LFP cells purchased on Alibaba

Dave

I wanted to post my experience with 64 LFP cells I purchased from Simon Lin of Dongguan Huanhuan Energy Ltd.n AliBaba. If there is a more appropriate place for this posting, please advise. I am not trying to badmouth, only to make my experience known in hopes of saving others from similar disappointment. 

I purchased 64 200Ah CALB cells in November 2019, arrived January 6th, 2020. They were connected in parallel and balanced overnight (not charged) and put into service in a 4P16S, 800Ah configuration January 7th.

Cells have been cycled lightly, typical usage cycle is between 85% to 96% SOC. Twice they were cycled down to between 50 and 60% SOC. Charge rate never exceeded 50A and was usually between 35A and 40A (<0.05C). Absorb is 3.45 or less for less than a half hour. The only time they have been above that until the capacity tests was for a one-time, full-bank, parallel top-end balance at 3.65V.

During a recent snowstorm and 3-4 days with limited recharge, they reached their low-end cutoff (3.065V, 49.0V bank). The cells were all within 10mV balance at this time thanks to a 10A active balancer.

At cutoff they had produced 644Ah. There is perhaps 6-7% capacity below 3.065V and the cells began this cycle at 97-98% charged. Therefore, 644Ah should represent roughly 92% of the capacity which indicated conservatively a full capacity around 700Ah, well below 800Ah.

I purchased two ZKETech EBC-A10H testers to evaluate the capacity of my cells. My tests are done at 20 degrees Celsius as follows:

• Charge cell to 3.65V, cutoff charging when charge current falls to 1A, immediately begin capacity test.
• Capacity test – 10A discharge (0.05 x 200Ah), 2.6V cutoff
• Recharge – charge at 0.001C (1.6A) to 3.1V, then 20A charge to normal.

I have now tested 17 of the 64 cells and the mean is 165.14 Ah standard deviation of 4.17. Assuming less than 1% capacity below 2.6V and an initial charge to 100% that means these cells are about 167 Ah cells, 83.4% of stated capacity. Even adjusting a couple percent for testing below standard temperature (20 vs 25), that’s still significantly below spec.

In hindsight, I should not have purchased these cells until I had the ability to capacity test them. Since the cells are usually cycled so lightly, I was not aware there was an issue until the deep discharge and shutdown. If there is something I am missing or have done incorrectly, please let me know. These cells were advertised as new, 1.5 year old batteries which shouldn’t be a problem for LiFePO4 cells. They were 3.29V when I received them and reasonably consistent across the set. The only explanation I have been able to come up with is that these cells had been used heavily before I received them. I have exchanged quite a few messages with Simon Lin since discovering this and have provided him test data and details but he has offered no explanation or resolution.

Please let me know if there could be something else that is causing them to perform so far below specification.

RCinFLA

You probably got used cells taken out of service when their capacity drop to 80%.

You need to measure their impedance.  Used cells have thicker growth of SEI layer which increases cell impedance and consumes available lithium reducing capacity.

You can get a good idea of battery impedance by measuring 120 Hz ripple voltage with 50 amps average DC load or more from sinewave inverter.  Voltage has to be measured within 10-15 seconds of applying heavy load to avoid to much extra voltage drop due to longer time constant cell kinetic voltage drop to move ions.

A new 200AH LFP should have between 0.25 and 0.4 milliohm impedance.  0.3 milliohms x 55.5 A rns will have 16.65 mVrms across cell terminals on a true RMS DVM.  

I would expect a well used 200 AH cell, but still with useful life, to have no more then about 1.5 milliohms impedance.  That would be 83.25 mVrms on true RMS DVM across cell for 50 amp average DC current to sinewave inverter.

see attached.

Some BMS's do this measurement and display Rs of each cell.  Just do it with higher current as the BMS accuracy is poor at lower load current.

mcgivor

Perhaps they are factory seconds, if they didn't come with a test report and do not have sequential serial numbers, or serial at all, they may be new cells which didn't pass the capacity test, the capacity test requires them to have a capacity of >/= 204 Ah.

Ballancing is generally not done mid bur rather top or bottom, if done mid the cells will become unbalanced at both top and bottom,  most off grid users do top ballancing. 

Dave

RCinFLA said:

You probably got used cells taken out of service when their capacity drop to 80%.

You need to measure their impedance.  Used cells have thicker growth of SEI layer which increases cell impedance and consumes available lithium reducing capacity.

You can get a good idea of battery impedance by measuring 120 Hz ripple voltage with 50 amps average DC load or more from sinewave inverter.  Voltage has to be measured within 10-15 seconds of applying heavy load to avoid to much extra voltage drop due to longer time constant cell kinetic voltage drop to move ions.

A new 200AH LFP should have between 0.25 and 0.4 milliohm impedance.  0.3 milliohms x 55.5 A rns will have 16.65 mVrms across cell terminals on a true RMS DVM.  

I would expect a well used 200 AH cell, but still with useful life, to have no more then about 1.5 milliohms impedance.  That would be 83.25 mVrms on true RMS DVM across cell for 50 amp average DC current to sinewave inverter.

see attached.

Some BMS's do this measurement and display Rs of each cell.  Just do it with higher current as the BMS accuracy is poor at lower load current.

I appreciate your reply and suggestions for testing. Unfortunately, I don't think I have the equipment or knowledge to do as you suggest. Is there an easy, idiot-proof shortcut to get an idea?

Dave

Does the attached picture look like a real CALB serial number? The first series always contains P8215 which makes me wonder if these were 215Ah when new.

Raj174

Hi Dave, 
More than likely it's a bogus label because the capacity should be indicated in nominal amp hours as it shows in this CALB literature concerning the serial numbers. The date information wrong also. In fact, I just looked at the SN on one of the SE180s that I bought from them back in April. It looks like it came off the same fake label roll.

Rick

Dave

wow rick, thanks a lot for looking into that. looks like we both got bogus goods from Huanhuan Energy. I am including this information in my latest communication to them. all they have offered to do so far is sell me more batteries for a few thousand more dollars. i pointed out that they were supposed to be warrantied for a year and that i contacted them within that year with this issue even though i didn't have the batteries until 2 months after i purchased them. did you say yours were about 7% under capacity?

Raj174

My 32 180ah cells had capacities fairly spread out between 179ah down to 168ah, averaging 172ah. So about 4.5% under capacity. Not happy with it, but I figured, what could I expect for 100 dollars per cell. If they are real CALB rejects due to low capacity, then CALB probably removes their label before selling them to trading companies.

mcgivor

Since the typical user in off-grid applications keeps away from the extremes, usually operating in the 20-90% SOC window, the 4.5% capacity differential would make little difference in real world useage and may actually be the cost effective way to go when the 35% initial cost savings of the cells is taken into consideration. 

The picture would be different if maximum capacity was a requirement, the cell voltage differential would begin to increase as the SOC decreases (in a top ballanced bank) this is where the LBCO voltage comes into play, naturally a BMS would be more accurate as it monitors individual cell voltage. Being consecutive is the key to getting the most out of the battery, this seems to be the general concensus.