Questions about low voltage in one cell.

[00X]

Hi. We have a single LFP battery. We have only had it installed for about six months. We have not been using the boat heavily so it gets used infrequently right now. Since its been installed we have only charged it about five times.

I have been charging it at 13.8 volts until I see all the individual cells hit 3.45 volts per cell. At this point i let it continue to charge for about half an hour once this voltage is reached on every cell. For the first few charging cycles this went perfectly. Charging and discharging has been even.

On the last re-charge I noticed that one of the cells would not get above 3.43 volts. While at this voltage, the highest cell got to as high as 3.692. I have no idea what the BMS high voltage disconnect is currently set to but the BMS did not disconnect. The battery has an internal BMS.

Anyways, I discharged the battery with the inverter down to about 80% SOC and tried re-charging at a higher voltage (14.2) and held this voltage for 45 minutes. The highest cell voltage reached over 3.69 again but then quickly settled to about 3.685. The lowest cell never got above 3.43 during this 45 minutes absorbtion.

This battery has passive balancing but this is almost a .3 volt difference. I know many people set their BMS with a high voltage of 3.65 per cell cutoff. I need to contact the manufacturer and ask what their BMS are set to but I would have assumed it would have cut off charging to protect a cell from overcharging, but am not sure what is an acceptable high voltage. Is a cell staying over 3.65 during charging a serious concern or more of a general practice to avoid? What would be considered an absolute high cell voltage you would start to worry about?

What sort of voltage ranges can we expect during charging LFP batteries that use passive balancing? Would you recommend regularly charging at a higher voltage of 14.2 ish instead of 13.8 for this type of LFP (passively balanced -vs- active) or is a higher voltage better at top end balancing? How do you determine if/when you have a cell that is becoming a problem? What sort of voltage range do you consider acceptable between individual cells for a high quality LFP during the end of its charge cycle?

Thanks for reading. Lots to learn...

[Pete7]

Assuming this a sealed battery and you can't get inside, there isn't much you can do about it. However, don't worry. The amount of capacity above 3.45v per cell is tiny and not worth fretting over.

Either one cell is weak or there is a poor connection. Don't worry about it and carry on.

[WE9V]

You didn't explicitly state that this was a "drop-in" or home-made battery. I'll assume drop in? If it is drop in, how are you getting cell voltages? Through a bluetooth app? If so, then you'd also be able to see what the high voltage cut-off is for a single cell (3.65 or ?).

The issue with drop-in packs is that you can't properly top-balance like you can with access to individual cells, and there's also a higher probability of getting unmatched cells. I recently purchases a couple 230Ah drop ins and saw about 150mV difference when one cell tripped at 3.65V. Because of that, I can't get anywhere near 14.4V before the BMS shuts down due to a single cell being 3.65. The other issue with drop-ins and passive balancers is that the balance current is negligible. For the JBD BMS in my drop in, they claim 50-150mA balance current. When you are still dumping in 5++ amps into the battery, the 50mA difference won't balance you out. It will take many charge/discharge cycles to balance.

And that's exactly what I did for my packs. I discharged about 20Ah out, then charged. Rinse and repeat about 10+ times. If you have the bluetooth access, you can also switch the setting for "balance only when charging" to off after it's fully charged. It will drain the high cell(s). Then, before turning the charger back on, turn that setting back to on. I now have an imbalance about 20mV, which is still higher than DIY batteries who properly top balanced, or those with active balancers.

Regardless of the battery manufacturer, I've found that the Overkill Solar app is the best for looking at and changing settings.

[sailorboy1]

Originally Posted by 00X

Thanks for reading. Lots to learn...

If I had a 6 month battery I would be asking my question to the manufacturer instead of a forum! They are the ones to advise you and it also puts on record your concern and their response.

With that I am not going to write my advice/thoughts

[wholybee]

I thought I responded to this before but now I don't see my post.

The BMS in the battery has failed or is setup incorrectly. It is neither balancing or stopping charging when a cell exceeds 3.65V. The cell that never gets higher than 3.43V is very far out of balance from the other cells. Who knows how or why that is exactly, but it's pointless to worry about it when the BMS is failed.

Your choices are to send the battery in for warranty if it has one, or cut it open and replace the BMS.

For a point of reference, I have a 300Ah battery with a JBD/Overkill BMS with passive balancing. The balancing is kept within 0.02V. It does this easily, and I originally set it at 0.01V but there is no advantage to that so I raised it to not work the passive balancers as hard.

