Lifepo4 cell balance problem

[larrylwill]

Its been a year since I and installed my 280 amp 4s Lifepo4 battery and 150A BMS. I also installed a Renogy 60A DctoDc. I have a blutooth phone monitor.
I didn't drive miles in the last 2 years untill last week. I drove about 1100 miles. I noticed the following. My Lifepo4 battery cells start withen 20mv of each other. As I drove I watched the balance rise up to 164ma. The voltages were. Temp= 24c, V diff=0.164, Cell1=3.453, Cell2=3.399, Cell3=3.560, Cell4=3.542. Bms Cell OverVoltage was set to 3.65, Pack Over Voltage was set to 14.4v.
On the return trip I set the BMS Cell OverVoltage to 3.4 andPack Overvoltage to 13.8v Readings an hour later were Temp= 21.7 Total Voltage= 13.8v, Voltage Diff=0.173, Cell1=3.422, Cell2= 3.477, Cell3=3.508, Cell4=3.597.

While driving the battery Amp out drops to 0v after it is fully charged, the only thing running on the Lithium battery is the Fridge which draws about 5.5 amps when running. I also have solar which when the sun was out kep the battery drain at 0v most of the time. After stopping or living off the battery and solar for a couple of days the balance eventually comes back down to 20-40mv.

Question: Does this look normal?
If not what should I do?

[Winston]

Originally Posted by larrylwill

. . . V diff=0.164 . . . I set the BMS Cell OverVoltage . . . Voltage Diff=0.173

We are uncertain what the significance of setting a cell "overvoltage" level is. What does your BMS do when this cell overvoltage is reached?

A "real" differential cell voltage of .164 and .173 would be very alarming to us. However, that you started, and ended, with a comparatively low differential cell voltage of 20-40mV suggests that your cells are probably well-balanced (as reflected by the more normal differential voltage values seen after a period of 'rest') and that the higher differential voltages, while curious, may be inaccurate and reflect temporary 'distortions' in cell voltages occasioned by very recent by charging/discharging of the cells.

[@Michael]

I'd suggest making sure that the terminals and buss bars are clean, no corrosion, properly torqued, etc.

[larrylwill]

Everything was new and I did a parallel charge of all cells when I installed them. So they were put on and removed at least 2 times.

When one cell gets too high, which it does the BMS turns off all chrging.

[FredA]

LarryLwil,

You are talking about battery amps in terms of V. Which would indicate volts. I hope you mean the battery amps drop to an indicated 0 amps.

While your cells were idle for 2 years it would be expected they have a small self-discharge that would take them out of balance with each other. All that means is that they do not all have the same quantity of electrons in them.

Balancing Li cells is often confusing due to the relationship between state of charge (SOC) and voltage. When the cells are between about 10% and 90% SOC the cell voltage is mostly dependent on the cell chemistry and the cell current, amps. So, when the cells are less than about 90% SOC, they will have almost the same voltage. You are seeing this as 20-40 mv when you started and after they are used for some time. But that does not mean they are balanced.

When the cells are charged above about 90% of their capacity the cell voltage climbs very quickly. It is then that you will see a difference in cell voltage. The BMS has a set point for the voltage when it will try to balance the cells. You indicated the balance current was 164 ma. That would indicate your BMS was working on balancing the cells at the end of your 1100-mile trip. But the cells were not in balance as indicated by cell difference of .167 volts. Probably the BMS did not have enough time to finish balancing. You have 280 amp-hr cells. A current of .167 ma is small compared to the capacity of your cells. I had a pack in my RV that got out of balance, and it took several days on a charger to bring it back into balance.

While the BMS is balancing the battery current will be very low and may indicate zero. Your Renogy can produce 60 amps and the balance current was less than 0.2 amps, almost zero compared to the charge current.

You can take a longer trip or fake out the Renogy to think the engine is running. One approach would be to put a battery charger on the engine battery and then put 12 volts on the Renogy D+IGN input by wiring it to the engine battery. I expe

[FredA]

The last sentence was lost in my prior posting

I expect the cell voltages would converge and the balance current decrease.

[larrylwill]

Originally Posted by FredA

LarryLwil,

You are talking about battery amps in terms of V. Which would indicate volts. I hope you mean the battery amps drop to an indicated 0 amps.

