Question | Good Sam Community

Kayteg1
Explorer II
Jul 14, 2016
Let's just say battery combiner will do it automatically and you might not even notice.
Older systems with insulator will require jumper like MrW did as the emergency relay is not 100 percent duty and you can't use it for long time.
Regardless generator, battery alone should allow for couple of hr of driving if you turn off unnecessary items.
When alternator failed on my Powerstroke, I drove the pickup 140 miles till I found a place with new alternators. Dual batteries still started the engine after alternator replacement.
big_jim_2
Explorer II
Jul 14, 2016
If you have voltage gauges on both sides measure voltage with generator off than measure with generator running. Make sure you wait long enough for generator to pick up the load. Two minutes should be long enough.
wa8yxm
Explorer III
Jul 12, 2016
Two answers depending on your RV

1: Many older RV's (And some newer ones) have a simple "one way" isolator.
In this type of system when you are driving the Engine charges the house batteries but when you park, the engine battery is NOT charged by shore/generator power

If you have that type of system the answer is NO, but (you can add a single jumper wire to do the job in many cases)

2: Many modern Motor homes (Last decade or so) have a BI-Directional isolator like Mine has

In this case, if EITHER engine alternator or house converter is active, ALL batteries get charged..

And the answer is YES.
YC_1
Nomad
Jul 12, 2016
OLYLEN wrote:
Just use jumper cables from the Coach to chassis batteries and you could go a long way. or use a cloths pin to keep the emergency start switch active. If a converter makes 12 volt supply from 120 and an inverter makes 120 from 12 volts why is a inverter/converter not the same as an inverter/charger.

LEN

Symantics I often use the term charger to make it easier for the target audience to understand things.
MrWizard
Moderator
Jul 12, 2016
My OEM chassis and house battery location is the same tray
I had an alternator failure four years ago during a shake down trip to grand canyon, after an engine repair in Flagstaff
I pulled over and put a jumper cable from house positive to engine positive, and then started the Oman
And continued on our way back to flagstaff and had the alternator replaced
bluwtr49
Explorer II
Jul 12, 2016
A lot of considerations that are coach dependent but many are the same in basic operation. The alternator charges both the house and chassis by means of a bi-directional relay or something similar, Shore power charges the house batteries via the inverter or converter. The chassis batteries are kept topped off by either an echo charger or something similar. Regardless of the specifics there is generally some mechanism to keep both battery banks charged from both the alternator or shore power.

If the alternatro fails than running the gen set will merely substiute for shore power and keep both bank charged. If one does not have an echo charger or such it's a simple matter to add on.

A last ditch effort is to stick a toothpick into the battery parallel switch to keep it engaged.....been there done that..and it works well.
OLYLEN
Explorer
Jul 12, 2016
Just use jumper cables from the Coach to chassis batteries and you could go a long way. or use a cloths pin to keep the emergency start switch active. If a converter makes 12 volt supply from 120 and an inverter makes 120 from 12 volts why is a inverter/converter not the same as an inverter/charger.

LEN
TominTampa
Explorer
Jul 11, 2016
Thanks so much for all the replies! This has been a very lively conversation! - Tom
Bill_Satellite
Explorer II
Jul 11, 2016
Onboard converter? No, there are no converters in many (most?) Class A motorhomes (There are inverter/chargers in many (most). SOME Class A RV's charge the engine and house batteries via generator but "most" do not. Despite that limitation, the DP does not use a lot of battery capacity to keep the engine running (unlike a gas/spark ignition) and allows dramatically longer travel times if the alternator should fail.
YC_1
Nomad
Jul 11, 2016
Any decent onboard converter would have no problem keeping up with headlights, radios, etc. The only thing when running the generator as I mentioned earlier is you need to use a voltmeter to see if the engine batteries are getting the voltage. You cannot assume the generator running will charge the engine batteries. These things are snow flakes and there are thousands of differences in setups.

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