Forum Discussion

oyly's avatar
oyly
Explorer
May 11, 2016

Fridge on 12V killing the batteries

Just came back from the shop where I got my new batteries after I toasted my first set. I got them replaced 5 months ago, and already they were performing poorly, even thought they were supposed to be real good quality deep-cycle batteries. So I just wanted them to put their battery tester on them to confirm my suspicions, and sure enough, they were no good. Very generously, they replaced them under warranty, even though I most likely damaged them with improper use. When talking to the staff, they asked me about my refridgerator. I said it runs three ways, on LP, 120V and 12V. They said I should never run it on 12V since this will damage the batteries. I tried to explain them that when the batteries are down, after driving the RV for a couple of hours, the batteries are fully charged, so even if the fridge pulls a lot of power, the alternator is more than powerful enough for both this and giving the batteries a good charge.

The still insisted it was not good to run the fridge on 12V. Now, I am an electrical engineer, and I cannot with my best imagination understand why it would be bad to run the fridge on 12V if the batteries are charged after a drive. The fridge should just lower the amount of current available to charge the batteries. And if that was a problem, then it should also be a problem to use a small charger to charge the batteries. I've never heard that it is bad for the batteries to be charged with a small current, only that it will take longer.

What do you think? Are the staff at this shop just superstitious, or do they have a point?
Class A

35 Replies

Show More
  • oyly's avatar
    oyly
    Explorer
    May 11, 2016
    MrWizard wrote:
    why were the batteries damaged if the fridge is only on 12v when driving

    what did you do to damage the batteries
    that you are NOT going to do anymore

    and you are sadly mistaken, that a few hours of driving will fully charge a depleted battery you might get back to 80 percent in a few hours... maybe
    but it could another 15 20 hrs in absorption mode to fully bring the specific gravity back to 100 percent charge


    I used the lights, carbon monoxide and LP sensors. I don't have power where I am at the moment, so I have to use the diesel generator to charge the batteries. I also have neighbors nearby, so running that generator for 15-20 hours is not an option.

    Excuse my wording about fully charging the batteries in two hours. I am not asking how to charge batteries. I was simply trying to make a point to the battery vendor that since the batteries have MORE charge after a drive and the fridge only draws 12V when the engine is running, then it can not be bad for the batteries: They are being charged, not drained.

    Clear enough?
  • MrWizard's avatar
    MrWizard
    Moderator
    May 11, 2016
    why were the batteries damaged if the fridge is only on 12v when driving

    what did you do to damage the batteries
    that you are NOT going to do anymore

    and you are sadly mistaken, that a few hours of driving will fully charge a depleted battery you might get back to 80 percent in a few hours... maybe
    but it could another 15 20 hrs in absorption mode to fully bring the specific gravity back to 100 percent charge
  • oyly's avatar
    oyly
    Explorer
    May 11, 2016
    n7bsn wrote:
    You should only run the refer on 12V when the engine is running, the 12V heater sucks too many watts to run it when parked.

    Most 3-way's are wired to only run when the ignition is on

    Obviously your University didn't offer any classes on battery power, but then I guess mine didn't either, but the US-Navy took care of fixing that lack.


    Glad to see an expert took the time to answer! Why do you think it's obvious I didn't learn anything about batteries in uni? What did I get wrong?

    My fridge only runs on 12V when the engine is running.
  • 2oldman's avatar
    2oldman
    Explorer II
    May 11, 2016
    I think you're fine. That's a 30a draw from your alternator.
  • n7bsn's avatar
    n7bsn
    Explorer
    May 11, 2016
    You should only run the refer on 12V when the engine is running, the 12V heater sucks too many watts to run it when parked.

    Most 3-way's are wired to only run when the ignition is on

    Obviously your University didn't offer any classes on battery power, but then I guess mine didn't either, but the US-Navy took care of fixing that lack.

About Motorhome Group

38,742 PostsLatest Activity: Aug 04, 2025