Battery Charging Question

[micpib1]

I have been running my Cummins/Onan 4000 generator to charge the batteries on my coach a couple of hours a day when boondocking; is there any advantage to using the motor on my motorhome to do the same type of charging? Is there any fuel difference in an idling V8 vs. a twin cylinder running at 3600 RPM? JH

[pnichols]

Some of the above discussion is the opposite of how we have been camping with our Class C motorhome now for about 9 years.

I just traded in a set of two 12 volt 100 amp hour Group 29 AGM deep cycle batteries that were around 8 1/2 years old and still had life left in them. I replaced them a couple of weeks ago with a couple of 12 volt 115 amp hour Group 31 AGM deep cycle batteries.

My coach batteries see weeks and weeks on the stock single stage converter at home for storage maintenance (the specified float voltage for the batteries is 13.6 - 13.8 volts). When camping, the batteries are charged for a few hours by the converter powered by the Onan or a small portable generator we carry along. Sometimes we idle the V10 first for up to an hour to hit the batteries with 14.XX volts and dump 25-35 amp hours into them before shutting down the V10 - and then finish bringing the batteries up to around 90% state of charge with the converter powered by the Onan or portable generator. We can do this several times on a long dry camping trip without battery damage because between dry campsites we may drive for several hours, during which the alternator will completely charge the batteries. I monitor all this with a coach battery voltmeter and ammeter mounted right on the cab dash. Amps going into the coach batteries from the alternator tapers to zero after around 4-5 hours of driving.

Our 130 amp alternator can deliver 60-70 amps with the V10 at idle, per the alternator's performance curve as published by Ford ... so it can deliver 30-35 amps into the coach batteries easily during about 1 hour of idling before I switch over to Onan or portable generator charging via the 13.6 volt stock converter. Nowhere on the 130 amp alternator's performance curve or footnotes does it say anything about the alternator not being rated for continuous use. Ford V10 powered vehicles sometimes idle the engine for long periods at incidence sites. Folks with scan gauges in these forums have reported the V10 as consuming about 0.7 gallons per hour when idling. It costs a little more, but I use only Mobil 1 EP (Extended Performance) synthetic oil in the V10.

Per the 4000 watt Onan specifications, I estimate it consumes only about 1 gallon of gasoline per 2 1/2 hours of runtime when powering the coach's air conditioner - so it consumes even less fuel when merely charging the coach batteries through the converter. Even though the Onan runs at a constant RPM, it's rate of fuel consumption depends primarily on the load on it, so this has to be taken into account when viewing it's fuel consumption rate as shown in it's specification - which is at a specific load. An RV converter charging coach batteries at 30-40 amps at 13.6 volts is only around a 1/5 load on a 4000 watt Onan. Using a multi-stage charger - which puts 14.XX volts on the batteries during boost stage charging - loads the Onan only slightly more. I use Mobil 1 EP synthetic oil in the Onan. When using the portable generator to power the converter for battery charging it consumes about 1/2 gallon of gasoline every 5 hours. The oil in it is Mobil 1 synthetic also.

The above procedures work fine for our style of a few days here, and few days there, of drycamping. It works under any condtions - no solar panels or sun required - shade or clouds is just fine. However, for extended drycamping days or weeks on end out in the sun - solar should be added into the mix ... although even then I would also make sure the RV had good generator powered battery charging capability on board because I no longer expect Mother Nature to act as she used to.

[time2roll]

Not the engine nor Onan charge the battery. Charging effectively will be the converter vs the alternator. I would put more faith into the converter if it is a proper three stage model.

[wa8yxm]

micpib1 wrote:
I have been running my Cummins/Onan 4000 generator to charge the batteries on my coach a couple of hours a day when boondocking; is there any advantage to using the motor on my motorhome to do the same type of charging? Is there any fuel difference in an idling V8 vs. a twin cylinder running at 3600 RPM? JH

Any advantage.. NONE at all

Any fuel difference Big honking V-8 at idle sucks about the same as Twin Lung ONAN at full load.. So yes. Big difference,, Way more gas.

[pianotuna]

add a solar system and generate no more.

[micpib1]

Thank you, that helps. JH

[captbru]

Plan to do dry camping three or four days at a time in NF, no hookups.
Have Onan 4000 gen, plan to run the generator a few hours in the morn and evening. Just plan on using existing house batteries, as we won't be doing a lot of dry camping.

My question is should I turn off the chassis battery while dry camping?

[Matt_Colie]

Opps

[Matt_Colie]

Micpib,

Just for starters, there is nothing good that happens to an idling engine. So, cross that one off now. You might also consider that the main engine's alternator will not put out rated load at idle. It can also abuse the house bank as that was never a consideration in the design.

Now, Forester was smart enough to include a Progressive Dynamics converter/charger. They don't get any better than that (there are others as good). That is what the APU (genset) will use to charge the house bank. It will never abuse the house bank. It should be a XX60 something. That makes it good for a 60amp charge if the house bank can take it.

It will still take a couple of hours to get a 50% bank back to 80%, but that is the nature of lead/acid batteries and has been that way with little change for over 100 years.

Before the depression, I did a lot of work on other people's expensive boats. This was a common issue and there are some small thing that can be done about it, but not very much.

Matt

[edatlanta]

Remember the best thing you can do to your generator besides regular maintenance is to run it. Generators are designed to run hard and long.

[Golden_HVAC]

Another thing to consider. Your RV will consume about 35 amp hours just to run the propane, CO meters, and refrigerator each day. That is about what one of my 120 watt solar panels can produce, or what the generator can recharge into the battery bank in about 1 hour.

So save the fuel, and install a pair of solar panels.!! You will be really happy with them once they are up and running - silently.

And my solar system saved the batteries in my RV. After 12 years, I replaced my original battery bank. I was also able to give them to a friend, as they still had a lot of life in them!

Also I consider it this way. The $3000 that I spent on the solar system (back in 94 - 99, when solar was much more expensive than today) I saved by saving $10 a night and dry camping instead of staying in a crowded RV park. Over 300 nights between 94 and 2005. From 2006 - 2008, I spent more than 500 nights dry camping! Paying for the system twice more!

Fred.

[Golden_HVAC]

The Onan can not use more than about 1 gallon per hour, under full load.

The V8 can use 3 gallons per hour, doing about the same thing. However you would be really loading up the 100 amp alternator, and not doing the front bearing or alternator any good to load it up that way. Every new alternator comes with a stern warning "Charge the battery before starting the engine, or warranty is void!" This is because charging a dead battery with a alternator is hard on the windings, and tends to overheat them. . .

The generator is designed for working hard, and is producing 120 volts, at up to 33 amps, while the alternator is producing 100 (+/-) amps at only 12 volts, that requires much larger wires and produces much more heat.

You are better off using the noisy Onan. Unless you are going to be out sightseeing anyway, when the engine will be run anyway, and will charge the battery fairly well on a 2 hour drive.

Best solution? Run a silent 140 watt solar panel. $229 at SunElec.com Much more quiet than the Onan, and less fuel used too! Only about 20 pounds, so lighter than an extra pair of batteries, and will last over 30 years.

My 1994 solar panels are still working great, some 21 years later! And I expect them to last another 20+ years, until I am well past 70 years old, and no longer can RV.. . .

Fred.