Is This Normal Battery Variability?

lyonkster
lyonkster Registered Users Posts: 10 ✭
in Off Grid Solar & Battery Systems #1
I just bought two 6V golf cart batteries from Costco. After charging them for a few days, and letting them rest, I measured the voltage and specific gravity of the two batteries.

Battery 1 is at 6.4V and 1.290 s.g
Battery 2 is at 6.3V and 1.265 s.g

Should this difference be considered "in the noise", or should I try to exchange Battery 2 for something more like Battery 1?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #2
    Re: Is This Normal Battery Variability?

    Trojan says to "equalize" a battery when the cells are 0.030 or greater difference...

    For your "first" inexpensive/training set of batteries, I am not sure I would worry too much--just log the "full charge" specific gravity and monitor from here on out...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • lyonkster
    lyonkster Registered Users Posts: 10 ✭
    #3
    Re: Is This Normal Battery Variability?

    Thanks Bill. I should have added that within each battery, all three cells have very consistent specific gravity, i.e. 1.290 in Battery 1 and 1.265 in Battery 2. So it's not a matter of equalization, at least not within each battery. Does equalization also work between cells of different batteries in the bank?

    I suspect that this is all in the noise and I am obsessing needlessly (a little knowledge is a dangerous thing, I suppose), but I just wanted to make sure that you guys would not recommend that I exchange the second battery, before I put it into service.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #4
    Re: Is This Normal Battery Variability?

    I am not a battery expert... But I would say the equalization is for all series cells in a bank (in theory at least).

    The issue is for cells discharging in series... You may have some cells at nominal of 225 AH, others at 230 AH and others yet at 180 AH of capacity (80% of nominal 225 AH--80% variation of capacity between cells in a bank is the normal worst case spread).

    When you discharge a battery bank--the first cell that discharges to 0% state of charge defines the "dead battery". If you where to continue to discharge that series string of cells, then that one cell would actually reverse voltage and begin to recharge.

    For a "12 volt battery", discharging to 10.5 volts (dead), beginning to reverse voltage and recharging that cell would take the battery voltage to ~8 volts (assuming going from +2 volts to -2 volts to "reverse" the cell on a 12 volt battery bank)... For normal operation with a 11.5 or 10.5 cutoff (such as an AC inverter)--taking a cell "negative" is not likely to happen. But if you have something, such as lighting, that simply connects to the battery bank, then it is very easy to do (lights/load left no, nobody to turn the loads off).

    Anyway--reversing a cell is an "easier" mistake to make in higher voltage battery banks (24 or 48 volts)... You could reverse a cell in a 48 volt bank and still have a "working bank" (48 volts - 4 volts for reversed cell = 44 volts. ~42.0 volts is a "dead" 48 volt battery bank).

    Anyway, if you avoid taking a battery very low (below 50% for normal cycling, and below 20% state of charge for very deep cycling)--You will avoid reverse charging a cell which is pretty much certain death for that cell (or the battery if part of a larger multi-cell battery).

    As a point of interest--for high performance rechargeable battery banks (such as for radio controlled model cars and aircraft)--they actually test individual cells so they can assemble a battery pack with equal capacity cells... Then, all cells "go dead" at the same time--and there is no chance of killing a cell by reverse charging (and they don't have to add electronics to protect the battery pack).

    Sorry--I tend to type a lot when answering detailed questions.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • lyonkster
    lyonkster Registered Users Posts: 10 ✭
    #5
    Re: Is This Normal Battery Variability?

    That makes sense, thanks. I certainly don't plan to ever let my batteries get below 50%, hopefully I'll keep them above 75% most of the time. So hopefully the voltage reversal will never be an issue.

    I just wonder is the difference between the two batteries is big enough to warrant taking the Battery 2 back, and trying to get one with a bit more capacity?
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  • RCinFLA
    RCinFLA Solar Expert Posts: 1,485 ✭✭✭✭
    #6
    Re: Is This Normal Battery Variability?

    Check the date code. I will bet the lower one is few months older then the higher one. Golf cart batteries are Antimony alloy plates which have a higher self discharge then Calcium alloy plates used in automotive and marine batteries. Animony alloy make them more rugged. Most all deep cycle batteries are Antimony alloy based.

    All lead acid batteries have a date code branded into the top edge of plastic case. Months are letter, A= January. Some manf. skip 'I' some don't. Two numbers following is year. For example, E11xxxx is May 2011. The numbers or letter following are lot codes by manf.

