Forum Discussion

Eber's avatar
Eber
Explorer
Aug 21, 2013

Amp/hour test is way off-

I have installed two NAPA 8145's rated at 6v and 226a/h each.
I'm testing to see how long they last with an Isotherm C-85, 3c/ft, 12v refrigerator, consuming an average .8amps.

I assumed it would run almost 10 days on a charge.
Adding variables and other minor loads, at least 4 days.

After 24hrs., my voltminder reads 12.29 with compressor running.

Other known loads are the voltminder, small fan on Nature's Head which draws milliamps (?), and a indicator light on the new 4 stage charger/converter from Best Converter.

At this rate, they would need charging after about 30 hours or so.

Am I wrong in expecting too much? Math error? Other?

All lights have been converted to led and there's a 100w solar panel coming to keep batteries up. My only concern was the actual use of the refrigerator.

Yes, refrigerator was prechilled and contained three, 2liter bottles of ice.

Thanks for any help-

Eber
  • McZippie's avatar
    McZippie
    Explorer
    Aug 21, 2013
    Here's a tip to monitor batteries; experience gained from decades of RVing and Power Boats.

    Mount separate large easy to read volt and amp meters in very visible location.

    Even though many use and like battery bank gauge computers, IMHO they are inaccurate, misleading, small displays and confusing.

    Large display easy to read amp and volt at a glance gives all the information need to monitor a battery bank.



  • MEXICOWANDERER's avatar
    MEXICOWANDERER
    Explorer
    Aug 21, 2013
    Don, may I borrow and change your statement to enlarge it a bit?

    "Voltage is the "great pretender". I'd suggest getting a proper temperature compensated specific gravity device and use that to determine state of charge"

    Third to last word can be changed to CALIBRATE. Meaning, it the specific gravity can be interpolated as a state of charge in low amperage discharge conditions, FOR A WHILE. This is not reliable as a month after month reference. But calibrating and correlating specific gravity to voltage is viable. Note my reference to "low amperage conditions" in honor of Mr. Peukert.
  • pianotuna's avatar
    pianotuna
    Nomad III
    Aug 21, 2013
    Hi,

    Overnight is probably not long enough for charging a brand new set of batteries.

    12.2 with no load after resting for 24 hours is a good number to recharge at if you wish the battery bank to last a long time.

    Voltage is the "great pretender". I'd suggest getting a proper temperature compensated specific gravity device and use that to determine state of charge. (from Mex's suggestion, then calibrate the volts on a chart that fits your personal battery bank)
  • mena661's avatar
    mena661
    Explorer
    Aug 21, 2013
    Eber wrote:
    Thanks for the replys.
    The batteries are new and were left on the trailer charger overnight.
    The initial reading was 13.62 and I had understood from some other thread that I should recharge at 12.2. The chart above shows I could go longer.
    13.62 is surface charge from the charging process. Full charge on a battery with NO loads attached is ~12.8V. This is why MEX says voltage readings are misleading.
  • MrWizard's avatar
    MrWizard
    Moderator
    Aug 21, 2013
    batteries in series ... you add the volts Not the amp hours
    2*6 at 225 is 12v at 225

    if you parallel them it would be 6v at 450 ampHrs

    you do not want to go dead aka use all AmpHrs

    staying at or above 50% is best for prolonging useable battery life
  • Rbertalotto's avatar
    Rbertalotto
    Explorer
    Aug 21, 2013
    Might want to look into getting a Trimetric 2025RV

    I love the one I installed in my camper....
  • MEXICOWANDERER's avatar
    MEXICOWANDERER
    Explorer
    Aug 21, 2013
    Using simply a volt meter and not an amp hour meter to measure spurious transient energy draw is like using a sundial in computing astrophysics. Good Luck.

    For fun one day, I sat next to a Danforth 24 volt freezer using an ACCURATE ammeter a clock, a stopwatch, a chair, a book, and a good dose of patience. The idea was to attempt to measure ampere hour draw over a four hour period. I was off more than eighteen percent (low). That learned me.
  • Eber's avatar
    Eber
    Explorer
    Aug 21, 2013
    Thanks for the replys.
    The batteries are new and were left on the trailer charger overnight.
    The initial reading was 13.62 and I had understood from some other thread that I should recharge at 12.2. The chart above shows I could go longer.
    The refrigerator states; Average Power Consumption:- 0.8 A/h @ 12v
    Max Power Consumption:- 3.5amps @ 12v

    Nobody seems to think there is anything wrong so, great.

    I did find there is a difference in voltage with the compressor running and not running (lower when running). Which number is significant for recharging?

    Thanks, Guys-
    Eber
  • MNtundraRet's avatar
    MNtundraRet
    Navigator
    Aug 21, 2013
    A compressor Refrigerator that size would be 3.8 amps on 12 volts, not 0.8 amps. Do you have a clamp-meter to check the actual draw? With your 24 hour reading your battery will be down to 40% in just two full days.

    Also; by sometime on the third day your converter could trigger "low voltage" cut-off. Start-up of the compressor is the "show-stopper". You had better test actual loads at home before going, or take a generator along just in case.
  • Salvo's avatar
    Salvo
    Explorer
    Aug 21, 2013
    You didn't show your math, so....

    I do see a possible error. 6V battery is rated 226AH.

    Total AH is 226AH, not twice.

    If you're pulling 1A continuous, we got 24AH/day.

    Number of days to 50% SOC is: 113AH/24AH/day = 4.7 days

    Your load current may be slightly more.

    Sal

    Eber wrote:

    Am I wrong in expecting too much? Math error? Other?

    Eber

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