lifePo4 State of charge

[dreuge]

Originally Posted by cutter3322

Hello, I currently have 3 lifePo4 batteries connected in parallel and I would like to know the state of charge of each. What device do you suggest to me? Thank you

I connect to my 560AH LiFePO4 battery directly via Bluetooth, but I also have a cheap $40 battery monitor which works surprisingly very well. It also tracks within a few percentage of what the BMS reports. It is a QWORK Battery Monitor with a shunt for 0-350A.

These are very affordable that you could install one on each battery or just monitor the whole.

I described my LiFePO4 upgrade on my blog: https://svjohannarose.blogspot.com/2...e-battery.html

Unfortunately I did not include the QWORK BM in the discussion as I originally installed it for my old FLA batteries. Try searching "QWORK Battery Monitor" on Amazon or try the link direct below.

https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

[Davidhoy]

Originally Posted by dreuge

This is very interesting. I also use the JBD-SP04S020 BMS. I just purchased a new one with Bluetooth and RS485 communications. I have been using the standard iOS app for Bluetooth communications and I am developing the RS485 to rasp pi for remote monitoring. As I'm learning along the way, I am looking forward to your open source project.

The nice thing about using Bluetooth is that it's wireless. My ESP32 device is powered off the NMEA 2000 bus, so you only need to connect one cable to it. And by mounting the device near the batteries, any concerns about Bluetooth range are largely irrelevant. You could do much the same thing with your Raspberry Pi with the addition of a CAN HAT, except that the Pi uses a LOT more power than the ESP32-based device, and takes a lot longer to boot (my device and code is up and running in less than two seconds, and most of that is the BLE scan and connection.

If you're interested in potentially working with me on this project (it sounds like you have some expertise on the matter), drop me a PM with your contact details and we can take this discussion offline.

Regards,
David

[Philtao]

Originally Posted by T1 Terry

If you want to see a photo of just what the inside of a quality Winston LYP cell looks like, let me know and I'll post a photo of one with the top off so you can see just how the plates are connected to the terminal plate ....

Yes, please I would like to see what is inside my Winston cells

[Philtao]

Originally Posted by T1 Terry

All active cell balancing equipment rely on cell voltage and this is what the balance from one cell to the next is, moving capacity from the high voltage cell to the low voltage cell. A cell that drops it's voltage lower than another cell in the series string while discharging, will constantly be topped up by the active BMS, even if the SOC was the same before the active balancer got involved, yet the same cell will have a higher voltage when charging causing the BMS to try and move capacity back out of the cell to the other cells. This can even show up as an issue between either charging or discharging and rest voltage. The cause is simply a different internal resistance, nothing wrong with the cell but that is not the way it looks when monitoring the cell voltages.
T1 Terry

This is why I have seen a number of good quality packs (Winston) that are out of balance after 12 months (up to 40% capacity loss!). Those BMS / Balancers that use the voltage to trigger charge and/or discharge of cells will unbalance the cells if there is a small variance in cell internal resistance (there re always). And if the cells are of poor quality with important variance in cell internal resistance, this can be very fast.

The BMS I use has a stable algorithm to estimate cell internal resistance (not the proper method using pulse, but good enough to have valid relative values - that are stable over time). It uses the open cell voltage of each cell (measured voltage compensated for current and internal resistance) to decide if balancing is required.

My opinion is that if your balancer uses the measured cell voltage for balancing decision, you might be better off without it. Unless it only balances above 3.50 or 3.60 VPC when the current tappers and the cell resistance has little impact on the voltage. But as battery does not stay at that voltage very long, only very little balancing can take place.

[team karst]

To measure Rs of a cell, it would seem to be necessary to need to load the cell (or charge the cell) a bit, and also measure Voc with zero current. Mathematically, solving two unknowns with two independent equations. Yes, could be pulse or steady state, presumably. Actually, you could do two different current states the get SOC, not necessarily zero current. One issue being the BMS is not "in charge" of what current is being drawn/charged, and must wait for external loads.

[EngineerRetired]

Doesn't really matter. Assuming the batteries are healthy and installed properly with equal length leads, the three will self balance their state of charge. Say one has a lower SOC than the other two, those two will in essence charge the third battery to the point all three have the same voltage.

[goboatingnow]

Originally Posted by EngineerRetired

Doesn't really matter. Assuming the batteries are healthy and installed properly with equal length leads, the three will self balance their state of charge. Say one has a lower SOC than the other two, those two will in essence charge the third battery to the point all three have the same voltage.

Of course cells exhibiting the same voltage doesn’t mean they have the same SOC.

[goboatingnow]

Originally Posted by Philtao

This is why I have seen a number of good quality packs (Winston) that are out of balance after 12 months (up to 40% capacity loss!). Those BMS / Balancers that use the voltage to trigger charge and/or discharge of cells will unbalance the cells if there is a small variance in cell internal resistance (there re always). And if the cells are of poor quality with important variance in cell internal resistance, this can be very fast.



