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Parallel LFP batteries drop voltage during absorbtion, what causes this?

Greendream

Off-grid Beginner
Joined
Jun 30, 2021
Messages
67
Location
Colorado
I am having a heck of a time with my bigbattery Llama's. Originally I had 3pc. They are oddball 14S batteries and charge to 51V. One of the batteries I have has a problem that bigbattery has said will be replaced after I ship it to them. There seems to be a problem causing the BMS to heat up to about 215F. The other two do not have hot BMS.

Now I am running on 2pc batteries in parallel. This is exactly why we got 3pc batteries for the redundancy.

I am watching my system like a hawk still because I have no idea how well these other two batteries are matched/balanced. Or if one of these two was causing the other BMS to work too hard to get so hot ??

Long story short, the batteries get to absorption, current tapers off very fast and 51V running to the parallel bank. Normal so far. Then I see the batteries drop down to 47.5V. Now I understand that's still quite high SOC but no AC loads were surging. This happened about 4 or 5 times during the 1hr absorption before it drops into float at 48V. While in float it never deviates from 48V by more than 0.1V.

What would cause this? Is it the BMS kicking in and disabling cells?

Bigbattery recommends 51V across the board for all stages. Low voltage cutoff is 43V. I've opted not to float so high but 48V is as low as my magnum will go from a 51V absorption charge.

If there is another problem battery I'd like to freight it back at the same time as the defective one.
 
I seem to have the odd scenarios.

For the past month we've been running on 2pc Llama's. I've seen the BMK show up to 160AH discharge between them overnight. The weird voltage jumping continues and the BMK AH meter never gets back to 0, sometimes it will cap out at -30AH sometimes -12AH.

This all has me wondering the cause. I decided to power down the whole system and disconnect the 2nd battery which I have suspected could have issues. Now down to one battery. When I turned everything back on the solo battery, I'll call battery #3, it did what I expected. It bulked in a timely manner and gained about 50AH then hit absorb and had some of the weird voltage drop and allowed a lot of current (72A) and then settled back into 0-1A. I let it absorb until just about float. BMK was reading +0.2-0.6A which I considered full after 45 mins absorb. I shut everything off again and unplugged this fully charged battery. I then plugged up the other battery, lets call it battery #2, it was at about 45.3V as that's about where it was when it was disconnected hours earlier. Bulk mode for about 45 seconds then over voltage and current is at 0. BMK shows +0.0A Voltage is now at 51.0V. BMK shows +0AH

Now its nighttime and the inverting discharging the volts quickly dropped to 45.2V and is decreasing.

Ive called BB but no call back yet. I am preparing to send the one smoked BMS unit back for warranty replacement but would rather send 2 back at once.

I wonder if battery #2 caused some harm to battery #1 with the smoked BMS? I wonder if the weird voltage dropping will continue every charge cycle on battery #3, and if it is even normal?

I have a lot of video showing the strangeness.

I am 100% off grid and luckily have 1? working battery though 115AH isn't enough but we can manage like we always do.

Frankly battery #2 , also seems defective to me but hoping someone may have an idea of what is happening?
 
I think #2 bms is seeing a high cell and stopping charge. Can you see cell voltage individually?

Thanks for your reply. That sounds very likely.

I can't unfortunately. It would likely require me to open the battery and I believe that would cause warranty issues.

Do you think it may balance out during discharge? The 14S pack is now at 44.8V discharging at 8A
 
The LVCO happened around 3AM this morning, BMK reported -60 AH. Solar began charging the battery. It was jumping voltage quite sporadically 44.3-49V. It accepted about 10AH and then hit over voltage 52V+ and settled back into 51.0V. BMK showed -49AH with the battery full. After turning the whole system off and powering off the battery and unplugging it (still see a reading on the display when it's OFF and unplugged, very strange), and the resting voltage reads 45.2V with 52A activity?

I am guessing one of these cells is way out of balance and the whole pack is pretty much useless to me at this point. I've emailed BB to see about a resolution. Frankly all 3 batteries are suspect at this point.
 
I received a call from BB today and they were very helpful.

Battery #3 seems to be working well for us. Ive discharged it at least 70% and the AH recovered hasn't been less than 1AH off.

They had me adjust my charge voltage to 50.4V and LVCO for 44.8V.

Ive been instructed not to charge these batteries over 50A current so I gotta tweak the chargers settings.

Battery #3 we are going to try and charge it 50.4V, then do a LVCO at 44.8V, then recharge and see if we can ger it to balance out.

