Recharging batteries: genset vs. chassis alternator

[jillhop]

Please bear with me--I'm new to the RV world (purchased my first RV this past summer). 🙂

I am planning a trip to Wisconsin this coming April where I will be dry camping in a hotel parking lot for 7 nights. This trip is for a dog event, and I will have access to the hotel for showers, etc, but no electrical hookups. The weather averages for the time I will be there is for low in the upper 30's, high in the upper 50's, so I am concerned about recharging my single 85AH battery after using the furnace overnight.

I've been doing some experiments to try to learn what it takes to recharge my battery here in the driveway. After fully charging the battery (12.83V) from shore power, I ran the furnace from 4pm to 8am the next morning. This depleted the battery to about 50% (12.11V). After 24 hours with the battery disconnected (no charging or use) the reading rebounded to 12.25V (about 60%).

I then ran the generator several times, taking readings 24 hours after each "run":

2 hours generator increased battery to 12.35V (70%)

Another 4 1/2 hours brought it up to 12.54V (~85%)

Another 3 1/2 hours brought it up to 12.75V (100% on my chart, but still not back to the original 12.83V charge from shore power).

So, that was 10 hours of generator use to bring the battery back to full charge from one night of furnace use. This was on a colder night (low 20's) than I'm likely to need on my trip, but still...

I'm wondering if running the truck engine (Ford V10) would charge the battery more quickly? I have been doing some research on solar, but I wouldn't be comfortable installing myself, so that would be a pretty big expenditure for me. While I do dry camp regularly, except for this trip doing so for more than 2-3 nights in one spot (without a day of driving in between) isn't likely to happen.

My coach has pretty much original equipment, so just a basic 1998 converter. I know from what I've read that I could benefit from a better converter, and probably adding a second battery would be helpful. But, if I don't have charging capacity to charge the single battery I have, I'm not sure how helpful that would be.

So, does my 7 nights of dry camping sound workable with the equipment I have now? I'm interested in any and all suggestions!

[mpierce]

jillhop wrote:

Is there an advantage to breaking up the 4 hours of generator use into 2 x 2 hour segments?
I did check the voltage on the battery when it was charging, and it was 13.6V.

KD4UPL and BFJ13: you recommended a adding a portable charger. Wouldn't I need to be plugged in to shore power to use one of these?

I did check the voltage on the battery when it was charging, and it was 13.6V.


KD4UPL and BFJ13: you recommended a adding a portable charger. Wouldn't I need to be plugged in to shore power to use one of these?

Yes, two segments are more efficient than one twice as long. The battery will accept more current each time.

13.6. You should have a convertor that is putting out 14.4 or so volts. 13.6 takes forever to fully charge.

No need for a power cord. Run your generator, and plug a GOOD charger/convertor into your camper!

I have a PD 9260 that I use as a portable battery charger. I put #2 cables on, and battery type alligator clamps on them to hook to the battery charger. Works very well.

You say room for only one battery. Any chance you could go to a little larger size? Also, they usually use a starting battery, getting the largest deep cycle you can fit in space will help.

If interested, look at portable solar panels. They come complete, just hook to battery. Example here: http://www.ebay.com/itm/Portable-KITs-100-W-Watt-100W-folding-PV-Solar-Panel-12V-RV-Boat-Off-Grid-2x50W-/231094509371?tfrom=191253885978&tpos=top&ttype=price&talgo=undefined

Then, run generator a couple hours in the AM, to bulk charge faster, and let the solar work all day long to top off. This is best idea. 100w panel for one battery, after bulk charged a couple hours, should top off the battery pretty good.

[pnichols]

We often idle our V10 for awhile before using a small generator plus our 13.8 volt stock converter to charge our AGM batteries. Our little generator is very, very quiet and runs 4-5 hours on 1/2 gallon of gasoline when charging our AGM batteries through the converter.

I agree with MEXICOWANDERER: Switch to an AGM battery. I recommend a Fullriver Group 31 AGM battery - their model DC115-12. Fullriver even recommends that their AGM battery be maintained when not in service using about 13.6 volts, so your stock converter can even maintain it while your RV is parked in storage. Note that an AGM battery can both be maintained and charged from the same low proper voltage because their chemistry makes them accept current better at all voltages.