The BMS I use has a stable algorithm to estimate cell internal resistance (not the proper method using pulse, but good enough to have valid relative values - that are stable over time). It uses the open cell voltage of each cell (measured voltage compensated for current and internal resistance) to decide if balancing is required.



My opinion is that if your balancer uses the measured cell voltage for balancing decision, you might be better off without it. Unless it only balances above 3.50 or 3.60 VPC when the current tappers and the cell resistance has little impact on the voltage. But as battery does not stay at that voltage very long, only very little balancing can take place.

But you only really need top balancing , there’s no need to keep cells in sync as they discharge

Bottom balancing is a nice to have in my opinion but since we try and avoid near discharged batteries it’s not that useful

[goboatingnow]

I’m confused here. I designed a passive cell balancer. In my case I have a “ balance voltage “ just below the HVC stop point.

The bms is in between each series cell

All I do is bleed charge from the first cell to reach the balance voltage

As all cells then reach the HVC charging stops.

This just goes a top balance.

Active cell balancing certainly fron an electronics definition merely means the waste energy from the bleed processes are put through a dc dc converter and Re used to charge either another cell or the series string. It doesn’t typically mean continuous balancing

[EngineerRetired]

Originally Posted by goboatingnow

Of course cells exhibiting the same voltage doesn’t mean they have the same SOC.

Key part of my statement, "Assuming the batteries are healthy and installed properly".

[team karst]

Yeah, popular confusion. Currently marketed SOC boxes need to have you enter the Ah of the bank. Deterioration of cells is not factored into the equations.
Now, you could design them to discharge the batts occasionally to actually measure Ah.

High end public safety handheld radio chargers do just that.

[Philtao]

Originally Posted by goboatingnow

I’m confused here. I designed a passive cell balancer. In my case I have a “ balance voltage “ just below the HVC stop point.

The bms is in between each series cell

All I do is bleed charge from the first cell to reach the balance voltage

That should be fine as the charge current is very small when the cell voltage is "just below the HVC stop point"... and you start with well top balanced cells... and the chargers are set to charge at (HVC * number of cells in series)

If you start with unbalanced cells, that method will not give much time to balance, and may not be effective.

Usually the bulk / absorption voltage for the chargers is below (HVC * number of cells in series). So the cell voltage should not reach HVC in normal charge. The BMS is here as a safeguard in case the charger fails - usually not as a charge regulator to replace the charger.

As all cells then reach the HVC charging stops

If the cells are not well balanced to start with, the first cell will be well above HVC in danger zone before the last one reaches HVC and the BMS stops the charge. That cell will age faster... its capacity will fade faster... its internal resistance will increase faster... and the unbalance will increase over time.
Good practice is to stop charging as soon as one cell reaches HVC.

[Philtao]

Originally Posted by team karst

Yeah, popular confusion. Currently marketed SOC boxes need to have you enter the Ah of the bank. Deterioration of cells is not factored into the equations.
Now, you could design them to discharge the batts occasionally to actually measure Ah.

High end public safety handheld radio chargers do just that.

Yes a good BMS has criteria to know when a battery is empty (like minimum cell voltage) AND the user needs to regularly (6 months?) empty the cells to that level so that the BMS can measure the actual usable capacity.

As we each use our charging parameters (13.8V - 14.4V?) a full charge and a full discharge must be performed with the standard operating conditions and set the nominal battery capacity equal to the measured capacity.
When capacity is measured again, that new measure is used for SOC calculation and any variance is reflected in the battery state of health (SOH)

[goboatingnow]

Originally Posted by Philtao

That should be fine as the charge current is very small when the cell voltage is "just below the HVC stop point"... and you start with well top balanced cells... and the chargers are set to charge at (HVC * number of cells in series)



If you start with unbalanced cells, that method will not give much time to balance, and may not be effective.



Usually the bulk / absorption voltage for the chargers is below (HVC * number of cells in series). So the cell voltage should not reach HVC in normal charge. The BMS is here as a safeguard in case the charger fails - usually not as a charge regulator to replace the charger.







If the cells are not well balanced to start with, the first cell will be well above HVC in danger zone before the last one reaches HVC and the BMS stops the charge. That cell will age faster... its capacity will fade faster... its internal resistance will increase faster... and the unbalance will increase over time.

Good practice is to stop charging as soon as one cell reaches HVC.

Hmm not really what happens

As the balance bypass trip voltage is just below the charge stop voltage , my experience is the current in the cell reaching the balance point is low and the bypass mechanism means the charge voltage can continue for as long as necessary without moving the out of balance cell to the charger trip voltage

Yes I stop charging if any series cells reaches HVC. But with the bypass mechanism active for all cells you’d never actually reach HVC. ( 2A bypass )

However I would always recommend a manual cell balance at least once every 12 months and always on new cells.

In practice rarely find the cell fully balances using the bypass technique anyway as there not enough time before the system loads start discharges.

[Paradox111]

Here is a youtube site testing LifeP04 (and comparing SOC) charging at 3.4v vs 3.5v
https://youtu.be/pijPu7t-akM