While your cells were idle for 2 years it would be expected they have a small self-discharge that would take them out of balance with each other. All that means is that they do not all have the same quantity of electrons in them.

Balancing Li cells is often confusing due to the relationship between state of charge (SOC) and voltage. When the cells are between about 10% and 90% SOC the cell voltage is mostly dependent on the cell chemistry and the cell current, amps. So, when the cells are less than about 90% SOC, they will have almost the same voltage. You are seeing this as 20-40 mv when you started and after they are used for some time. But that does not mean they are balanced.

When the cells are charged above about 90% of their capacity the cell voltage climbs very quickly. It is then that you will see a difference in cell voltage. The BMS has a set point for the voltage when it will try to balance the cells. You indicated the balance current was 164 ma. That would indicate your BMS was working on balancing the cells at the end of your 1100-mile trip. But the cells were not in balance as indicated by cell difference of .167 volts. Probably the BMS did not have enough time to finish balancing. You have 280 amp-hr cells. A current of .167 ma is small compared to the capacity of your cells. I had a pack in my RV that got out of balance, and it took several days on a charger to bring it back into balance.

While the BMS is balancing the battery current will be very low and may indicate zero. Your Renogy can produce 60 amps and the balance current was less than 0.2 amps, almost zero compared to the charge current.

You can take a longer trip or fake out the Renogy to think the engine is running. One approach would be to put a battery charger on the engine battery and then put 12 volts on the Renogy D+IGN input by wiring it to the engine battery. I expe

You are talking about battery amps in terms of V. Which would indicate volts. (please explain your where)
I hope you mean the battery amps drop to an indicated 0 amps.
(I thought thats what I said)

The cell difference is 20-40 mv after resting after a drive.

Im talking Amps when I refer to amps and Volts when I refer to V. After any trip I plug in the RV and a battery charger charges the battery untill the charger is satisified that the battery is full, or not if the battery is already fully charged.
Why would i connect a lithium to a AGM battery and charger, The charger will stop charging when the AGM is full. The RV has 2 alternators, one for the engine battery and one for the House battery.

[FredA]

Larrylwill,

The first line of your second paragraph is "While driving the battery Amp out drops to 0v". That is the basis for my amps vs volts statement. No matter if we are talking the same language.

In your original post you did not mention the shore charger or the second alternator. So, I assumed you were using the Renogy to charge the Li battery from the engine power. That is what Renogy 60 amp manual describes as its use.

Your saying the cells are matched after a rest is what is confusing to me. What do you mean by "rest". The cells should balance at the end of the absorption cycle when the pack voltage is at max charge voltage.

My coach has a 4s6p battery system with each series string having an individual balancing system. The absorption voltage is set to 14.2 volts on both the inverter/charger and the three stage alternator controller. When the battery voltage is 14.2 the min and max cells voltages from all 24 cells is 3.53 and 3.57 volts. So, mine are balanced within 40 mv at a charging voltage. As soon as the pack voltage drops to 13.6 volts the min and max cells are 3.38 and 3.40. The cell voltages converge at the float voltage.

One can only see an out of balance condition in Li cells when the cells are at the maximum charge voltage. As soon as they discharge a small amount, they will read very close to the same voltage.

The first graph in this link https://www.powerstream.com/lithium-...ge-voltage.htm shows the discharge voltage as a lithium cell is discharged after being charged at different voltages. Since they were charged at different voltages, they have a small difference in the state of charge. All the ones that were charged at 3.4 volts or above have the same voltage for most of the discharge cycle. That is why the balance can only be determined at the maximum charge voltage.

[larrylwill]

Ok I see how that could be misconcrewed. Here it is in other words. Before driving the cell difference is 20-30mv. While driving it gets > 160mv. After stopping after an hour or 2 it goes back to 20-40mv. I got the Renology because yje 2nd alternator puts out 280amps, but the BMS is only 150Amps. I did not set up the BMS or the AC charger for max. Somewhere around 14V. zif I set the BMS to 14.6v one cell always cuses the BMS cell voltage to shut down the charge, I never recorded the voltage but I know it was close to the top. I guess I need to do some more testing. I will change the BMS to what you suggested and also the house charger and the Renology and take more notes.