Accuracy of Overkill BMS SOC

wholybee

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Feb 11, 2021
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I have cycled my cells several times over several days and the ah's hardly changes. It's always within a couple of ah's but this is with low C rates. The built in coulomb counter might not be the most accurate but it is consistent in my case.

Based on my testing it might be off by a few ah's and this was compared to a DROK I have which uses a hall effect sensor. I don't have a Victron or anything that would be more accurate and I don't need it. The BMS is good enough for me and comparing the voltage to the ah's used I believe it is fairly accurate.

As was pointed out, changing any of the parameters will throw off the SOC and coulomb counting and the battery must be fully cycled in order to get accurate readings.
It has been several weeks since I hit full charge. My battery has ranged between 45% and 65% according to my Victron during this time. So neither the Victron or the BMS has had a chance to re-sync. As I write this, the Victron reads 56%, and the BMS 74%. That is a really big difference. I believe the Victron to be closer, but understand neither is going to be correct.

During a single cycle, if I discharge to a relatively low SOC and then fully charge, the Victron and BMS will be within about 2% of each other.

I agree, the BMS is probably fine for most people.
 

wholybee

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I can only speak to the BMS I have which came in the small 100 AH prebuilt battery I ordered .... I'm pretty sure it's the JBD (Overkill) BMS since it uses the same app.
When the BMS is initially powered up .... or any time a parameter setting is changed... the BMS calculates a guesstimated SOC based on the pack voltage. This guesstimate is derived from the cell 20-80% Cap. Voltage parameters ... and these parameters can be adjusted.
Once one cell reaches the cell full voltage, the BMS resets to 100% SOC and starts tracking via charge and discharge current ... which is coulomb counting.

I don't know if there is an internal shunt .... or if it is hall effect.

I had to calibrate the zero, charge, and discharge current thru the config menu in order to get the tracking to be accurate, and as someone else stated, it's not good at detecting small current draws and accuracy can suffer because of that if it sits with small loads for a period of time .... but ..... once fully charged again, it resets ... and now that mine is calibrated, it tracks pretty good. I just use that pack to power my boat electronics and the SOC tracking is working very will for that application.

Other shunt based BMS do the same thing .... some just do it better than others. I know my Chargery BMS, which is shunt based works pretty much the same way and has the same issue with not measuring small loads accurately. They have improved that, but it still isn't as good as it should be ... maybe in the new BMS if they ever get around to releasing it.

The Victron seems to be the gold standard for SOC tracking .... but my round Chinese knock off that looks a lot like the Victron works very good also and measures very small loads.
Probably the best explanation of what it does that I have read.
 

wholybee

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Feb 11, 2021
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The coulomb counting in the Overkill works ok, but isn't great. My guess is that over a few days it wasn't accurate, and it reset to what it thought was more accurate. Or, maybe you used more power than you think.

I also have a Victron 712, and after a week, the BMS is 10% different than the Victron. When I recharge, the battery reaches full charge and charging stops just as the Victron meter reaches 100%. The BMS is about 90% at that time, but will suddenly jump to 100% when it realizes that it is full.
Following up on my own post. I decided to try and fully charge my bank today, just to see how the SOC readings compare. Right now, the Overkill is reporting 100% charge, with a cell voltage of 3.39v. Very clearly not 100% charged, and very clearly not using the voltage to determine SOC. The Victron is 84.6%, and both the Victron and BMS agree that charge rate is currently 27A.
 

MisterSandals

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I am mostly just following along to learn what I can.
I do have a question that i cannot find an answer to:
The coulomb counting in the Overkill works ok, but isn't great.
How could "counting" not be very accurate? I have garnered from the explanations of how the Overkill "counts coulombs" as calculating amp flow over time. This is pretty straight forward math.

If its inaccurate, either the amp measurement or time measurement (really easy and straight forward) would need to be the cause, right?
 

Bob B

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Sep 21, 2019
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I am mostly just following along to learn what I can.
I do have a question that i cannot find an answer to:

How could "counting" not be very accurate? I have garnered from the explanations of how the Overkill "counts coulombs" as calculating amp flow over time. This is pretty straight forward math.

If its inaccurate, either the amp measurement or time measurement (really easy and straight forward) would need to be the cause, right?
There are 2 factors I have seen that will affect the accuracy.

1. The amp in and amp out or zero calibration is off .... is that really three things?
2. The BMS sometimes doesn't start registering current til over a certain amperage .... for instance maybe .5 or 1 amp.
So, over time with small parasitic loads, the SOC will be off. That would be the case in the example above where the BMS thinks the SOC is higher than it actually is.
 

wholybee

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Feb 11, 2021
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There are 2 factors I have seen that will affect the accuracy.

1. The amp in and amp out or zero calibration is off .... is that really three things?
2. The BMS sometimes doesn't start registering current til over a certain amperage .... for instance maybe .5 or 1 amp.
So, over time with small parasitic loads, the SOC will be off. That would be the case in the example above where the BMS thinks the SOC is higher than it actually is.
Those are as probably the most significant factors. In my case, my Victron and the BMS report exactly the same voltage and current, at least over .5 amps or so. But, are a small fraction off at very low currents. My only charging source is solar, and my loads are light, so fractions of an amp are the norm for me, with Solar powering my load all day and 10ths of an Amp trickling in or out of the battery.

Small almost insignificant factors for Lithium, but still there, are efficiency, and Phuket. When charging, there is some loss to heat. Lithium is better than 99% efficiency, so it is small, but over a long time without resetting to 100%, it adds up. Also Peukerts law, which says that at high C rates, a battery has less capacity than low C rates. Again, with Lithium, this is a very small effect, and can usually be ignored. But over a long period of time without resetting, it will add up. Victron allows user setting of these parameters, and I have set them to recommended values for Lithium.

I also have a Blue Sea SOC meter, which the Victron has replaced. The Blue Sea meter only has lead acid profiles and doesn't allow manual setting of Peukert or Efficiency. Not surprisingly, the Blue Sea meter is uselessly inaccurate.
 

joeblack5

Solar Enthusiast
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Dec 27, 2020
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I am very curious about the voltage accuracy of the JBD bms. anybody compared the cell voltages with a good dvm?
Preferably a 48V system

Thank you.
Johan
 

Alkaline

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Jul 26, 2020
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JBD have a value called remaining AH it stores an AMP remaining, pretty sure it uses this as part of its calculation
 

Alkaline

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Jul 26, 2020
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Yes it has an AMPS remaining register in the memory. but this isn't the only thing it looks at when calculating e.g. if you switch from LIFE to LION profile it will change the remaining percentage.
 

wild01

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Feb 18, 2021
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I double checked mine with an rms clamp meter, I have my bogart shunt hooked up as well on it's default settings. the bms was dead on with the clamp meter, but the bogart was only dead on in one direction, and off by a few amps in the other, I think this is because the bogart takes round trip efficiency into account, and displays current based on that math. I know I can reprogram it's efficiency % but I haven't gotten around to that yet. I also notice that the bms charting (click battery voltage-brings up chart) is only accurate if I leave the app running, doesn't seem to track internally in the bms.
 
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