AH capacity lie

bc buckbc buck Posts: 51Registered Users ✭✭
I installed a inverter charger and TRI-METRIC TM-2025 in my cabin. I read about AH exaggeration of battery manufactures and took that into consideration when buying battery's. I bought two Energizer 125 AH deep cycle trolling motor battery's from Sam's club. Wired in parallel I am only getting about 50AH at temp of 20 deg.F. I checked each battery with a automotive load tester and they look OK. My amp load averages about 12 to 17A. There is no draw that does not go through the instrumentation. I'm thinking that at temp of 70 deg this will be about 30% of what manufactures AH rating. Has anyone else had this problem.

Comments

  • BB.BB. Posts: 27,694Super Moderators admin
    Re: AH capacity lie

    Some questions... There is a setup for the Trimetric for what size Shunt is used/etc... I assume that all agrees.

    Next, can you use a hydrometer on the batteries and measure they state of charge?

    What kind of charger are you using and how is it configured? Is the Bulk/Absorb voltage around 14.2-14.5 volts?

    Does the controller have an Absorb timer/Float transition setting? Is the float voltage set around 13.2 to 13.7 volts?

    Does the controller have a a remote battery temperature sensor? As batteries get cold, the charging voltage needs to be increased. Below 20F, you are probably looking at nearly adding 1 volt to the charging setpoint for "full charge".

    Have you verified the charging voltage as reported by the charge controller agrees with the charging voltage at the battery bank as measured with a known good volt meter that is accurate, hopefully to 14.45 volts--at least two digits to the right of the decimal point (with a heavy charging current)? If you have a long cable and/or small diameter/gauge wire from the charge controller to the battery bank, you can have significant voltage drop there too (more than 0.1 volt drop under heavy charging current is probably about the recommended maximum for a 12 volt battery bank).

    How do you know the battery capacity is only XX Amp*Hours? Is the battery voltage collapsing below 11.5 volts or 10.5 volts?

    Very cold lead acid batteries do have a significant reduction in AH capacity--but not that much:
    Battery capacity (how many amp-hours it can hold) is reduced as temperature goes down, and increased as temperature goes up. This is why your car battery dies on a cold winter morning, even though it worked fine the previous afternoon. If your batteries spend part of the year shivering in the cold, the reduced capacity has to be taken into account when sizing the system batteries. The standard rating for batteries is at room temperature - 25 degrees C (about 77 F). At approximately -22 degrees F (-27 C), battery AH capacity drops to 50%. At freezing, capacity is reduced by 20%. Capacity is increased at higher temperatures - at 122 degrees F, battery capacity would be about 12% higher.

    Anyway, a couple batter FAQ's to read:

    Battery FAQ
    www.batteryfaq.org

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • bc buckbc buck Posts: 51Registered Users ✭✭
    Re: AH capacity lie

    I am using a 500a shunt programed into the TM 2025. I do not have access to a hydrometer. The Trace Inverter charger is set up for wet cell with bulk of 14.7,absorption 14.7, float 13.3v and uses a remote RTD for temp. Did compair Volts using multimeter at battery to instrumentation volts but not during charge cycle. The conductors are 3' long and shielded for the instrumentation and the battery conductors are 2/0 and about 13' long if you only measure one wire. The TM-2025 logs about 53AH used by time battery bank reaches 11V. I appreciate your help.
  • BB.BB. Posts: 27,694Super Moderators admin
    Re: AH capacity lie

    With substantial current flowing, measure the voltages around the system (connections, use a section of cable like a shunt to see if the batteries are sharing current or not, etc.).

    After the batteries reach 14.7 volts--It should hold that voltage for around 2-4 hours or so...

    Can you keep the batteries warmer? (insulation, heater block powered by generator to get batteries warmed up after a long cold soak, etc.?

    Otherwise, I don't see any major issues...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • icarusicarus Posts: 5,108Solar Expert ✭✭✭✭
    Re: AH capacity lie

    First,,, I would suggest that you buy a good hydrometer and use it.

    Second, a 12-17 amp load is a pretty hefty load on a small(sih) 225 ah battery, some where in the neighbor hood of 150-200 watts,, and that is a stated average. Do you have peak loads that are larger?

    I would run my batteries down to say 75% according to the Bogart meter. Then would let them sit for several hours. I would test the idle voltage, as well as the specific gravity and see if they all generally agree. I would then recharge according to the inverter/charger until it says full/ and the Bogart says full. I would then let them sit again, check the SG and the voltage and see if they all generally agree. If not then perhaps you have a problem.

    Just for the record, we have 4 T-105's wired in series/parallel to make 450 ah @ 12 volts. We draw about 50 ah/day and see resting voltages in the morning of ~ 12.4-12.5(that is with a few small perpetual loads going 24/7) So that is ~ 11%.

