Lifepo4 302ah cells deterioration.

[Gordon Bland]

HI All. My original battery was 12v 302ah (4s). It had been working fine however we were finding it a bit small. So I bought another 4 x 302ah cells (secondhand) to double our capacity to 604ah. I assembled the battery using 2 original, 2 recent,  2 original, 2 recent to make a larger 4s2p battery. However the battery wasn't performing as well as expected (capacity wise). 

So I purchased a yr1035+ tester to check out all 8 cells. The results are my original cells are on average 0.26M ohm internal resistance (ir) whereas the recent acquisition (4 cells) range from 1.47 - 1.75Mohm.

So my options are 

1) Reassemble the pack putting in parallel a low ir and high ir cell. Not too keen on this as the ir difference maybe too much?

2) Make a separate pack out of the high ir cells and use my 20A dc-dc charger to put another 20A into my good pack when possible.

3) Throw them away as they're too far gone and potentially a fire risk?

Thinking option 2 is the way to go? What are your thoughts about these cells? I haven't been able to find anywhere on the internet what is a max ir before cells become unusable?

[Dacian Todea (electrodacus)]

The way you want to build a pack made of cell with different ages or characteristics is to have one original cell in parallel with a recent cell that way all the 4 groups will be much more similar.

That said the internal impedance of those recent acquired cells is way higher than can be expected even from used cells. My best guess is that those cells where abused maybe over discharged.

If they are not swollen and look in good condition use option 1 for connection as that is the best connection option in all conditions.

Maybe do a capacity test to see what is the total capacity of your new battery.

[Gordon Bland]

Hi Dacian.  Thanks for the prompt response. The recent cells look to be in good condition and have no swelling whatsoever. I'm charging the cells now to do a capacity test. Once I have both capacity test results (original and recent) I'll be able to set a realistic battery capacity in the sbms0 instead of the 600ah it's currently set at.

[Dave McCampbell]

If you have cell tester/load like a ZKETECH you can do a capacity test on each cell then do a top balance.  That will move all the capacity of each cell to the top of the charge.  We just did that with our new EVE 280 ahr cells.  Still, as Dacian said, those high internal resistances will cause problems during future charging and discharging.  New cells are relatively inexpensive right now, and EVE has warehouses in the US and Europe so cells are readily available there.  EVE cells are excellent.

[Gordon Bland]

Thanks Dacian and Dave for the replies. I have used one of the small capacity testers and checked 1 of each cell. The results are

Original cell: 297Ah capacity. I charged up to 3.55V and drained down to 2.8V. So very pleased with that result.

Recent cell: 140Ah capacity. Charged and discharged the same way as the original cell. So not even half their capacity left. They also were much harder to charge and sagged/dropped much quicker under the 15A load. 

So the capacity comes out at 440Ah instead of 600Ah with 4 cells not performing so well. As the 4 recent cells are so bad, is Option 1 still the way to go? or am I better to use Option 2 and just use it as a supplemental battery supplying additional charge to the original battery? Eventually 4 replacement cells could be sourced. 

1) Reassemble the pack putting in parallel a low ir and high ir cell. Not too keen on this as the ir difference maybe too much?

2) Make a separate pack out of the high ir cells and use my 20A dc-dc charger to put another 20A into my good pack when possible.

Dave, I wish that the cheap cells Eve cells were available here! Unfortunately 4 new 306Ah no name cells currently cost about $1,000 her in New Zealand :( 

[Dave McCampbell]

Wow, just wow!!  Four no name cells with half rated capacity for $1000!  EVE top quality similar cells are about $240 US plus shipping.  Shipping from the China warehouse is by air.  Our 12 EVE cells got to us in Malaysia in less than 10 days for $400.  Hopefully you can return those very low capacity cells under warranty as defective. 

[Dacian Todea (electrodacus)]

Yes 297Ah for 3.55V down to 2.8V is normal capacity so those original cells are in excellent condition.

The recent cells at 140Ah are super low capacity (maybe they are significantly out of balance) But if that is the capacity for those cells not sure how that will be possible.

Not sure in what way where those cells abused and if they are safe to use.  If I was sure the cells where safe to use (not sure how that will be possible at 140Ah capacity and super high internal impedance) then it will not have been any problem to parallel them with so option 1

For option two you need an additional BMS and not sure is work getting a BMS for this battery.

Not sure you can have no name 306Ah cells (They will need to be one of 3 or 4 manufacturers of such cells and they are all decent brands).  I'm guessing the prices is 1000NZD (about 560USD) Current prices in China about 50 to US$60/ cell so depending on shipping options you may be able to get them from China at 240USD + shipping and eventual import taxes thus it may not be that far from the prices you get from local dealers.

Just checked Amazon Canada and price seems to be around 400CAD to 550CAD  (285USD to 400USD) for 4x 280 to 4x330Ah cells with shipping included but they ship from China so there is no local intermediary to try and make some profit.

[Gordon Bland]

Thanks Dacian and Dave.  The 140Ah test was for a single cell. So definitely not a balance issue.  I bought them secondhand off Facebook marketplace.  So they're not worth continuing with and I will dispose off them in e-waste. It's a shame that I didn't have my ir meter at the time as a quick check would've shown that they weren't worth purchasing. Fortunately we're heading into summer so the original battery will be fine until I can get 4 new cells for $1,000 nzd and double my capacity. 

I've looked into importing cells however they're treated as dangerous goods incurring fees, taxes and paperwork.  I've read stories about customs blocking them at the border and returning them back to the sender if the paperwork etc isn't correct. 

[Dave McCampbell]

If you do decide to buy new cells contact Amy at Docan Power who sells the excellent EVE cells.  They are a big, very well known and well regarded LFP battery company in China.  They have a good website and she will answer your emails with any questions you may have.  They probably have customers in NZ, and I know they do in Australia.  Good luck.

[Gordon Bland]

Hi Dave. Thanks for that! Ironically lifepo4 are much easier to ship to Australia than New Zealand because of our uptight deranged regulations.  However when I'm ready to purchase I'll definitely contact Amy because she'll know whether they can send to New Zealand or not? I'd prefer to purchase them from Docan as they should be slightly cheaper and really good quality. Thank you for the information!