Understanding Battery State of Charge

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OffGridRory
OffGridRory Solar Expert Posts: 26 ✭
in Solar Beginners Corner #1
I've found myself wondering the % charge state of my battery bank... When I put in about 100Ah/day and the charge controller seems to damper down the 400w input from the solar panels to about 50w (assuming this is because it is in absorbtion mode as it claims) my voltage drops to about 13v shortly after dark... Is this considered "full" for a 12v battery bank? After a couple hours of moderate usage my voltage drops to about 12.8 and sometimes 12.7... I'm trying to not use more than 110Ah of my 440Ah battery bank a day so I do not exceed the 25% usage cycle very often so these batteries last me longer... The problem is that I'm not sure exactly when I hit the 75-80% remaining charge on my batteries.

It would be GREAT if someone could "dumb-down" roughly what % my battery bank is down to according to the voltage reading on them... Is 13v "full" 12.8 75%? etc.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
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    Re: Understanding Battery State of Charge

    You mean like in the Battery FAQ's:
    http://www.windsun.com/Batteries/Battery_FAQ.htm

    A "12 Volt" battery is normally about 12.7 Volts at rest (no loads going out, no charge coming in) when fully charged. The best way of determining SOC is with a hydrometer, of course. But you can't stick a hydrometer into an AGM.

    If you want a useful gauge for determining how much power you've got while your using it battery monitors are a good idea: http://www.solar-electric.com/metersmonitors.html
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  • nsaspook
    nsaspook Solar Expert Posts: 396 ✭✭✭
    #3
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    Re: Understanding Battery State of Charge
    OffGridRory wrote: »
    I've found myself wondering the % charge state of my battery bank... When I put in about 100Ah/day and the charge controller seems to damper down the 400w input from the solar panels to about 50w (assuming this is because it is in absorbtion mode as it claims) my voltage drops to about 13v shortly after dark... Is this considered "full" for a 12v battery bank? After a couple hours of moderate usage my voltage drops to about 12.8 and sometimes 12.7... I'm trying to not use more than 110Ah of my 440Ah battery bank a day so I do not exceed the 25% usage cycle very often so these batteries last me longer... The problem is that I'm not sure exactly when I hit the 75-80% remaining charge on my batteries.

    It would be GREAT if someone could "dumb-down" roughly what % my battery bank is down to according to the voltage reading on them... Is 13v "full" 12.8 75%? etc.

    Because the State of Charge is a chemical process, you can only get so much information from just voltage, even the rest voltage of a battery is a rough estimate of SOC. A battery monitor must measure current , voltage, know the rated Ah of the battery, know the Charge Efficiency Factor (how much current is stored as usable energy when charging.) , know the Peukert factor (A fast vs slow discharge factor) and several other factors like temperature and internal resistance to make a educated guess. Invest in a quality monitor if you really need the information or make your own if you enjoy electronics and software.;)
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  • NEOH
    NEOH Solar Expert Posts: 74 ✭✭
    #4
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    Re: Understanding Battery State of Charge
    OffGridRory wrote: »
    It would be GREAT if someone could "dumb-down" roughly what % my battery bank is down to according to the voltage reading on them... Is 13v "full" 12.8 75%? etc.


    SOC Voltage
    ==== =====
    100% = 12.7v
    90% = 12.5v
    80% = 12.42v
    70% = 12.32v
    60% = 12.20v
    50% = 12.06v
    40% = 11.9v
    30% = 11.75v
    20% = 11.58v
    10% = 11.31v
    0% = 10.5v

    Do you have a Digital Volt Meter with an accuracy that is better than "+/-1% Full Scale" on the 20 Volt scale? If not then your DVM reading of 12.32v (70% SOC) means the battery is actually somewhere between 55% SOC and 90% SOC. Not that good!

    You'll need a DVM with an accuarcy of "+/-0.5% of Actual Reading" to do what you are attempting to do. Now your DVM reading of 12.32v (70% SOC) means the battery is actually somewhere between 65% SOC and 70% SOC. Very good!
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #5
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    Re: Understanding Battery State of Charge

    neoh,
    you've just confused soc and dod which are exact opposites. that 12.32v is at 70% soc and not dod. 70% soc is the same as 30% dod.

    also, these readings are the at rest readings, meaning the battery has not been charged or discharged in about 3 hours or preferably more. a battery being given a charge or discharge will sway the voltage + or - from the actual at rest voltage. that means if you are discharging with say a 20% of c load that the voltage will dip farther than normal. once that load is disconnected the voltage will start to rise again and will settle in time to the actual voltage known as the at rest voltage.
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #6
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    Re: Understanding Battery State of Charge

    As Niel suggests,, all voltage readings are taken 'at rest', neither charging or discharging, hence the problem. One can almost never have a time where a battery is neither being charged nor discharged.

    Get a Battery monitor like a Tri-metric from Bogard, use the %full and the ah in/out gauges to gauge your own battery. After a while, it becomes sort of routine.

    Also, consider the following link:

    http://www.batteryfaq.org/

    Tony
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  • OffGridRory
    OffGridRory Solar Expert Posts: 26 ✭
    #7
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    Re: Understanding Battery State of Charge

    Thanks for the tips. I probably do not even need a battery monitor quite yet as I usually get home from work around 3-4 hours after dark every day so I can take a look at the voltage reading before I flip the switch on the inverter. In the future I plan on getting a battery monitor though, sounds like it will become part of my routine to know exactly where my battery SOC is at... I've found since I've set the system up that I have become rather obsessed with looking at the charge controller every 10 minutes on my days off to see whats coming in etc. So I'm sure a battery monitor will just give me something to look at during dark hours :)
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