Ive got a direct email with the rep so I am hopeful we can get all of the batteries replaced and/or working.
 
I have a LAMA 48V from BigBattery. It has never worked as expected.

I finally got it to charge to 50.4V (3.6V), it quickly bleeds down to 47.6V (3.4V). I asked BigBattery and they said they consider 3.4V 100% charge. Which seems to be the case with LFP in general?

They had me reset the BMS. After it shutdown for the 10th time and I emailed "WTF". The side panel information is ... questionable.

I've measured the output after charging it to 50.4V until the BMS cuts out 44.8V (3.2V). When this happens I have to jump it with a charger to get it to come back to life.

The panel says 40V is cut off voltage.

End of story after numerous emails with polite but useless BigBattery support, I get less than 2.6kW out of the 5.13kW 48V LAMA.

With the QC issues reported here, I think BigBattery is 100% avoid at all costs and good luck with them honoring their warranty.

I assumed I could get 3.5-4kW out of this battery, not the case.
For $2000 for 2.6kW usable, that's 0.77/watt ..... good times ...

I have 14kW of Chinese batteries in 16S that I charge to 55.2V (3.45V) with solar and can pull down to 49.6V (3.1V) without issue. And they will go as far as 43.2V (2.7V) for LVCO.

For $4400 for 11.5kW usable, that's 0.38/watt ...

And the Chinese even know the voltages! Imagine that!
For 48V system, please set according to the following parameters.

Charging
Charging Limit Voltage: 58.4V
Over Voltage Disconnect Voltage: 60V
Over Voltage Reconnect Voltage: 56.8V
Float Charging Voltage: 55.2V

Discharging
Low Voltage Disconnect Voltage: 43.2V
Low Voltage Reconnect Voltage: 46.4V
Under Voltage Warning Voltage: 49.6V

Let's Go BigBattery!
#FBB
 
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No idea whats going on with your battery, sounds like weak cells or out of balance?.

I have 1 of 3 that's close to spec. 44.8v cutoff gave me 91AH discharge It has a bit of weird activity when it gets to the top of charge but eventually balances out and tapers the current as it should during absorb. I wish I could be confident in any of the three but at least this one can keep my SCC running.

I've emailed them nearly a week ago requesting an RMA for 2 batteries and haven't heard back.

I wouldn't recommend them at this point. I have spent almost $6k and within the first year I'll be spending hundreds more on freight hoping they deliver me with working batteries. If this becomes common having to RMA batteries the cost savings up front aren't worth the downtime and added expense shipping big batteries back and fourth.
 
The LVCO happened around 3AM this morning, BMK reported -60 AH. Solar began charging the battery. It was jumping voltage quite sporadically 44.3-49V. It accepted about 10AH and then hit over voltage 52V+ and settled back into 51.0V. BMK showed -49AH with the battery full. After turning the whole system off and powering off the battery and unplugging it (still see a reading on the display when it's OFF and unplugged, very strange), and the resting voltage reads 45.2V with 52A activity?

I am guessing one of these cells is way out of balance and the whole pack is pretty much useless to me at this point. I've emailed BB to see about a resolution. Frankly all 3 batteries are suspect at this point.

After reading your post again. The voltage jumping around happened to my 48V LAMA after the first BMS shutdown and BB told me to charge with 48V external charger the first time it kicked off after about 1.5kW of use.

The voltage read 44.5 on the battery side of the BMS, and 4.5V on the plug side.
I put 50.4V on the plug as recommended by BB and BMS started charging.

BB support couldn't tell me voltages.

Good to know the LVCO is 44.8V ... not the voltage range of 40V-51V.

If you are getting 90AH (4320W) that is 78% of rated.

I pull under 200 watts an hour off it, including inverter losses,etc. It doesn't last the night.

I should be getting something like below.
5300W * 0.80 = 4240W
4240W / 200Wh draw = 21.1 hours

I convert everything to watt hours out of habit.

To be honest, BB is paying shipping back and refunding me or I'm filing a fraud complaint/suing them in small claims. My 48V LAMA hasn't worked since I received it the end of August '21. I'm not even sure the cost savings is there with BB, if they are hard limiting to 3.2V. Imo, BB should have the LVCO set to 3.0V or 3.1V. Leave 10% as the LVCO.
 
I should have stated that the 44.8v, I have set in my charger/inverter (LBCO), not from the BMS. I misspoke with LVCO.

Oh .... my 48V LAMA goes dead/unresponsive without a jump below about 45V.

