Hypothetical DC electrical Question

[TominTampa]

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

[Kayteg1]

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]

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]

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]

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]

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]

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]

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]

Thanks so much for all the replies! This has been a very lively conversation! - Tom

[Bill_Satellite]

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]

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.

[Bill_Satellite]

A DP uses very little battery power once started so you likely can drive for quite a distance without the alternator. I made it from Ontario, CA to Las Vegas. You just don't want to turn on your headlights and you want to turn off all other 12V devices that you can.
As mentioned above, "if" the generator powers some source that charges your engine batteries (mine does not) you likely will be just fine but still want to try to reduce as many other 12V draws as possible as the charging source is likely not going to put out as many amps as your engine alternator would.

[gatorcq]

My rig has he ability to charge ALL batteries whether on Shore power, generator or alternator.

Now in this day and age, you can buy a unit that will automatically charge the engine to coach. It is a microprocessor that is connected between the engine and coach Via 50 to 100 amp cables. It senses which battery is above 13 Vdc and allows the transfer of voltage.
If this is an emergency, jumper cables IF your coach battery is very close to your engine battery

[wolfe10]

As already posted, it MAY work just fine.

Yes, absolutely, your converter, charger or inverter/charger will charge the HOUSE BATTERIES. On SOME COACHES, it also charges the CHASSIS BATTERIES-- by no means on all.

Some coaches have a "boost/combine" switch that closes a relay/solenoid so that the two banks are combined. May be an intermittent duty solenoid or constant duty one.

Some coaches have a low amp combiner (Xantrex Echo Charger for example), but likely not sufficient to keep up a chassis battery with loads like fans, lights, etc.

BUT, you can manually combine your banks. Whether at a battery isolator, battery boost/combiner solenoid or other place where you have large-gauge battery cables from both battery banks meeting at a common place. Merely take the large wire off one side and ADD IT to the other side. Presto, both banks being charged by your converter, charger or inverter/charger.

Amp output of the charging device needs to be considered. Said another way, a 30 amp converter is NOT the same as a 2000 watt inverter with 100 amp charging capability.

[bdpreece]

I would not take a chance that the on board systems would automatically supply charging voltage to the engine batteries. Some only do so after they sense that the house batteries are at full charge. What I have done in the past is just carry a set of jumper cables. I start the generator then jumper from the house battery plus post to the engine battery plus post. No need to jumper the negative since they share a common ground. Over the years this has gotten me home more than once.