Renogy 100Ah LFP cell imbalance on discharge reducing capacity - anything I can do?

[caseyzak24]

Hey all,

Apologies in advance for the long/sob story but I'm hoping you all can point me in the right direction....

I bought and installed a Renogy 100Ah LFP battery w/ bluetooth (RBT100LFP12-BT) for our Casita travel trailer in late summer 2022. I got it on sale, I believe because it was their older (1st gen?) model. Shortly after, we went on the road full time and the battery worked without issue for about 9mo. In that time, IIRC, we took SOC down into the single digits a few times without issue (one of the great things about LFP). Around July of 2023, the battery shutoff around 30% while discharging in the evening, and when it kicked back on the next day the BT app showed at least the cell undervoltage warning had tripped. This happened again, so then I forced it and got a screen shot showing it was tripping the cell undervoltage protection at 2.5V (attached). One thing that was different around this time was that, we were running our 3 way fridge on DC because it was summer with great solar resource. I mention that in case it matters but even with the fridge the load was <20A. Of course, first thing I did was reach out to Renogy, filed a warranty claim, etc. It took me a while to get them the battery since we were on the road, but I finally did in early September 2023. It took 4 months to get a replacement and it was kind of a nightmare; first they told me the battery was bad and they would replace it, then they told me the battery was good and they were sending it back to me. Eventually they sent me a new battery, I think just to get me to stop calling.

I received the new battery in mid December but only got a chance to start messing with it recently. I charged it up to full charge using the LFP-specific converter on the Casita and let it sit there for a few days. Yesterday I unplugged the charger and turned on every load I could think of (fridge, lights, fan etc). According to the BT app it was a draw of 22A. Came back a few hours later and this new battery had shutoff at 20%! Once it turned back on with the charger it showed the cell undervoltage warning and one of the cells was at 2.3V, so pretty sure its the same issue. Since I already suspected the high/long draw from the fridge might matter, I charged up a few percentage points and then tried again with the fridge off, reducing the draw to 7A. I hoped this might allow things to balance out but this time the battery made it to 18% before shutting off (sigh) though that could be because I hadn't completely "zeroed-out" the previous imbalance.

Having now looked around on the forum a bit, it seems like cell imbalance is part of the deal with bargain plug-and-play LFP batteries, but most folks encounter it at the top of the charge, and in this case it is reducing the usable capacity that I thought I paid for. What's more, the manual even describes a "typical" discharge as a constant draw of 30A! So I'm not even loading it at a typical level. At this point, I'm unwilling to go through the rigamarole with Renogy again, so depending on what you all say I am willing to live with the reduced capacity, but I am wondering:
- Is this normal? Renogy eventually said that my first battery was tested and was fine. This one is brand new and experiencing a similar issue. Is this actually expected and I am just misunderstanding the spec/manual?
- Could this be a result of how my setup is wired? For context, I have a completely separate solar+battery setup in our tow vehicle. It uses the same model of Renogy battery and it has never had an issue with cell undervoltage. I have a DC plug and 3 way switch in the camper so I can run off either system. But in the camper, the wiring setup is a little wonky due to space constraints. There are some long runs that use either 2 or 4AWG. Could differences in wire resistance on the positive and negative terminals lead to imbalance? Doesn't seem likely but...
- Is there anything generally I can do to improve the situation?


Thanks!

 

[DIYrich]

What are you charging it up to? What is the cell balance at full charge?

 

[caseyzak24]

I wasn't paying that close attention on the new battery given that I thought I was performing a sanity check/being paranoid. I'll put it on the charger right now and report back

 

[caseyzak24]

OK, as an update, after putting the battery on the charger, I wasn't able to get it up to 100% or get the cells balanced (see app screenshot). After playing with it a bit, I can tell the charger isn't going into bulk mode, so right now I am trying to drain the battery low enough to get it to trigger. I am now cautiously optimistic that this was the issue. The battery had been sitting a while even after I got it (and I'm sure before), and if the charger hasn't yet done a bulk charge that might be the issue. I'm also wondering if that could be the cause of my issue generally. If neither of my chargers (solar or AC) aren't holding bulk/absorption long enough then maybe the cells aren't balancing at the top and it becomes an issue at high DoD.

Even when not in bulk/absorption, I measured a charger voltage of 13.7 and a battery terminal voltage of 13.3. I have some longer runs in there. I can't change the bulk voltage for the AC charger, but I can for my Victron Solar charger. We mostly run on the solar charger, so perhaps just bumping up the absorption voltage (currently 14.4V) would minimize the issue?

 

[DIYrich]

Not enough resolution in the cell voltages to tell much.
13.3 resting voltage is about right.
I would charge up to the max it can with the AC charger before morning, and let the Victron continue to charge the battery all day.

 

[mikefitz]

measured a charger voltage of 13.7 and a battery terminal voltage of 13.3.

Battery is in protection mode due to cell overvolts or you have excess volt drops if these voltage readings were made with current flowing. There will be an Inbalance nearing full charge, but with only one decimal place via the app, its not very clear as to the degree of diference between cells. The display of cell volts only to one decimal place, is further evidence of Renogy poor engineering.

bumping up the absorption voltage (currently 14.4V) would minimize the issue?

Try the opposite, lower the charge voltage to 13.8 or slightly higher if protection is not triggered, with a long absorbtion period. This will give a longer time for balancing to take place. Review the situation after several days. For normal charging don't exceed the Victron default, slightly lower is less stressful. Note that balancing will not take place unless the cell volts exceed , typically, 3.4 volts.

Even with better balance at full charge, the weaker cell will hit low cell volts first. When you get low cell protection , what voltages are the other cells?

this new battery had shutoff at 20%!

I would not regard the SOC value given by the Renogy app as super accurate. A quality battery monitor should give a more accurate indication.

A top balanced battery will ensure the cells have similar voltages at full charge, but this has no effect on the cells actual capacity, thus if the battery has been built with cells of significantly different capacity, its possible low cell volts could trigger protection with a small amount of capacity remaining in the battery.

Your issues with the first battery, performing well for some time before causing problems, indicates that either a faulty cell developed, or serious inbalance built up . From reports of other issues with Renogh batteries it seems the BMS used in Renogy batteries may not be able to successfully balance the cells. This could be or due to the BMS itself, or its inability to deal with problem cells .

There are some long runs that use either 2 or 4AWG. Could differences in wire resistance on the positive and negative terminals lead to imbalance?

Ideally the cable length, charger to battery, should be as short as practical . If there is significant voltage drop at typical charging currents, the charging may be compromised. What cables and length connect the solar charger to the battery?