Point is, passive balancers should be able to keep a 280Ah battery balanced just fine, assuming it's balanced to begin with. And no cell should ever be allowed higher than 3.65V. (there are a few BMS's that cutoff at 3.7V, it is possible you got one of those)

[00X]

Originally Posted by wholybee

I thought I responded to this before but now I don't see my post.

The BMS in the battery has failed or is setup incorrectly. It is neither balancing or stopping charging when a cell exceeds 3.65V. The cell that never gets higher than 3.43V is very far out of balance from the other cells. Who knows how or why that is exactly, but it's pointless to worry about it when the BMS is failed.

Your choices are to send the battery in for warranty if it has one, or cut it open and replace the BMS.

For a point of reference, I have a 300Ah battery with a JBD/Overkill BMS with passive balancing. The balancing is kept within 0.02V. It does this easily, and I originally set it at 0.01V but there is no advantage to that so I raised it to not work the passive balancers as hard.

Point is, passive balancers should be able to keep a 280Ah battery balanced just fine, assuming it's balanced to begin with. And no cell should ever be allowed higher than 3.65V. (there are a few BMS's that cutoff at 3.7V, it is possible you got one of those)

Thanks. Its an Epoch 12v 300AH V2 battery. Its also has a JBD BMS as well. The bluetooth app really sucks but it does show individual cell voltages and has three battery temp sensors. The temps while charging never get above 84 degrees. The first four or five charge cycles that was my experience as well. The voltage stayed within .02 volts near the top of the charge cycle.

I will contact Epoch and try and find out what the cell cutoff voltage is. They did originally recommend some really high charging voltages when their batteries were first being sold so its possible my particular BMS cuts off at 3.70. The BMS can handle pretty high continuous discharge amperage so it may have a higher cutoff for this reason. Not sure if there is any correlation though.

The passive ballancing does eventually bring the cells to within .002 volts of each other if i leave it alone overnight with no loads on it. I have been depleting it with the inverter under modest loads (20 amps) after the cells finally balanced and all the cells are staying within .002 volts during discharge. I did notice on their website they now have some charging information, and they recommend a low amperage charge (15 amps) for the first few charging cycles. I was charging with 50 and all seemed well though.

Thanks for the reply.

[00X]

Originally Posted by sailorboy1

If I had a 6 month battery I would be asking my question to the manufacturer instead of a forum! They are the ones to advise you and it also puts on record your concern and their response.

With that I am not going to write my advice/thoughts

Thanks. I will do this now. I just wanted to get a group consensus before bothering them with what could have been a non-issue. I didnt want to waste their time due to my ignorance or lack of full understanding. Seems like it might be prudent now to check with them.

[00X]

Originally Posted by WE9V

You didn't explicitly state that this was a "drop-in" or home-made battery. I'll assume drop in? If it is drop in, how are you getting cell voltages? Through a bluetooth app? If so, then you'd also be able to see what the high voltage cut-off is for a single cell (3.65 or ?).

The issue with drop-in packs is that you can't properly top-balance like you can with access to individual cells, and there's also a higher probability of getting unmatched cells. I recently purchases a couple 230Ah drop ins and saw about 150mV difference when one cell tripped at 3.65V. Because of that, I can't get anywhere near 14.4V before the BMS shuts down due to a single cell being 3.65. The other issue with drop-ins and passive balancers is that the balance current is negligible. For the JBD BMS in my drop in, they claim 50-150mA balance current. When you are still dumping in 5++ amps into the battery, the 50mA difference won't balance you out. It will take many charge/discharge cycles to balance.

And that's exactly what I did for my packs. I discharged about 20Ah out, then charged. Rinse and repeat about 10+ times. If you have the bluetooth access, you can also switch the setting for "balance only when charging" to off after it's fully charged. It will drain the high cell(s). Then, before turning the charger back on, turn that setting back to on. I now have an imbalance about 20mV, which is still higher than DIY batteries who properly top balanced, or those with active balancers.

Regardless of the battery manufacturer, I've found that the Overkill Solar app is the best for looking at and changing settings.

Drop in. via bluetooth app. The app has very, very limited info. Its a pretty gimmicky, crappy app and the only useful info it provides is individual cell voltages, battery temps and alarms. There is no info about cutoff voltages etc...Im sure i would see the alarm if it did disconnect and then know what that voltage was, this hasnt happened yet though so no idea.