[MEXICOWANDERER]

The Ford 4G develops 113 amperes @ 105c at a rotor speed of 1, 600 RPM. That is a ratio of 2.6 of crankshaft speed. My custom hydraulically driven 10 HP alternator test bench saw enough of them to become familar with their behavior. An hour's idling could serve to restore approx EIGHTY amp hours to a group 29-31 AGM battery. I fooled around with my LIfeline and the poor 50/120 MoPar Nippondenso was subjected to an 87 ampere charging rate.

A Magnatek can't walk and chew gum. The F105 Ford reg stll bats out 14.05 volts at an incredible 110c. Using the generator in a heads-up contest would be a joke. When the OP has a budget for a converter, then thought should be given to type and size. But a 10, 000 amp converter is only going to stuff around 30 amps into that single flooded battery at 13.6 voltsrating.

[cgmartine]

I was just thinking that if your main use of that single battery is to run the furnace, I would consider firing up the generator during those times. When I had my class c with a single house battery, I used it sparingly. Using it to run the furnace would draw more power than you can spare for other uses. What I did, was to convert the nearest outside storage area to install two six volt T-105 Trojan batteries. You might also try alternating, like I used to, such as running the furnace off the batteries, then running the furnace off the generator, after that, running the furnace off the engine, all in order to keep up the power level of the battery. But my best suggestion would be to just bundle up under some heavy blankets and leave the battery for other things like lights. Good luck.

[time2roll]

If you go AGM think Lifeline not Optima.

[BFL13]

"KD4UPL and BFJ13: you recommended a adding a portable charger. Wouldn't I need to be plugged in to shore power to use one of these?"

The charger has battery clamps and a 120v cord to plug it in. With your generator on, you have your own 120v to your receptacles inside and usually one outside. Clamp the charger to the battery posts and plug it in (extension cord if necessary)

Do replace your battery with an AGM like Mex said. It sounds like you have an ordinary starting battery in there, not even "marine/RV" type.

Here is the charger I meant. It would do well with the AGM too. You should be able to keep your gen time to two hours a day with one of these.

http://www.boatandrvaccessories.com/black--decker-vec1093dbd-12-volt-40-amp-battery-charger.html

[jillhop]

Well, when I bought the RV this past summer, it had what appeared to be the ORIGINAL 1997 coach battery... I purchased a regular flooded lead acid battery, but I might be persuaded to swap it out for an AGM, and/or try to find space for a second battery. Sounds like increased battery capacity would be the necessary first step.

This is really a wonderful forum--you guys are so helpful. I'm enjoying learning about my new toy ...

[Golden_HVAC]

You can run your generator for a few of hours on one gallon of gasoline. The V10 will use a couple of gallons per hour, and running at less than 1,800 RPM, then there is not a lot of oil volume being pumped by the engine, and upper valve area, and cam might not get enough splash lubrication, so that can cause problems.

It is better not to idle a larger engine.

And the "130" amp alternator is 'rated' in a LAB, not on a real engine in the real world, and only while it is cold. Once over 130F (such as under the hood of any vehicle) the rating goes way down, and at idle, the rating falls off to only about 85 amps.

So expect the engine alternator to put out say 100 - 110 amps at 2,000+ RPM, and at normal under the hood temperatures.

What your battery can accept is another matter. Your charger might be rated at say 45 amps, or even 55 or 60 amps. Probably only putting out around 15 amps into the 85 amp hour battery. So all that gas is mostly wasted to charge the battery. If you double the amp hour rating of your battery bank, then you can put back twice as many amp hours per hour of generator run time.

So even with my 4 golf cart batteries, and my 70 amp inverter/charger, hooked up with huge battery cables to the golf cart batteries, my charger is not putting out anything near 50 amps for more than a couple of minutes.

I have a 'E-Meter' and it carefully measures the amps going into and out of my battery bank. So one morning I was at -120 amp hours (watching a lot of TV) and ran the generator at breakfast. I was charging at around 45 amps for the first 10 minutes. After breakfast, it was less than 35 amps (so 30 minutes of charging total time) and I shut off the charger. I was only putting back about 35X14 = 250 watts. Hardly worth the noise and expense to run a 4,000 watt generator.

I let my 400 watt solar system finish the charge, at around 20 - 22 amps per hour until near sunset, when it stops charging (normally full by 3 or 4PM).

You will be much better off charging the battery twice a day. As you have already found out, you get the most amperage from the first hour of charging. Second hour charging is much slower.

Your battery will be depleted by 35 amp hours just to run the CO meter, refrigerator and propane leak detector for 24 hours. Run the 6.5 amp furnace fan for 5 hours, and take away another 32.5 AH. It will take about 90 amp hours to replace that 70 used power. So three times running the generator for about 1.5 hours each day should do the trick.

