Solar Power Off-Grid Battery Bank Testing – Discharge


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A simple capacity test on a 450 amp-hour battery bank. The results should scale for larger loads and capacities. My batteries are Trojan T105s.
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28 replies
  1. Peter Dorn
    Peter Dorn says:

    As your battery voltage drops, the inverter draws more current to supply the same power output. As you discharge your battery, the density of the electrolyte drops and the internal resistance increases. Nothing was going on in the inverter… it was going on in the battery! BTW… A simple capacity test is not simple, it requires a recording watt meter and the battery needs to be fully charged at the start of the test and fully discharged at a constant load.

    Reply
  2. Terry Pullen
    Terry Pullen says:

    I love your videos but I can only watch for a short period of time because the motion makes my stomach queasy.  Please use a tripod.  
    Anyway thanks for the videos.  

    Reply
  3. realvanman1
    realvanman1 says:

    Also, my inverter (a Trace Engineering SW4024) has several fixed steps in the ratio of transformation, such that output voltage vary somewhat with input voltage. As input voltage drops off, subsequently higher transformation ratios will be selected. So output voltage will gradually drop (during discharge) to a point, then jump up when the next ratio is selected, then continue to again drop gradually, and so on. This may explain why you were sometimes getting higher output power readings.

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  4. realvanman1
    realvanman1 says:

    Realize that, in order to determine state of charge from open circuit voltage, the battery must first stand idle for 24 hours. This allows the specific gravity of the electrolyte adjacent to the plates to equalize with that in the rest of the cell.

    Reply
  5. hdidane00
    hdidane00 says:

    these are apparently the batteries of choice for a lot of people for solar setups but since they are not AGM and they are not deep cycle batteries either i presume and are flooded batteries and need maintenance how often do you have to top off the acid to keep them running good?

    Reply
  6. Sean White
    Sean White says:

    So quick question, unless I'm missing something, which is absolutely possible….Shouldn't you test the bank with out power coming in from the panels? Your vid shows voltage coming into the bank while you are trying to measure bank capacity.

    Reply
  7. Zachrey Helmberger
    Zachrey Helmberger says:

    8 hours at 20 Amps is 160 amp hours. The battery should be left in an open circuit condition for about 30 minutes to allow the chemistry to reach equilibrium before reading the voltage. It looks like the camera was turned off for some time and then it came back on to read the battery voltage, but this rest time was not specified in the video. Also most voltage charts are for 25C ambient temperature so you may need to compensate for that to find true state of charge.

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  8. Zachrey Helmberger
    Zachrey Helmberger says:

    Remember that the inverter is supplying a constant amount of electrical power to a constant load. Power = voltage x current so as the battery voltage falls, the inverter (Assuming it maintains a constant output voltage at all times) must draw more current to meet the power demand of the constant load.

    Reply
  9. Zachrey Helmberger
    Zachrey Helmberger says:

    The clouds very close to the sun act like a mirror that concentrates the light onto the panels and surrounding land. Also, the clouds that are not so close to the sun are scattering white light instead of the fainter blue light scattered by oxygen and nitrogen. So, in a sense, it's like the sun got bigger, or your panel became a hundred miles wide but with a much lower efficiency because the clouds scatter the light in all directions.

    Reply
  10. Richard Lewis
    Richard Lewis says:

    amps rise as battery volts drop…when charging. But when drawing power from a battery, lower voltage through a constant resistance would mean current would have to fall..ohm's law. Something else must be going on. Maybe the door was opened, the room got cooler, the light bulbs were cooled somewhat and lowered the resistance, therefore drawing more current.

    Reply
  11. Linda Cap
    Linda Cap says:

    Let me pose a problem that I need solved.

    I see people taking s small stand up freezer and putting a Ronco controller on the thermostate to turn it into a efficent fridge.For example an off grid couple on you tube found that the daily draw on a stand up fridge was 2000 watts.

    They moved over to the freezer which is half the size of the fridge for a draw of 140 watts due to cold air being heavy .They did not put R-10 around the freezer and plywood for a lower watts usage.

    Could you do

    Reply
  12. lindosland
    lindosland says:

    So you got about 8 hours at 11A from fully charged to near empty? That's 88Ah from a 250Ah rated battery. Shouldn't you conclude that it's faulty and you should be sending it back? I've tested several makes of deep-cycle batteries and concluded that they lie about the amp-hours. I've also found that they don't hold up for anything like the number of cycles stated (for a given depth of discharge). My four 250A-h 48V Elecsol 12V give 40Ah after 2 yrs (they say 1000+ cycles).

    Reply
  13. dirt4b
    dirt4b says:

    i am now using 4 awg welding cable to connect my 8 batteries in paralle, what is youre thought about using thick galvanized metal to take the place of the cable?my thought would be it would be better than cable. cant seem to find out much info on the subject. thanks for any info.

    Reply
  14. SouthernPioneer1965
    SouthernPioneer1965 says:

    Pure sine wave inverters use a bit more energy to produce the same amount of usable current as a modified sine wave inverter,also you might want to rethink running your 105's down to 50% it will reduce battery life, think 20% discharge that gives you 90 amp hours…

    Reply
  15. helloman1976
    helloman1976 says:

    Almost forgot, the T105-RE version, if you have that version, is designed to go to 80% discharge and has a 7 year warranty and they are designed specifically for solar applications. They will last 10+ years if you drain them to 50% or less as long as the 50% isn't daily or constant, just occasionally. My batteries don't get monitored so I drain mine down to 10V constantly and they are still running strong, got them from Walmart and it's been a year now. 🙂

    Reply
  16. helloman1976
    helloman1976 says:

    Your amps probably went up because your voltage went down, focus on watts. Also, the reason your inverter is using more power is likely because of induced heat and loss of efficiency due to heat. My 2 cents anyway…

    Reply
  17. TheKilog69
    TheKilog69 says:

    Nice job on all of your testing all diffrent ways.Having watched all this I,m trying something new.I just got 2-90watt sopray mono panels that match the 1 on top of my tracker.I,m am building a new improved tracker for these 3 macthing panels.I will the hook them to a single 300 watt GTI,and go full time that way with this build,i think from what i have learned from your tests this could be a wise use off power,as ihave plenty to recharge my daily use of 1.25kwh thru my battery,&Inverter.TKS G.W

    Reply

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