The battery is an Epoch 300AHV2. Good quality batteries, but i did get one of the very early rounds of shipments. Maybe something got rushed but so far no one has complained yet about them being out of balance. They use grade A automotive EVE cells. Still possible to get a bad one though.

Thanks, ill check out the overkill solar app. I did try a couple other apps but they didnt connect to my BMS for some reason.

[00X]

Originally Posted by WE9V

You didn't explicitly state that this was a "drop-in" or home-made battery. I'll assume drop in? If it is drop in, how are you getting cell voltages? Through a bluetooth app? If so, then you'd also be able to see what the high voltage cut-off is for a single cell (3.65 or ?).

The issue with drop-in packs is that you can't properly top-balance like you can with access to individual cells, and there's also a higher probability of getting unmatched cells. I recently purchases a couple 230Ah drop ins and saw about 150mV difference when one cell tripped at 3.65V. Because of that, I can't get anywhere near 14.4V before the BMS shuts down due to a single cell being 3.65. The other issue with drop-ins and passive balancers is that the balance current is negligible. For the JBD BMS in my drop in, they claim 50-150mA balance current. When you are still dumping in 5++ amps into the battery, the 50mA difference won't balance you out. It will take many charge/discharge cycles to balance.

And that's exactly what I did for my packs. I discharged about 20Ah out, then charged. Rinse and repeat about 10+ times. If you have the bluetooth access, you can also switch the setting for "balance only when charging" to off after it's fully charged. It will drain the high cell(s). Then, before turning the charger back on, turn that setting back to on. I now have an imbalance about 20mV, which is still higher than DIY batteries who properly top balanced, or those with active balancers.

Regardless of the battery manufacturer, I've found that the Overkill Solar app is the best for looking at and changing settings.

Wow! I just downloaded the Overkill app. Im at work but have a friends brand new Epoch battery here with JBD BMS. Its giving me full access to change all the BMS settings. The high voltage cutoff is 3700 mV on these batteries, which are similar to mine so confirms why my battery probably never triggered a high voltage shut down.

This is great, thanks for mentioning it. I'll be at the boat tomorrow and will be able to get a deeper dive into my battery. The Epoch bluetooth app is like a first graders toy compared to this one. Im sure thats by design since they really dont want just anyone going in and wreaking havoc with their BMS but some of us need better toys

[CaptainRivet]

Originally Posted by 00X

Wow! I just downloaded the Overkill app. Im at work but have a friends brand new Epoch battery here with JBD BMS. Its giving me full access to change all the BMS settings. The high voltage cutoff is 3700 mV on these batteries, which are similar to mine so confirms why my battery probably never triggered a high voltage shut down.

This is great, thanks for mentioning it. I'll be at the boat tomorrow and will be able to get a deeper dive into my battery. The Epoch bluetooth app is like a first graders toy compared to this one. Im sure thats by design since they really dont want just anyone going in and wreaking havoc with their BMS but some of us need better toys

Be careful what you are doing. All cell voltage mention here from everyone means right at cut off or right after absorption is finished and NOT later as the cell drop to their resting voltage where all seems equal but in reality isn‘t so you need to watch your app and make screenshot right away when this happens.
I would first contact EPoch as this is an expensive battery and with modifying their parameter you void the warranty. If they set overvoltage to 3700mV they tested that with the cells they use and is ok, I know batteries they even have 3800mV.
You also not really know what you are doing and also not follow epochs arge recommendations. Don‘t touch the parameters of the BMS, just figure out where they are and post!!!

Your issue is origined by charging only to 13.8V which is 3.45V and the balancer starts to balance at 3.4V. As it’s passive it’s only doing this during charging and has no time to balance the cells, means each cycle they drift more and more appart. 300AH battery with 200mV deviation means you have a difference about 5AH (lowest is 295AH while highest 300AH) that your tiny 50-150mV passive balancer need to burn off the highest cell to to get the lowest up to the 300AH. That will take about 2 month.
What you need to do is set your chargers to 14.2V absorption with 3h duration and float to 13.7V with a rebulk of 0.05mV and reduce your chargers output to 5A. What this does is it keeps the battery exactly at the start of the knee where the balancer operates and give it max time to operate. Do this over 4 weeks.
Then set the charge source to 14.6 absorption with 30min and charge it normally. It will most likely cut off with an high voltage but that’s what we want to see how high the weak cell comes up. If the weakest cell is below 3,6V while highest cut of at 3.7V go back to the prior settings 14.4V absorption with 3h and 5A and do this another 4 weeks. Can be it cuts off due to high voltage of one cell, then reduce to 14.3V or even 14.2V…just try the highest value when it’s not cutting off.
Then the 14.6V charge again. If the difference of the lowest to the highest right at cut!!! (Not later, you need to be there and best make a foto) off is then >100mV it’s time for a warranty replacement request as clearly one cell has bad quality or connection is faulty.

