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jillhop's avatar
jillhop
Explorer
Jan 03, 2015

Recharging batteries: genset vs. chassis alternator

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!
  • MrWizard's avatar
    MrWizard
    Moderator
    Jan 03, 2015
    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's avatar
    DrewE
    Explorer II
    Jan 03, 2015
    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's avatar
    KD4UPL
    Explorer
    Jan 03, 2015
    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's avatar
    time2roll
    Nomad
    Jan 03, 2015
    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.
  • Fishinghat's avatar
    Fishinghat
    Explorer II
    Jan 03, 2015
    The alternator on your engine is designed to bring the battery back up to 80% faster than the generator and converter. That said, the alternator on the engine can overheat if you don't run the engine at 1200 rpm or so. It just wasn't designed to recharge an almost dead battery at low rpm.

    The suggestion to use your generator for two hours in the morning and two hours in the evening is a good one. That's what we've done. It won't bring your battery up to a full charge, but good enough. Adding a second battery is also a good suggestion.
  • BFL13's avatar
    BFL13
    Explorer II
    Jan 03, 2015
    Some 1998 MHs have a PD 9100 converter. If your Class C does, then just get a Charge Wizard for it so you get decent amps while charging on Generator.

    Otherwise you need a higher amp portable charger like the VEC1093DBD or a deck mount converter that will do more amps in the 14.4 volt range or better. You can leave the existing 13.6v converter in place either way. Your charging is taking so long now because it only goes to 13.6 instead of 14.4 or higher.

    A second battery would be a good idea too.
  • 2oldman's avatar
    2oldman
    Explorer II
    Jan 03, 2015
    It's difficult for any of us serious boondockers to ever get fully charged batteries. It just takes a long time.

    I'm sure there is someone more familiar with truck alternators to answer, but I'd like to know the amperage rating of your convertor.

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