Winston LiFePO4 battery problem?

[SOLAR SUPPORT]

Originally Posted by Eirik

Hi,
In my boat I have the following LFP battery bank: 8 Winston 300 Ah in a 2P4S setup, yielding 600 Ah/13V. The bank/setup is further described further below.

During 2019 I have noticed that battery cell set #4 (i.e. 2 x300 Ah cells) has started to behave a bit off: During charging it has a higher voltage than the other cell sets, and during discharge the cell set’s voltage is lower. The difference increases in relation to the rate of charge or the discharge. If the charge current is around 6-10 A (i.e. 0.01-0.02 C), cell set #4 is about 10-20 mV above the other sets. (i.e. 3.165V vs. 3.15 V), but if the charge current is 70 A (i.e. around 0.12 C), the voltage difference increases to 100 mV (3.25V vs 3,15V). The same differences occur when discharging, but then of course in the other direction, and with roughly the same magnitudes.

Furthermore, the voltage on cell set #4 also seems to flutter a bit, i.e. at times (lasting around 10 sec) the difference is reduced, but then increases. All this is measured by the BMS’ volt meters, and possibly follows the refresh rate of the BMS.

Finally, as the battery bank gets fuller when charging, the difference in voltage increases further. This has also resulted in the BMS kicking in and triggered the HVC contactor (to stop charging) because the cell set reached 3.7V (max cell threshold), whilst the others were at around 3.37V (i.e. 13,8V), and the bank hence not being fully charged, with a reported SOC around 80%.

I am thus wondering what the issue could be. Are the cells out of balance (which doesn’t make that much sense since the voltage goes in both directions depending on charging or discharging), or could it be that one or both of the cells in the pack are damaged? Open for suggestions for figuring out the issue and possible remedies.

Information about the bank:
The bank was commissioned in a sailing boat in May 2018, and then top balanced at 3,65V per cell. It has not been balanced since commissioning. I use the REC Active BMS to protect the bank, which also does active balancing. Charging is mainly done via the Mitsubishi alternator (standard on VP engines), which I have reduced to 80 A (Using VRC 100 from Nordkyn), and after deducting from the boat’s overhead, rarely giving more than 70A to the bank. In addition, I have 2X100W solar panels, yielding rarely over 10-12 A (Victron MPPT 75/15 controller), and a Victron Multiplus 1600-70-12 which is rarely used for charging. Max voltage on all charging sources is 14.0 V, and “float”/”hold” is set at 13.3V. Absorption (14.0V) is at max 30 min. In practise, it is the alternator that fills up the bank (solar charging will usually at best cover current consumption, and the Multiplus with shore power is hardly used).

The bank has (apart from the issue described above) worked nicely and also provided plenty of juice for both the usual 12V DC applications as well as 230V AC applications via the Multiplus. The latter is regularly used, and may then draw up to 130A (0.2 C), but usually less. The SOC is rarely brought below 50%, and only occasionally up to 100%. Usage has been for coastal day sails, weekend sails and a few 1-3 weeks sails during summer season. All charging and discharging sources are usually disconnected when the boat is at the dock, and the bank is usually kept at an SOC of 70%-80% (and 50% in winter, when it is not being used).

SOC is calculated by both the BMS and a Victron BMV 712 (peukert 1.05), and they remain very close, usually within 1-2% percentage points. I have not done a capacity test of the bank.


Thanks,

Eirik

I experienced a similar problem with 2P4S, 2x200Ah drop-in Lifepo4 battery bank. You have the tool, active balancer to solve the problem. But you need to use the right charger which has voltage and current adjustments can be made manualy.

My active balancer, which I added to my batteries later, had a balance current capacity of 1A. I used this charger to bring the 13.7-13.8 volt Hvcutoff threshold back to its original level, which the batteries had decreased in a year. First of all, I separated the batteries one by one to balance them. It is necessary to charge the battery with a charging voltage of 0.05-0.1 volts above the bms Hvcutoff voltage with a current of 1A. Each step was completed by monitoring the cell voltage balance obtained through the smart active balancer for 2-3 hours and increasing the charging voltage by 0.05-0.1 volts at each step. It took about 24 hours for the active balancer to complete its job.

After this procedure hvcutoff level of the batteries is arround 14.4-14.5 volts. Each battery has its own active balancer that communicate through the Bluetooth with the phone app. When at sea, I wait around sunset for a top balance charge of Lifepo4 bank. The cells are balanced at the target peak voltage with the charging current taken from the solar energy system at a level of 2A, occasionaly.

[CaptainRivet]

Originally Posted by CatNewBee

Hi Erik,

Sad to hear that. You need to track down and troubleshoot your setup first and then find a solution.