    By comparison, your 50 ah our of 225 is ~ 22% I would expect to see your at rest voltage to be in the 12.3- 12-4 range With colder batteries,, the effect is that they well have smaller capacity (although longer life, all else being equal!) so that instead of your batteries being drawn ~ 22%, the net effect is that they are being drawn closer to 30 % or more.

    11.0 volts under a 12-17 amp load after drawing out ~ 50 ah does not sound too out of the ordinary.

    It's doesn't sound like you have much a problem.
  • nielniel Posts: 10,311Solar Expert ✭✭✭✭
    Re: AH capacity lie

    bc buck
    what are the battery manufacturer's charge set point recommendations so we don't assume that is what you are using?

    if they are charged properly to full capacity then your average of say 15ah for a 250ah battery bank represents 6% which should not be considered a heavy load, but is just a tad greater than the 20hr rate. does the manufacturer rate these batteries at 125ah for 20hrs or is it 100hrs? if it's at 100hrs then by introducing a 16hr or 17hr rate will drop the capacity a bit from that 125ah which you list. add to that a bit more loss being not at its rated temperature and this battery is now way down in capacity. if they stretched the truth on the capacity then that would add to the dilemma you face.. try placing some insulation around the batteries and be sure to give space above the batteries if insulating there and do not use foil faced insulation as this could cause an accidental short.
  • techntrektechntrek Posts: 1,366Solar Expert ✭✭✭
    Re: AH capacity lie

    Plugging in your figures into Smartgauge's Peukert calculator (250 ah, 14.5 amp load), you have 119 amps available @ 50% DOD. Then to correct for your 20F battery temperature, you should have about 89 amps available. At your full 17 amp load your capacity drops to 85.5. If you are only getting 50 amps then I agree that something is either wrong or your shunt is providing incorrect data.
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
  • bc buckbc buck Posts: 51Registered Users ✭✭
    Re: AH capacity lie

    The rest V is about 12.7. These are the biggest trolling motor Battery's Sam's sells but they weren't labeled if the rating was 20 or 100 hours. I changed the parameters on the TRY-Metric so it takes a 1 hour 2A float to read 100% charge. The trace charger can usually get this done in less than 5 hours . I will see if I can get a amp reading with my multimeter to com pair to the TRY-Metric. The battery bank is in a cooler with added insulation but is vented so I don't think they get very warn with 20 deg. ambient temps. These are $80 battery's and maybe I got what I paid for.
  • RandomJoeRandomJoe Posts: 472Solar Expert ✭✭✭
    Re: AH capacity lie

    This brings up a question I've occasionally wondered about: Just how accurate can the shunt measurements be?

    I work with HVAC control systems, and 2-3% accuracy is pretty good. REALLY good - and markedly more expensive - sensors are 1%. Moreover, if you get your scale way off and are operating down in the first 10-20% of range, the errors are typically even worse.

    Transferring that to a 500A shunt (which I use as well, only thing I can use with my Outback system AFAIK) 1% is 5A! If the meters have 1% accuracy, then a 12A reading could really be anywhere from 8A-17A. If the shunt consistently reads low, it's quite possible to be using (and adding back) more AH than you think.

    And if the shunt / meter accuracy is only 2-3%...!

    That said, when I've checked with my Fluke, the Outback measurements seem to be pretty accurate. I wasn't able to find any specs on my equipment, though. It just amazes me that something so (relatively) cheap could be all that accurate way down at the low end of range like that.

    And then we get to resolution... What particularly got me was that a 100A shunt is usually 100A=100mV. But a 500A shunt is only 50mV?!? So the meters have to measure in 0.1mV - or 100 microvolt! - increments just to get 5A resolution. Tens of microvolts to handle 1A! That's just crazy! :confused:
  • bc buckbc buck Posts: 51Registered Users ✭✭
    Re: AH capacity lie

    I cant use a clamp amp accurately because it is designed for AC. I think I should be able to check across shunt using multimeter on DC A setting to compare to instrumentation. This shunt is new high quality resistive device that I am hoping should have been calibrated at factory. If I charge till the charger says 100% (not the TRY-METRIC) and with the multimeter reading the same as TRY-METRIC amp draw. I should be able to consider the AH used to 11v reliable. The 50 AH on new 250ah rated battery bank don't seem right.
  • AntronXAntronX Posts: 462Solar Expert ✭✭✭✭✭✭
    Re: AH capacity lie

    What is manufacturing date of your batteries?
  • BB.BB. Posts: 27,694Super Moderators admin
    Re: AH capacity lie

    There are DC clamp current meters.
    RCinFLA wrote: »
    Sears 'Craftsmans' labeled version of MA220.

    http://www.sears.com/shc/s/p_10153_12605_03482369000P

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • bc buckbc buck Posts: 51Registered Users ✭✭
    Re: AH capacity lie

    I purchased the battery's in Oct 2010 and were manufactured Aug 2010. I will be in trouble when I start loosing AH because of age.
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