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


Off-grid Beginner
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.


Off-grid Beginner
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?


Off-grid Beginner
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


Off-grid Beginner
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.


Off-grid Beginner
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.