The voltage difference while charging and discharging, resulting imbalances etc. always results of differences in resistances. And this means not only the obvious cell inner resistance, but also all contacts, bridges, screws, BMS contacts on the cells etc. The smaller cells like the one you use have only a single contact per battery pole and are very vulnerable to contact errors.

The other thing is, the LiFeYPO4 cells are non linear regarding cell resistance on the shoulders, when a cell reaches this SOC, the resistance rises almost exponentially, small changes result in big jumps.

Cells between 20 and 90% SOC have usually some mOhm, while the contacts have some 10 to 100 mOhm because of surface contact and pressure, material, size of the bridges, a little more or less tension, unclean or uneven surface on a single connection can lead to less charge on a paralleled cell, what will cause in capacity imbalance. Just draw a circuit diagramm of yout battery and add on the battery cell and each connection a symbol for a resistor, you will see a meshed resistor matrix, small changes lead to different voltages and currents.

So empirically you can start by firing up your inverter under heavy load and touching each screw and bridge on the battery by hand, try to feel temperature differences. If there is a warmer connection, tighten the bolt a little more or clean the contact. If all cells are equally warm, you likely have a problem either with a cell or the BMS.

To sort out the bms you can use a clamp amperemeter and check there is a balancing current when charging above the start balance voltage on one cell. If it is, the BMS is fine.

Next would be to check each individual cell, you need to disconnect the pairs and do a test. If you have a notorios low cell, that always is charged and one that always is discharged, you can swap the partner cell, to make both equal capacity.

I have similar issues when the battery is near full, my inner cells are always a little different then the outer cells of the pack, on high charge current about 200A (0.2C), and full battery it can be 90..100mV, but this is normal, very little SOC differences lead to dramatic resistance changes, with smaller currents the voltage difference is smaller. Your BMS balances with 2A, it is one percent of this current, you see the voltage jump of few millivolt in this SOC area on the BMS when the balancer is active or turns off to measure. It is normal there, but it should not lead to disconnects. If your BMS disconnects, there is either a misconfiguration or a big imbalance. You have to allow some margin for balancing, but 3.7 vs 3.37 is definitely out of range.

Eric,
Thats exactly the right way to check what is described above. I faced a similar issue with cell pack 1 with my 272AH Lishen cells in a 4P4S bank and tackle.d it yesterday. Its most likely a contact issue

Best way is if you have an IR temp meter, if not they are 20Euro in home depot and a must have tool on board.
Then as cat new bee suggested prepare high loads in the 200A area, i used the hot water boiler and the convection oven via inverter. Also how much torque does manual state, if remember right they are M8 so around 14NM.
Before you apply the big loads measure all busbars and especially the cell studs in temp with the IR meter. Are they all even or differ already.
Well if they differ and where its hotter you have contact issues.
No apply the big loads and measure especially the busbars and cell studs.
With 200A you will quickly see where the issues are as where you have contact issues it will heat significantly up which your IR meter shows you.

What i found in mine that work over 16 month up to 500A without issue before and how i fixed it. Repeat my way and most likely problem solved.
1) Shunt, that side that is conntect directly to therminal heats up above temp, the bolt connecting the terminal to shunt even more like a candle.=> M10 bolt loosened over time, tighting it down with 20NM twice solved the issue. Fixed it with a drop of nail laquer on the corner of the bolt. secures it from opening again but you can easy remove it.
That heat was transferred to the cells busbars and cells and made it worse. All gone.
2) in cellpack 1 the cell studs of cell 1 and 2 heated up like candles. Tighting 3 times the m6 flangenuts down with 4,5 and last 6NM made it a bit better but didn't solve it. Fluctuation got better..dismantled the busbars, sanded the terminal and busbar surface on all 4 cells with 1000 grid wet, cleaning with 98% alcohol and applied No-al-Ox paste and screw all down again with 4 and 5NM. Then measured again, all even temp.
3) now the one i wouldn't have found if i didn't check all, cellpack 3: cell 2 was above 2 degrees but cell 4 3 degrees below all the others in temp. Obviously cell 4 has the most capacity of all and it backed up cell 2, so nothing obvious to the outside, clear downside of having 4p. I tightened down cell 2 and nothing changed. Took busbars of and sanded all 3 terminals and busbars and applied No-Al-OX but left cell 4 untouched. Now cell 2 is even with the others, cell 4 still 2 degress less then all others.
Now i charged it completely full which took longer then expected, looks like there was some significant AH missing due to above and BMS was fooled. Now when pulling large loads they all heat up even,except pack 4 cell 4 which is lower. Fluctuating very small and cell voltages quite even, clear the above screwed up balancing, so i run it via shorepower charger and active balancer in the 98%-100% overnight and that brought them back in balance.

Key is apply 200A load and measure, wait 3 min and measure again, and wait another 3 and measure 3rd. So things heat up and you can also measure more precise whete the heat comes from. Then let all cool down and fix the issues you found.
Then start this procedure again.