Forum Discussion

ctrout's avatar
ctrout
Explorer
Jun 15, 2016

Battery question

I have three 110ah 12v agm batteries connected by 2/0 wire. The specs are here. I will eventually be getting an inverter sized to run anything in the rv up to and including the 1350 watt microwave. No, I have no plans of running the ac at all or the microwave in conjunction with all of the other electrically powered items so the largest load that I will ever run will be the microwave by itself. Expected run time should not exceed 5 minutes at a time for a total of no more than 15 minutes in any given day.

I'm planning on getting the Samlex SA-1500 inverter which I'm pretty confident will manage the load. I'm also confident that the 2/0 cables from Northern Arizona Wind and Sun are sized appropriately to handle the load. Please correct me if I'm wrong on the inverter or cable sizing. My concern is the batteries. The draw on the batteries will exceed 100 amps. Will a 100 amp continuous draw for up to 5 minutes be a problem? I know that lifepo4 batteries could sustain that kind of draw with no problem but I didn't have $6000 to invest in lifepo4 so I have agm. Can a 330 ah agm bank support 100+ amps for 5 minutes?

Incidentally, my solar panels were pushing 61 volts to the charge controller the last time I read them in full sun.
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  • Mandalay_Parr's avatar
    Mandalay_Parr
    Explorer
    Jun 15, 2016
    My guess is your draw will be more like 200 amps using the microwave.
    Mine draws 200.
    I run my generator generally for my microwave.
  • MrWizard's avatar
    MrWizard
    Moderator
    Jun 15, 2016
    i have (5) AGM at 135Ah (each) total = 675 amphrs
    and i still prefer to run the generator when i use the MW

    and my use is typically 1 min at a time , except for baking potatoes
    which it is 6 or 7 minutes

    your looking at 34 ? amps per battery
    34 amp minutes per battery 102 amp minutes total use or 1.5 Amp hrs
    and that does not include purkert effect at that rate your batteries are good for 2hrs max to total discharge aka dead

    technically by the listed specs, yes your those batteries will support that when new and fully charged

    how will they act after a couple of week ends doing that 3 times a day
    IDK
  • time2roll's avatar
    time2roll
    Nomad
    Jun 15, 2016
    ctrout wrote:
    I will eventually be getting an inverter sized to run anything in the rv up to and including the 1350 watt microwave.
    Is that 1350 input power or cooking power?

    The battery will be fine although this will max them out. AGM does better than wet cell batteries at high load.
  • ctrout's avatar
    ctrout
    Explorer
    Jun 15, 2016
    I'm pretty sure it's input power. I just checked the sticker and it's actually 1380 watts. The sticker says 11.5 amps, 120 volts, 1380 watts. (11.5 x 120 = 1380). I'm seriously considering just skipping the microwave. The next largest thing that I would run is a tv at 40 watts. That would greatly reduce the cost of inverter that I would need. Of course that would also mean that I have grossly overbuilt my power for this rig. At 330 ah and 300 watts of solar, I could literally leave every electrical device in the camper on 24/7, including the tv but excluding the microwave and ac and I would never run out of power.
  • pianotuna's avatar
    pianotuna
    Nomad III
    Jun 15, 2016
    Hi,

    A 2000 watt pure sine wave inverter would be needed. My microwave draws 1580 watts and sometimes did not start on a 2500 MSW inverter. I occasionally would have to have another load on first. Since I moved to 3000 watt PSW there are zero issues with starting.

    A 1500 watt may not start the 1380 watt microwave reliably unless it is one of the Panasonic units that have low power settings that actually work. Most microwaves cut back on the heating power by running the unit flat out--but for differing slices of time.

    i.e. power setting 1 might be 1/10 of Magnatron on time per second during run time.
  • profdant139's avatar
    profdant139
    Explorer II
    Jun 15, 2016
    You might want to post this on the tech forum, so that you can get several different and conflicting answers, all vehemently expressed. ;)

    Just kidding.

    My own two cents (probably worth less than that) would be not to run a high-draw appliance like a microwave using only the batteries -- a very fast drain puts a lot of stress on the components. I would be more inclined to use the generator. But really, this is just my impression, since I have never tried a fast drain on deep cycle batteries. Who knows? Maybe the batteries will enjoy the extra exercise, like a racehorse finally able to gallop. ;)
  • time2roll's avatar
    time2roll
    Nomad
    Jun 15, 2016
    ctrout wrote:
    I'm pretty sure it's input power. I just checked the sticker and it's actually 1380 watts. The sticker says 11.5 amps, 120 volts, 1380 watts. (11.5 x 120 = 1380). I'm seriously considering just skipping the microwave. The next largest thing that I would run is a tv at 40 watts. That would greatly reduce the cost of inverter that I would need. Of course that would also mean that I have grossly overbuilt my power for this rig. At 330 ah and 300 watts of solar, I could literally leave every electrical device in the camper on 24/7, including the tv but excluding the microwave and ac and I would never run out of power.
    Yes a 150w sine wave inverter is a bit easier to install and saves money. The 1500w could come in handy for other uses and should run the MW just fine. Your call. Too much power can be barely enough at times...
  • BoonHauler's avatar
    BoonHauler
    Explorer
    Jun 16, 2016
    pianotuna wrote:
    Hi,

    A 2000 watt pure sine wave inverter would be needed. My microwave draws 1580 watts and sometimes did not start on a 2500 MSW inverter. I occasionally would have to have another load on first. Since I moved to 3000 watt PSW there are zero issues with starting.

    A 1500 watt may not start the 1380 watt microwave reliably unless it is one of the Panasonic units that have low power settings that actually work. Most microwaves cut back on the heating power by running the unit flat out--but for differing slices of time.

    i.e. power setting 1 might be 1/10 of Magnatron on time per second during run time.


    I agree with Don.

    The 2/0 wire I think would be okay as long as the wire run does not exceed 5 feet in length. Also you'll probably exceed 150 amps when the MW starts.
  • Gjac's avatar
    Gjac
    Explorer III
    Jun 27, 2016
    I have a 1000 w MSV inverter and 2 6 v GC batteries. After about a min the inverter alarm goes off when using the MV. If I am driving I can use the MV on the inverter. However I look for ways to use the genset so I don't have to exercise it monthly so MV on the generator is a good thing.
  • pnichols's avatar
    pnichols
    Explorer II
    Jul 03, 2016
    Off topic ... but just as a side note: We have to run our MW using our built-in Onan generator - as we don't have a high power inverter on board.

    HOWEVER ... I am concerned about the wear and tear on the Onan with a lot of short-run startups and shutdowns. In general, isn't this supposed to be very hard on a gasoline engine (at the very least hard on the starter and it's gears)? Should this be a concern with Onan generators? The Onan manual says to run it, after being started, for at least one minute before placing a load on it and to run it, after removing a load, for at least one minute before shutting it off. We violate this rule of thumb all the time whenever using the MW for a few minutes here and there.

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