If it’s below 100mV difference, set your charge parameters to 14.2V absorption with 1.5h and float at 13.5V with rebulk of 0.1V and run it for a month.
Then charge tom14.6V absorption and check if the delta high to low is still <100mV. If it is reduce absorption 60min at 14.2V, run for a month and same game charge to 14.6V and see if delta grows or reduces.
If it’s reduces or stays the same set absorption to 14.0V and same game.
I assume 14.2V with 1hor 1.5h absorption is what you end with.

[CaptainRivet]

To add:
How big is your inverter, how high of loads does this 300AH see?
If that’s a lot absorption time at 14.2V needs to be even longer as your balancer has to do more work.

You most likely got a not well top balanced battery and made it worse when charging with 50A instead 15A as this gives the passive balancer even less time to do its job.
A good quality battery like the epoch you have to except a deviation of max 30mv at 3.65V when properly top balanced and that’s what you slowly do over the next 2month as the only way you have that the tiny passive balancer burns off the higher cells till they are even.

[00X]

Originally Posted by CaptainRivet

Be careful what you are doing....duces or stays the same set absorption to 14.0V and same game.
I assume 14.2V with 1hor 1.5h absorption is what you end with.

Interesting, thanks for all the insights, i will give this a shot and see how things progress. In the future I think i will charge at a slightly higher voltage. The problem was that when i bought my battery last year, Epoch hadnt really provided specific charging parameters so a lot of us were probably setting the voltage too high or too low. They have since updated this for their batteries which is helpful.

I have managed to close the gap between the cell voltages at the top end of the charge to just about 100mV now over the past week. Inspite of this the low cell will charge to just under 3.43 volts, no higher and then immediately drops to 3.4 volts for the duration of the charge. The high cell settles to 3.5ish volts now and the other two rest somewhere in-between. It seems to have stabilized at these voltages.

Over the past week i have also been discharging with my inverter under various amperages, many times now, ranging from 20 amps to 100 amps and observing how the cells are behaving. During heavy 100 amp discharges, all the cells are now staying within .020 mV of each other. Its even less if its a lower amperage draw staying within .005mV when discharging with 20 amps. Once loads are removed, every cell settles to within .002 mV of each other. Also, i have noticed that after fully charging, all the cells settle at 3.33 volts if let to sit overnight with no loads.

So now i wonder why the low cell will not go above 3.43 volts and if this is really a problem considering how the cells are behaving while discharging and recharging? Is a cell that wont go above 3.4 volts really a problem? technically anything above 3.33 is considered full. The three temp sensors are reporting normal temps, just a few degrees above ambient. Is it possible the cells are all good and that i just got a poorly matched pack?

The pack did capacity test to 316 Ah when it was new. Im kind of curious now what its at.

[00X]

Originally Posted by CaptainRivet

To add:
How big is your inverter, how high of loads does this 300AH see?
If that’s a lot absorption time at 14.2V needs to be even longer as your balancer has to do more work.

You most likely got a not well top balanced battery and made it worse when charging with 50A instead 15A as this gives the passive balancer even less time to do its job.
A good quality battery like the epoch you have to except a deviation of max 30mv at 3.65V when properly top balanced and that’s what you slowly do over the next 2month as the only way you have that the tiny passive balancer burns off the higher cells till they are even.

3000 watt inverter. Occasional 100 amp loads, maybe once every other week to make a coffee. mostly the loads are between 5-20 amps while im at the boat on the weekends.

I did come across Epochs updated data sheet after my first initial post and did do a recharge from 70% SOC using 15 amps. It definitely seemed to help as mentioned in my last post, i have closed the gap to around 100mV so far.

