Wrong percentage JBD/Overkill

[JerzyR]

Hi!
I have a problem with readings in my Xiaoxiang/Overkill BMS that I'm using with DIY lifepo4 280 ah pack.
Below is a screenshot: cell 3.31, 92%. And according to the default Overkill settings it should be a little over 40%.
I have two questions for you:
Why BMS gives percentages that are inconsistent with the saved settings.
And is 3.31 more like 90% or 40%. There are many different charts for lifepo4 on the internet and I don't know which one is real.
Thank you all!

 

[Bob B]

Those settings are only for when the BMS isn't doing coulomb counting .... As long as you get to 100% once in a while, the coulomb counting is more accurate than the voltage settings which are just a guesstimate.

 

[time2roll]

OK let the battery rest 10 minutes disconnected and check voltage again.

 

[JerzyR]

Thank you for your replies! Information that coulomb counting is more accurate makes sense! As long as I get to 100% - what should be 100%? I think I have never seen anything over 3.5 on a single cell.
After the rest it was like this:

 

[SupraSPL]

Yes it is in the high 90s. These are the cell voltage.settings I am trying now.

 

[MisterSandals]

Can you describe your charging? Did it charge to some voltage (what voltage?) and stop or is there an absorb stage?

What charger and pic of settings very helpful too.

 

[42OhmsPA]

Thank you for your replies! Information that coulomb counting is more accurate makes sense! As long as I get to 100% - what should be 100%? I think I have never seen anything over 3.5 on a single cell.
After the rest it was like this:

Mine haven't seen 100% in days / weeks. They are all out of whack ?

I would set full somewhere around 3.4V per cell.
There isn't really any need to take them to 3.5V / cell, there is minimal capacity at the top of the knee / charge profile.

Yes it is in the high 90s. These are the cell voltage.settings I am trying now.

How are the they working for you?
Any reason you have full and 100% different?
I've messed with mine so many times, I can't get them accurate. I think the coulomb counter is way off... It doesn't help I have 3 packs in parallel and the house idles around 300w...

 

[MisterSandals]

I think the coulomb counter is way off..

It isn’t really coulomb counter. It is a calculation with available numbers for current, voltages and time to simulate what a real coulomb counter does.

I think they are low accuracy, high durability (yeah!) electronics that are used in most of these. I suspect BMSs have a reduced sampling rate than a real meter (less accurate) to reduce power consumption too. This is especially important in low voltage scenarios where the battery might be in LVD but the BMS still drawing power.
This ends badly in many cases.

 

[1201]

It isn’t really coulomb counter. It is a calculation with available numbers for current, voltages and time to simulate what a real coulomb counter does.

I think they are low accuracy, high durability (yeah!) electronics that are used in most of these. I suspect BMSs have a reduced sampling rate than a real meter (less accurate) to reduce power consumption too. This is especially important in low voltage scenarios where the battery might be in LVD but the BMS still drawing power.
This ends badly in many cases.

This is something I wonder about. The bms knows voltage at all times, amps in and out at all times -if it has that information shouldn't it know exactly how much power is coming in and going out?

 

[42OhmsPA]

It isn’t really coulomb counter. It is a calculation with available numbers for current, voltages and time to simulate what a real coulomb counter does.

I think they are low accuracy, high durability (yeah!) electronics that are used in most of these. I suspect BMSs have a reduced sampling rate than a real meter (less accurate) to reduce power consumption too. This is especially important in low voltage scenarios where the battery might be in LVD but the BMS still drawing power.
This ends badly in many cases.

That last part is concerning... One of these days I'll have to clamp mine. Somewhere I remember reading about current drawn from the contactor but I can't remember if it was open or closed...

 

[Bob B]

Thank you for your replies! Information that coulomb counting is more accurate makes sense! As long as I get to 100% - what should be 100%? I think I have never seen anything over 3.5 on a single cell.
After the rest it was like this:

It will reset to 100% when any cell reaches the CELL FULL VOLTAGE setting .... but, any time you write a setting change, the BMS will revert to the voltage guestimate again til it cycles to the cell full voltage again.

If you are occasionally getting to 3.5, set the cell full voltage to 3.45

 

[Bob B]

Mine haven't seen 100% in days / weeks. They are all out of whack ?

I would set full somewhere around 3.4V per cell.
There isn't really any need to take them to 3.5V / cell, there is minimal capacity at the top of the knee / charge profile.

How are the they working for you?
Any reason you have full and 100% different?
I've messed with mine so many times, I can't get them accurate. I think the coulomb counter is way off... It doesn't help I have 3 packs in parallel and the house idles around 300w...

When I first got mine, I had to calibrate the idle, charge, and discharge currents to get the SOC during coulomb counting to be accurate.

