Resting Voltage

keyturbocars
keyturbocars Solar Expert Posts: 375 ✭✭
in Off Grid Solar & Battery Systems #1
Is the resting voltage (after charging) a good indicator of battery condition?

I've been conditioning some batteries by cycling them with a BLS desulfator, and I've seen some improvement. For example, resting voltage (5 hours after charging stopped) on one battery was 12.72V. Before conditioning it for 8 hours, it was 12.58V.

Not sure if resting voltage is really a good indicator. I can't check specific gravity on these Concorde Lifeline AGM's.
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  • RCinFLA
    RCinFLA Solar Expert Posts: 1,485 ✭✭✭✭
    #2
    Re: Resting Voltage

    It's better then nothing. Does not give you assurance on capacity or series resistance of battery.

    Do about a 30-40% expected capacity discharge. Let sit rested for a few hours and see if the open circuit voltage matches the expected state of charge.

    High current load test for short period will give indication of series resistance.

    Another test is for leakage. An AGM should keep the 12.7v for at least a week.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #3
    Re: Resting Voltage

    From this thread:

    Specific Gravity versus voltage:

    From an earlier battery discussion thread:

    Well mixed electrolyte is important--so if you notice all cells not coming back to "full charge" level of s.g., then there is a good chance that "mixing" (equalization) will bring the s.g. back up to your logged readings (keep these as your "good batteries" / "fully equalized" numbers.

    The amount of variation you are seeing is not that great (less than 0.030 between all cells). And having "high" s.g. is not always a good thing either.

    I ran across this page on why different types of batteries have different starting s.g. fills... Is pretty interesting:
    Specific Gravity vs Applications
    1.285 Heavily cycled batteries such as for forklifts (traction).
    1.260 Automotive (SLI)
    1.250 UPS – Standby with high momentary discharge current requirement.
    1.215 Geral applications such as power utility and telephone.

    As mentioned earlier, the specific gravity (spgr.) of a fully charged industrial battery, or traction battery, is generally 1.285, depending on the manufacturer and type. Some manufacturers use specific gravities as high as 1.320 in an attempt to gain additional Ah capacity, but at the cost of a shorter cycle life.

    ...

    Higher Gravity = vs Lower Gravity =
    More capacity / Less capacity
    Shorter life / Longer life
    Higher momentary discharge rates / Lower momentary discharge rates
    Less adaptable to "floating: operation / More adaptable to "floating" operation
    More standing loss / Less standing loss
    Also on that page is the formula between cell resting voltage and specific gravity:
    Specific gravity = single-cell open-circuit voltage - 0.845 (example: 2.13v – 0.845 = 1.285)
    Or
    Single-cell open circuit voltage = specific gravity + 0.845.
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • keyturbocars
    keyturbocars Solar Expert Posts: 375 ✭✭
    #4
    Re: Resting Voltage

    Thanks for the good info guys.

    I just ordered a 500A carbon pile load tester the other day. Harbor Freight has it on sale for $49.99, and on they had a special coupon (25% off any single item) on their website on Thanksgiving Day. Got the tester for $37.49. No shipping cost either, because I bought some other stuff and qualified for free shipping.

    http://www.harborfreight.com/500-amp-carbon-pile-load-tester-91129.html

    So, the load tester checks out resistance and doesn't tell anything about battery capacity?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #5
    Re: Resting Voltage

    Battery resting voltage (corrected for temperature) is a good analog for specific gravity.

    Certainly measuring battery resistance is an important test... Fractured plates, corroded grids, excessive sulphation, etc. all affect battery performance (and value to you).

    You may be looking at battery resistance on the order of 0.005 (5 milliOhms)--So you will need good sized loads and accurate voltage and current measurements.
    • V=I*R
    • V=500 amps * 0.005 ohms = 2.5 volts
    To give you an example of the values you may see (if you can draw 500 amps from a reasonably charged battery):
    • R=delta V / R = 0.005 ohms = (12.5 - 10.0 volts) / 500 amps
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • keyturbocars
    keyturbocars Solar Expert Posts: 375 ✭✭
    #6
    Re: Resting Voltage

    That's interesting Bill. Does that mean that if I applied a 500a load, then the voltage would drop 2.5V. If I saw 10V on the meter, then the battery would be considered "good". Am I understanding this correctly?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #7
    Re: Resting Voltage

    Depends on the size and design of the battery... Typically, one would expect a flooded cell battery to support a surge of C/2.5 (~40% of rated Amp*Hour capacity)... So, a 12.0 resting voltage (~50% state of charge) drops to 11.5 volts and leaves 1.0 volts for wiring drop to 10.5 volts...

    AGM's should support C*4 discharge rates...

    Or, compare with known good battery and look for 2-10x more resistance for a "bad battery"--Just guessing here--but that is where I would start.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #8
    Re: Resting Voltage

    be careful as you don't want to do a 500a test on lifeline batteries. their website will specify the actual load you will need and for how long. this is a strenuous test for the batteries though and if any of them are shaky or near the end of their life it could put them over the edge just so you know.
    interesting tester and might do well for many applications being it is adjustable, but i have difficulty believing it can do a 500a test long term without burning up something internally. at a minimum the resistor must handle 500a x 12v = 6000w. that's around 19000btus of heat!!!!! they don't specify the wattage of the resistor.
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  • keyturbocars
    keyturbocars Solar Expert Posts: 375 ✭✭
    #9
    Re: Resting Voltage

    Thanks for the warning Neil. I think I read somewhere that the test duration is 15 seconds and the load tester shuts down or sounds a buzzer when the test is finished. I'm curious to get the tester.

    I just read the tester manual online and it sounds a buzzer at 15 seconds at which point readings are supposed to be taken. Then the load must be manually shut off. They say that 1 minute cool down required between tests and a maximum of 3 tests in 5 minutes.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #10
    Re: Resting Voltage

    if you get it let us know how it does for you.
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  • keyturbocars
    keyturbocars Solar Expert Posts: 375 ✭✭
    #11
    Re: Resting Voltage

    OK. After it arrives, I'll try to let you guys know how it works.
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