As an aside, i was told about an active balancer made by Fogstar batteries that may work with my specific battery. They sell an identical copy of the Epoch battery with the exact same plugs and ports, same cells and also a JBD BMS. They sell an active balancer for their version of my battery and was told there is a good chance it will work perfectly well with my battery. Not 100% guarantee but its an inexpensive piece of kit to test so i may give it a try. It would plug into the external port so i would not have to open up the battery.

[wholybee]

Originally Posted by 00X

Interesting, thanks for all the insights, i will give this a shot and see how things progress. In the future I think i will charge at a slightly higher voltage. The problem was that when i bought my battery last year, Epoch hadnt really provided specific charging parameters so a lot of us were probably setting the voltage too high or too low. They have since updated this for their batteries which is helpful.

I have managed to close the gap between the cell voltages at the top end of the charge to just about 100mV now over the past week. Inspite of this the low cell will charge to just under 3.43 volts, no higher and then immediately drops to 3.4 volts for the duration of the charge. The high cell settles to 3.5ish volts now and the other two rest somewhere in-between. It seems to have stabilized at these voltages.

Over the past week i have also been discharging with my inverter under various amperages, many times now, ranging from 20 amps to 100 amps and observing how the cells are behaving. During heavy 100 amp discharges, all the cells are now staying within .020 mV of each other. Its even less if its a lower amperage draw staying within .005mV when discharging with 20 amps. Once loads are removed, every cell settles to within .002 mV of each other. Also, i have noticed that after fully charging, all the cells settle at 3.33 volts if let to sit overnight with no loads.

So now i wonder why the low cell will not go above 3.43 volts and if this is really a problem considering how the cells are behaving while discharging and recharging? Is a cell that wont go above 3.4 volts really a problem? technically anything above 3.33 is considered full. The three temp sensors are reporting normal temps, just a few degrees above ambient. Is it possible the cells are all good and that i just got a poorly matched pack?

The pack did capacity test to 316 Ah when it was new. Im kind of curious now what its at.

The reason the one cell stays at about 3.4V is because it is in the "flat" part of the voltage curve. For most of the charge cycle, the voltage of LFP doesn't change much. So, at 60% SOC, about 3.4V(well, maybe just a little less). At 70%, also about 3.4V. It isn't until you get to around 90% that the voltage starts to noticeably increase. So, while you are charging the battery, the SOC is increasing, but in that cell it is still below 90% (how much below we can't be sure) while the other 3 cells are over 90%, and thus their voltage is increasing.

Because there is such a large area where the voltage is "about" 3.4V, the voltage difference at anything other than fully charged doesn't matter. At lower than 80-90%, the voltage will be almost identical, even if one cell is at 65% and another at 75%.

There is a setting in the overkill app "Balance only when charging." You want to change that to off, so that the cells will continue to balance, even after charging has stopped. That will get them balanced faster so you don't have to wait 2 months. One caveat however, on _some_ batteries, that switch works backwards from how it is labeled. So whatever it is, watch the balancing and see how it behaves, and if it stops balancing when the charging stops, toggle that switch and try again.

Someone else on here had a similar problem recently, and toggling that switch balanced his battery overnight.

[00X]

Originally Posted by wholybee

The reason the one cell stays at about 3.4V is because it is in the "flat" part of the voltage curve. For most of the charge cycle, the voltage of LFP doesn't change much. So, at 60% SOC, about 3.4V(well, maybe just a little less). At 70%, also about 3.4V. It isn't until you get to around 90% that the voltage starts to noticeably increase. So, while you are charging the battery, the SOC is increasing, but in that cell it is still below 90% (how much below we can't be sure) while the other 3 cells are over 90%, and thus their voltage is increasing.

Because there is such a large area where the voltage is "about" 3.4V, the voltage difference at anything other than fully charged doesn't matter. At lower than 80-90%, the voltage will be almost identical, even if one cell is at 65% and another at 75%.

There is a setting in the overkill app "Balance only when charging." You want to change that to off, so that the cells will continue to balance, even after charging has stopped. That will get them balanced faster so you don't have to wait 2 months. One caveat however, on _some_ batteries, that switch works backwards from how it is labeled. So whatever it is, watch the balancing and see how it behaves, and if it stops balancing when the charging stops, toggle that switch and try again.

Someone else on here had a similar problem recently, and toggling that switch balanced his battery overnight.

So it sounds like the only way to know for sure is to do a capacity test? I do have a tester that can do a 20 hour rate.

K, switched it off. Will see what happens.