Have you tried taking it to 3.1V * 14 ( 43.4V )?

Mine will not.
 
Ive not discharged the one battery down that low but I did discharge all 3 in parallel to 43.0V at one point as they had that voltage recommended on the parameter sheet online when the batteries were bought. Now they have it listed at 44.8V for 80% DoD.

I have no doubt this 1 Llama is within spec but the jumping around in voltage after absorption makes me concerned. Perhaps it will balance out eventually. Lately we've been discharging about 51AH overnight and sitting at 45.5V.
 
Finally getting the ball rolling on RMA for my 2 batteries. Communication has been difficult but helpful when it happens.

They require me to re-box the batteries and ratchet tie them to a pallet. They arrange the freight pickup which I have to pay, they do the repair and replacement and pay to ship them back.

I'll update as this progresses.
 
Ever since I started emailing BB through my non yahoo email account I've been getting better correspondence. Both batteries were carefully packaged up and secured on a pallet. Pickup was scheduled for this past Monday but they didnt get picked up until Thursday (due to our location and the local freight line's available equipment specifically for liftgate). Anyhow, the batteries are on their way back to BB for repair/replacement. I am on the hook for the shipping charges from my location, BB claims they will pay the shipping back to my location.

The one battery I am currently running on is performing very well, and I have all the confidence in the world with it. We are only using about 45% capacity overnight and have already had 2 overcast days where the batteries were in the mid-to-low range capacity the entire time.

I'll update once the batteries are returned to me. I am sure I will be needing to charge them all up individually before hooking them back up in parallel, and then top balancing them all again.
 
I received my returned Llamas from big battery last week. I unboxed them yesterday. Both batteries look in good condition. These appear to be the batteries I sent back as one of the small phillips screws are still stripped in the head like when I returned it. The batteries have labels and logo stickers on them now.

I charged each battery with my SCC no more than 50A and when they hit float at 50.4V I disconnected them. Once all at what I expected to be 100% SOC Llamas I connected them in parallel. I was able to get 1AH into the bank before SCC went into Absorb.

I noticed still with all of these batteries one cell will hit over voltage at 50.5-50.6V and then it will drop back to 50.4V and after some time, maybe a couple minutes the voltage will drop to 49.0V and absorb more current. Eventually the battery will stop doing that and we will see a standard taper of current during absorb instead of this voltage dropping and then going over voltage.

We used 85AH overnight and saw a voltage of 46.0, 46.0, 45.9V on the batteries LCD.

The BMS on the units do not get hot at this time. The other really jumpy cell that was thought to be out of balance seems to be working, at least like the other two while charging.

I hope these batteries are now fixed and can be relied upon for the next 9+ years.
 
I received a Lamma last week that is giving me some issues. It seems that it was not charged, so out of the box its 43.8v! When connected to the Growatt, the voltage jumps all over the place on the display and as a result its not really charging it. I tried to charge with my golf cart charger and the voltage goes all over the place on the BMS and causes the smart charger not to do very much. Do you guys recommend something? Ive been in contact with BB going back and forth and rather try to sort this out. Any tips would be very appreciated! My other one has been working 24/7 flawlessly for a entire year.
 
I received a Lamma last week that is giving me some issues. It seems that it was not charged, so out of the box its 43.8v! When connected to the Growatt, the voltage jumps all over the place on the display and as a result its not really charging it. I tried to charge with my golf cart charger and the voltage goes all over the place on the BMS and causes the smart charger not to do very much. Do you guys recommend something? Ive been in contact with BB going back and forth and rather try to sort this out. Any tips would be very appreciated! My other one has been working 24/7 flawlessly for a entire year.

If the voltage on the display is jumping around during charging it sounds like the cell(s) are out of balance. I had a similar issue though the battery would charge to a point. It would hit the high voltage cutoff early as a result, and many of the cells were left uncharged. I had tried doing a LVCO discharge at 44.8V for over a month and it didn't help. I had to return my battery for repair since the warranty is void by opening up the battery.
 
If the voltage on the display is jumping around during charging it sounds like the cell(s) are out of balance. I had a similar issue though the battery would charge to a point. It would hit the high voltage cutoff early as a result, and many of the cells were left uncharged. I had tried doing a LVCO discharge at 44.8V for over a month and it didn't help. I had to return my battery for repair since the warranty is void by opening up the battery.
Yeah I figured they were out of balance, is there any other way to balance them? I actually haven't tried doing the LVCO, I think its set on my inverter to 42, maybe I'll try to increase that?
 
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