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SoC% false accuracy? State of Charge
#1
I just read this on Battery University
Source 1: https://batteryuniversity.com/learn/arti..._a_battery
Source 2: https://www.mpoweruk.com/soc.htm

" The EBM (electronic battery monitor) works well when the battery is new but most sensors do not adjust correctly to aging. The SoC accuracy of a new battery is about +/–10 percent. With aging, the EBM begins to drift and the accuracy can drop to 20 percent and higher. This is in part connected to capacity fade, a value most BMS cannot estimate effectively. It is not an oversight by engineers; they fully understand the complexities and shortcomings involved. "

This brings up the question:

Why use a more expensive BMS systems or hardware like shunts to measure SoC% on battery packs with second life Li-ion cells?

Concluding from the quoted text, it seems that measuring SoC% on second life batteries gives false accuracy for a high cost.

thoughts and ideas??

edit 1: added another source with similar statements around SoC inaccuracy for aged cells.
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#2
More expensive BMSs tend to use other, more accurate, means to calculate SoC, such as coulomb counting.
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#3
Coloumb counting with proper calculation and it works down to 1% accurazy or something
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#4
(03-14-2019, 02:15 PM)GeneralDJ Wrote: The EBM (electronic battery monitor) works well when the battery is new but most sensors do not adjust correctly to aging..

"Most sensors" is the key term here. Most are usually cheap and are just based on voltage curves. More advanced and expensive ones will use a coulomb counter. The key to the coulomb counter is that a full charge and discharge are usually required to calibrate them every once in a while. Most phones have a coulomb counter as well.
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#5
Issue with typical joule / coulomb approach is it's your battery cycle efficiency that limits the accuracy unless you reset the counter each charge cycle.

You may put 5600Wh of energy in, but your litmium cells may only return say 5500Wh, after that one cycle your count position is out by 100Wh. This efficieny loss is dependant upon the cell loading so will vary cycle to cycle. Coulomb counting always drifts unless the counter can correct for losses at a given loading or charge rate.

Cycle that 10 times without calibration and your accuracy is possibly close to 20% out.

There is a different much simpler way... voltage to Wh lookup table.

The relationship between charge and volts is not linear and is different for different battery chemistries. But a single controlled cycle can give you an accurate lookup table. Much simpler. Cheap as a piece of A4 paper stuck on the wall next to a volt meter.

That's what I use as a means of calibration for my Wh meter...
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If you can't quantify how much they cost, it's a deal, I'll buy 5 of them for 3 lumps of rocking horse ......
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#6
(03-15-2019, 12:17 AM)completelycharged Wrote: There is a different much simpler way...  voltage to Wh lookup table.

The relationship between charge and volts is not linear and is different for different battery chemistries. But a single controlled cycle can give you an accurate lookup table. Much simpler. Cheap as a piece of A4 paper stuck on the wall next to a volt meter.

Isn't battery voltage also related to temperature & current at the time?  And with LiFePo4, the "curve" is very flat.

My 100% SoC readings are set up to get reset most charge cycles when the cells have been at "100%" for a while & current dropped away.
For longer term SoC battery total capacity loss with years & use, etc, I guess this would require maybe a once a year measured full cycle to empty (by cell voltage), then recalculate the slightly reduced battery total Ahrs & reset SoC devices accordingly?

I have both Batrium & a Victron BMV-712 tracking SoC (both do coulomb count method) & they don't seem to be drifting in SoC + they usually match pretty well.
Eg, right now they are reading 85.9% (BMV) & 87.0% (Batrium)
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#7
Voltage wont work on LiFePo4 as example. They are flat in the middle and drift perhaps 20mV when it goes 10% SOC.. And this is in resting state and left for hours... So impossible to do it properly.
LA is same thing. If you look at that during a discharge you will see the SOC jump all over the place on those using voltage. Voltage is just fine during resting and on the chems that have a pretty linear curve and not Life Smile


I use coloumb as well and had Victron before and now I use 3x Batrium. They for sure do not drift so it gets noticeable at all.

The Victron ran on LA that do have easy 10-30% loss during charge and that one did cope with it pretty well due to it "resetting" during charge up keeping track on where it was.
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#8
If there is a reason to use the state of charge, then I would, but my PIP doesn't care about the SoC. All it cares about is the voltage, therefore I really don't see the real need to calculate the SoC.

It is quite a problem, because during heavy loads, like >1500W, my voltage will drop by 1V. So if I was to set the cut-off voltage at 25V, my battery would bounce back to 26V. At rest 25V should be 0% SoC, but at 26V is about 28% SoC. So it is a bummer that the PIP doesn't use SoC.

In any case, with laptop chemistries, the SoC can be determined with a fair accuracy. The graph is quite linear between 80% and 20% SoC, going from 4V to 3.5V.

So I just worked up some math to see if it's possible:

Battery Max = 28.5V
Battery Min = 25.0V
Voltage Drop @ 1000W = 0.5V

Current Status
Current Voltage = 27.5V
AC Usage = 200W
PV OUtput = 1200W

Voltage Compensation = (PV Output - AC Usage) / 1000 * Voltage Drop@1000W = (1200 - 200) / 1000 * 0.5 = 0.5V

SoC = (Current Voltage - Voltage Compensation) - Battery Min / (Battery Max - Battery Min) = (27.5-0.5) - 25 / (28.5-25) = 57.14%
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#9
Voltage is doable as long as you have resting voltage yes. Or close to. Thats how You do in RC. During load is something else. Then you need a counter and then you have voltage as a dead end check and in case over current too.

My MPP inverter do SOC based on voltage but it jumps all from 20-80% when used. Due to it goes by voltage Smile

On RC you do the same. Voltage on resting is ok but during running the system you generally do it with a counter instead for better accurazy. But as you know the counter need to take into account the current for it to be close enough.
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#10
Uh if you bothered to read the rest of my post that's why I have a voltage compensation during load in my formula. All you have to do is figure the voltage drop at 1000W load and it should extrapolate the rest.
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