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Cell Drift Under Load

Mrdanielmh

Solar Enthusiast
Joined
Apr 19, 2020
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177
Hey everyone, I'm looking for some advice!

I have a 12v 280ah DIY LifePo4 battery bank that I have top balanced a while ago. I have finally fitted it into my RV and attached some load. I have noticed that one of my cells drifts massively when load is applied to the battery, but when the load is reduced the cell comes back in line.

I wondered if there is anything I can do about this? They were top balanced to 3.6v before being put into 12v 4S. Then sat for around 4 week at around 70% capacity. I fitted the battery to my RV this morning and let the MPPT charge them to 14.2 Absorb and then I attached a 12v fridge which pulls are 3/4amps every now and again.

When the fridge is idle, all 4 cells are matched closely, however under load cell 1 drifts a huge amount which has got me worried.
 

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Verify, good idea. If verified, unfortunately, it doesn't look good. Since the cells are in series, a bad connection is unlikely to be the culprit, and the bms is not involved since the cell returns to normal with no load. You could test for internal resistance (maybe the bms provides that value), and assuming it's bad, that will give you more ammunition for a warranty claim. :(
 
Ok I've I managed to measure the cells under 4amp load and they all return 3.30v, so they appear to be matched even under load. Does this mean that there might be a connection issue going into my BMS?
 
All you can do is check the connections and start there eliminating a possible issue, if there something loose check cell 1.

Did you move wires during these 2 different voltage checks?

4a (.01c) on a 280ah battery isn't really a load.

I'll take the BMS cables off tomorrow and re-seat them all. None of the cables moved during my voltmeter checks, no.

True regarding load, but its all I have! It's for my RV so I don't have much in the way of load really!
 
I forget and being lazy to look at your other threads, are you charging with solar?

Add a signature to your post with your system. :)(y):)
 
Sorry ill do that now! Yes charging via a 315w panel through a 100 30 victron mppt
 
To clarify, you measured with a voltmeter and got 3.30v on all four cells, even when #1 is reported as 2.9v by the bms? That's good news!
 
That seems really odd to me .... not sure why the BMS cell reading would change based on load .... especially such a small load.
 
To clarify, you measured with a voltmeter and got 3.30v on all four cells, even when #1 is reported as 2.9v by the bms? That's good news!
Yes that's right, i just used the voltmeter across tje individual cell terminals and got the same reading at all 4 cells while the fridge was active. Ill connect my balance meter up todsu amd see what thst shows
 
So i tool everything apart this morning and noticed one of my bolts was slighlty loose. I have tightened everything up and attached a separate balance meter. Everything now appears much better under load! Screenshot_20200807_075312_com.jiabaida.bms.jpgIMG_20200807_075318.jpg

Any tips on making sure that the bolts done come loose during travel?
 
Also, how much drift is normal and ok? Ive been watching throughout the day and im noticing some drift again. Charging via solar, 12v fridge as load
 
So i tool everything apart this morning and noticed one of my bolts was slighlty loose. I have tightened everything up and attached a separate balance meter. Everything now appears much better under load! View attachment 19285View attachment 19286

Any tips on making sure that the bolts done come loose during travel?

Loctite, nyloc nuts, lock washers.
 
Also, how much drift is normal and ok? Ive been watching throughout the day and im noticing some drift again. Charging via solar, 12v fridge as load
Not all cells are created equal even within a production run.
What you and many other new users are seeing and calling drift is a normal reaction within the cell chemistry when charging or discharging. Cells are in flux all the time during these processes some more than others.

I just finished making breakfast using the microwave. 70% SOC solar contributing 25a, microwave load inverting at 148a (besides my other draw item, inverter itself 2a, humidifier 4a, fridge fans and parasitic items 2a).
Looking at my cell display screen, it shows 20 cells in constant flux under these loads 3.26-3.30 changing continuously every 1-2 seconds. When the heavy load is finished and solar is covering the normal daily items (inverter itself 2a, humidifier 4a, fridge fans and parasitic items 2a) now some cells are still fluctuating and others not while the battery bank is being charged.

Now with all that said if you see a cell doing radical nose dive or spike compared to the others than you might have a cell problem.

You probably wouldn't rest at all watching 20 cells fluctuating. ;)
 
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