    It is not catastrophic but whenever I buy batteries to put in series I try to get them all with the same date code. (actually I cherry pick to get the newest date codes) I also have a little pocket DVM that I calibrated and check terminal voltage on each battery.

    The batteries sit there on the shelf and have self discharge.

    Battery 1 is at 6.4V with 1.290 s.g looks like a 'virgin' as the SG is a little high. Probably has not fully formed the positive plate yet. Run a couple of cycles to 70% state of charge and SG will likely drop at full charge.
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  • lyonkster
    lyonkster Registered Users Posts: 10 ✭
    #7
    Re: Is This Normal Battery Variability?

    Thanks, those are good tips. I actually did pick two batteries with the same (and recent) date code of 5/11, so I don't think that this is really the issue.

    But I am curious, when you talk about batteries self-discharging, shouldn't they come back to full charge when recharged (assuming of course that they did not self discharge to the point of getting damaged)?

    And I see what you mean about Battery 1, so perhaps it's not that Battery 2 is weak, rather Battery 1 is not at a level that it will maintain in use? Then again, isn't 1.265 sg (Battery 2) only about 90-95% of full capacity?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #8
    Re: Is This Normal Battery Variability?

    Final "Full" charge is always relative to state of battery when it was "dry charged/sealed" and the acid strength when filled at the distributor/store.

    I don't have a opinion on whether your should switch batteries or not--other than it is probably within the margin of error for those batteries.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #9
    Re: Is This Normal Battery Variability?

    Battery #1 is the aberrant one. Its electrolyte appears to be a bit rich.
    But here's the thing: you bought them from Costco so how much did you pay? The difference between the two isn't significant in my opinion, and they'll both last your money's worth with care.
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  • lyonkster
    lyonkster Registered Users Posts: 10 ✭
    #10
    Re: Is This Normal Battery Variability?
    Cariboocoot wrote: »
    But here's the thing: you bought them from Costco so how much did you pay? The difference between the two isn't significant in my opinion, and they'll both last your money's worth with care.

    That is true, I was just being my usual over-obssessive self...

    So I stopped by Costco and exchanged Battery 2 for Battery 3, with the same 5/11 date code. Took it home, charged it for a day, let it rest overnight, and measured the following:

    Battery 3: 6.4V, 1.290 s.g.

    Which is the same as Battery 1, which makes me very happy to have two "identical" batteries.

    Now, as an aside, I guess I have a battery chemistry 101 question. What determines a battery's amp-hr capacity? In other words, if both of these batteries have a nominal capacity of 220 A-hr, but one has 10% less s.g. than the other, does that mean that one battery has 220 A-hr and the other one has less? or does it mean that they both have 220 A-hr capacity, but one of them can never be charged to 100%? The latter question is of interest because I obviously want to avoid discharging a battery below a certain DoD, so if I am starting out at 90%, I would have less capacity before I reach that threshhold. Or am I totally misuderstanding how s.g. relates to A-hr capacity and SOC?
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  • RCinFLA
    RCinFLA Solar Expert Posts: 1,485 ✭✭✭✭
    #11
    Re: Is This Normal Battery Variability?

    If a battery truely has high end of mix tolerance of electrolyte it is a 'hot' battery.

    High SG makes the discharge more active and recharging less so. It gives it a lower effective internal resistance for discharge. During discharge, plates have to find acid. During recharge, plates have to find water. A high current discharge will cause a local starvation of acid near plates as the electrolyte remixing lags behind.

    In other words, play hard and die young.

    The other end of extreme is like a telecom battery which is mixed with lower SG. This gives it longer longevity but higher internal resistance (which in telecom is not necessary since peak current draw is low).

    AH capacity is a combination of factors. AH capacity rating does not have anything to do with battery quality or longevity. Generally for good longevity, you want the electrolyte acid depletion to determine end of charge. This prevents too much degradation of plates. Higher starting SG will 'eat' more lead (turn it into lead sulfate) before voltage collapses. Every time a lead acid battery is discharged and recharged its plate mutates a little. The deeper you cut, the greater the scar on recharge.

    Some batteries have high electrolyte reserve to reduce maintanence cycle periods. Good deep cycle batteries have thicker plates, particularly positive plate, which eats away over cycles and time and drops to bottom of battery. Large quantity of thin plates gives lower internal resistance for high current draw but are not good for deep discharge (vehicle starting battery).

    I don't want to alarm you too much. A starting SG as high as 1.300 is not that bad.

    http://giantbatteryco.com/GLOSSARY/Specific.Gravity-Industrial.Batteries.html
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