 

[42OhmsPA]

When I first got mine, I had to calibrate the idle, charge, and discharge currents to get the SOC during coulomb counting to be accurate.

Thanks. That could be a pain with multiple packs. Added to the project list.

 

[Bob B]

Thanks. That could be a pain with multiple packs. Added to the project list.

Yeah, once they are in service it's a lot harder. Might be able to do it with a clamp meter if the charge and / or load is fairly stable.
I did the calibration while controlling the charge and discharge current.

 

[bronxfire]

Please pay attention to its ability to sense discharge under 30 watts. Mine and many others dont sense any discharge under that amount of draw. So in theory if u only draw 25 watts for 2 weeks straight it would be 8400watts depleted and this stupid bms would still think 100%??. Its sole purpose is to manage battery and ita not accurate. Sorry im not satisfied with this bms

 

[SupraSPL]

I just measured the JBD SP16S020 V1.1 at .015A @ 26.4V so idle power draw with bluetooth dongle running is about 0.4 watts. After 60 seconds of no activity (no charging, discharging and with BT ap closed) it shuts and goes below anything I can measure with DC clamp. Even if the ap.is running in the background of my phone the Bluetooth remains active.

I just found the calibration section in the Overkill BMS ap but haven't messed with it yet. My clamp meter always reads about 5% low so I'll have to try that with actual inline Fluke ammeter. It will be interesting if there is a way to improve it's low power draw sensing I'm sure that is the main cause of drift. From what I have seen the charge and discharge readings seem accurate from JBD.

@42OhmsPA Im trying the 100% at 3.35 to try and get a more accurate reset when resting and that should be easy to dial in (99.5% resting basically).

I noticed the the generic lifepo4 normal and lifepo4 lite 20% 40% 60% 80% voltage settings are way off compared what JBD set from the factory. This is a screenshot of the custom settings JBD sent me, which is much more accurate and precise. If we can get a general consensus on these numbers we will have a much better chance of gauging SOC at any time with a quick rest.

 

[OffGridForGood]

Its sole purpose is to manage battery and ita not accurate. Sorry im not satisfied with this bms

actually the SCC has the job to manage the battery, the BMS is there to protect the cells if the SCC doesn't, or one single cell gets too high/lowells get too hot/cold / but the SCC doesn't know this information, it only knows the total pack/ESS voltage.
I had some issues with my batteries and SOC until I bought the Victron 500A smart shunt, then my life changed.

 

[SupraSPL]

This is something I wonder about. The bms knows voltage at all times, amps in and out at all times -if it has that information shouldn't it know exactly how much power is coming in and going out?

True and the voltage readings have proven to be accurate and precise. The current readings seem very accurate to me also, comparing with my MPPTs and clamp meter etc. If I can't get the BMS dialed in then I will just have to get more familiar with those precise voltage readings after a quick rest.

@42OhmsPA I have the same situation, December is not topping up my batteries often, there's are four in parallel and house idle power is 175-250. What are your voltage settings in Overkill ap?

 

[SupraSPL]

@JerzyR The settings in your picture are the generic settings that I noticed when I clicked some of the presets in the xiaoxiang ap. I think you will get better results when resetting capacity based on the ones I posted and I will keep working on it and see if I can figure out a repeatable way for us to get all of our JBD BMS dialed in better like Bobs.

 

[Bob B]

I just measured the JBD SP16S020 V1.1 at .015A @ 26.4V so idle power draw with bluetooth dongle running is about 0.4 watts. After 60 seconds of no activity (no charging, discharging and with BT ap closed) it shuts and goes below anything I can measure with DC clamp. Even if the ap.is running in the background of my phone the Bluetooth remains active.

I just found the calibration section in the Overkill BMS ap but haven't messed with it yet. My clamp meter always reads about 5% low so I'll have to try that with actual inline Fluke ammeter. It will be interesting if there is a way to improve it's low power draw sensing I'm sure that is the main cause of drift. From what I have seen the charge and discharge readings seem accurate from JBD.

@42OhmsPA Im trying the 100% at 3.35 to try and get a more accurate reset when resting and that should be easy to dial in (99.5% resting basically).

I noticed the the generic lifepo4 normal and lifepo4 lite 20% 40% 60% 80% voltage settings are way off compared what JBD set from the factory. This is a screenshot of the custom settings JBD sent me, which is much more accurate and precise. If we can get a general consensus on these numbers we will have a much better chance of gauging SOC at any time with a quick rest.

I doubt there is any way to improve the threshold required to trigger power draw ..... I had the same problem with my Chargery BMS.
I haven't experimented enough with the JK BMS to figure out how much low of current it draw it will detect.
I think the higher the amperage the BMS will handle, the less sensitive it is going to be for low current draw situations ..... also don't know how low of current the Victron shunts will measure ... but, they are probably better.