I would still recommend another battery.

Also a Olympic Catalytic heater. It has a pad to prevent formation of Carbon Monoxide, a killer by-product of burning propane or natural gas (or gas or diesel). You will still need to leave a roof vent open, and window slightly cracked open. Just like while cooking. I have a 6,000 Btu model and it keeps my 30' Bounder with good insulation warm at 30F outside air temps. Below 30, and I still need to run the furnace once in a while to bring that warm air into the basement, and circulate the warm air around the RV!

I have my heater hooked up to a quick disconnect, under my stove, where I installed a 3/8" flare Tee.

Good luck!

Fred.

[time2roll]

If two batteries will not fit consider a larger group 27 or 29 for more power.

Magnetek is a huge roadblock at 13.6 volts. Best to get a new converter but a 25 to 40 amp portable will also serve well.

[MEXICOWANDERER]

The 4G 130-amp alternator will survive quite nicely at idle. It has dual internal fans. Believe it or not the issue here is the battery itself and it's charge acceptance ability. You can kill about 5 birds with one stone. Replace the battery. If a larger battery will fit choose a group 29, 30, 31 which are all the same size. But AGM. Absorbed glass mat.

Without touching anything else you start the engine and presto you almost double the amperage going into the battery from the engine alternator. Next, is the point you can SAFELY Discharge an AGM to 20% of capacity 80% discharged a few dozen times in it's lifetime without impacting it's superior lifespan.

When budget affords it then go for a larger converter but for now it would be danged tough to get what a larger AGM battery offers. It's a fast and easy fix and you compromise nothing after the show is history and you continue RV'ing.

[jillhop]

Thanks for the great info, everyone. I'm heartened that several of you think that I will be able to get by with ~4 hours of generator use per day, and that I shouldn't expect/have to get the battery back to 100% each day.

Is there an advantage to breaking up the 4 hours of generator use into 2 x 2 hour segments?

I certainly won't rely on the engine alternator, but I hadn't thought about the fact that I can run the truck heater to warm up the camper at the same time--that is also an advantage.

I'm not sure exactly which converter I have (it's installed under the dinette, which now is the platform for my dog crates, so not really easy to access unless absolutely necessary). My owner's manual is for MagneTek 900 series, but there are several models listed. I did check the voltage on the battery when it was charging, and it was 13.6V.

There isn't room in the current battery compartment for another battery, without some modifications, so I'm not sure how that would go.

KD4UPL and BFJ13: you recommended a adding a portable charger. Wouldn't I need to be plugged in to shore power to use one of these?

I really appreciate the input. It does sound like you guys think I can make it through my event with what I have, just will require lots of generator use.

[MrWizard]

It's not just capacity to charge..
But it's also about battery accepting the charge
When you get to 80% or above the batteries any battery will take hours to get to 100%
More batteries mean more reserve and less depletion
Even having two would mean starting out around 70% maybe higher and would take about THREE hours to get to 90%
Many people due 50~90% then plug in and charge to full charge
You did not include info on the converter..charger
Please post model number etc..
If your converter is a low power model you can replace it, or buy a portable charger to use
Many of us keep a portable charger for backup use, or extra charging capacity

[DrewE]

I think you'd be best served by installing an upgraded converter/charger. If I understand the nature of battery charging properly and am doing math about right (both of which are questionable assumptions!), you're getting maybe 7-10A charging current out of your current converter/charger...not a whole lot.

The main engine (at idle) might end up being a bit quicker than the existing converter/charger, but it won't be super speedy. The alternator's output is comparatively limited at idle speeds, and some of it is going to power the engine systems like the fuel pump etc., and some more is going to recharge the chassis battery.

[KD4UPL]

I don't know what converter you have but I suspect it's most of your problem. The factory converter in 2003 camper will only put out 10 amps to the battery. It's rated for 45 amps I think supplying lights, pump, etc. but only 10 charging the battery. I don't understand that design but it is what it is. If I camped more often without electric I would upgrade but I don't do that enough to justify and I have 100 watts of solar anyway.
You're converter, being even older, is likely no better. A good aftermarket stand alone battery charger would probably help things considerably. Or, upgrade your converter.

[time2roll]

You need to find space for 1 or 2 more batteries for best results.
Was the battery charging at 14.2+ volts? or some trickle charge at ~13.6?

200 watts of solar would make a huge difference.