Fogstar Seplos Mason 280L - 1 cell out of whack

[swwils]

I have the 280L with the 16 x Envision 305Ah Grade B; one of the cells the terminals snapped off when I setting up for top-balancing (hand tight, so bad laser weld); fogstar sorted me a replacement and this replacement cell, which is has different stickers but is meant to be "305ah" is all over the shop:


Does anyone have any advice? All my other 15 cells are within 2mV and working nicely; this one has been drifting away from the pack, the closest I got it was within 54mV. I cant even get close to my balance open voltage at 3.4v. It seems to jump to 3.65 very quickly then comes way down under any load at all, at rest its all lower than the others.

When I was top balancing with my lab supply it by itself it never seemed to get to 3.65v either; despite A draw tailing off to 0.05C,

Is it a naff cell?

 

[Quattrohead]

Yep no good. Try again.

 

[kommando]

Best you go back to Fogstar with that info and see if they want any more testing but it looks like a dud. My 16 Envision grade B's have never missed a beat, Currently at 53.96v and a delta of 0.005v.

 

[swwils]

Best you go back to Fogstar with that info and see if they want any more testing but it looks like a dud. My 16 Envision grade B's have never missed a beat, Currently at 53.96v and a delta of 0.005v.

Ah that's a bummer.

It's extra annoying because this is a 2nd replacement, the first they shipped was dented and the other cells are so good. I really like them for the price.

 

[kommando]

I have 16 280's and 16 305's from them and they were all good, Fogstar can be slow in responding but they seem to respond well.

 

[swwils]

Fogstar confirmed baddy cell. Replacing. Thanks fogstar!

 

[wpns]

I really like them for the price

Except for the 3 bad ones? 17% failure rate? I'd mark that vendor as 'dodgy: no not use' and move on.
[Well, unless they swap it for free and you get a good one...]

 

[swwils]

Except for the 3 bad ones? 17% failure rate? I'd mark that vendor as 'dodgy: no not use' and move on.
[Well, unless they swap it for free and you get a good one...]

They have been good with swapping so far.

First cell had a bad laser weld, this probably could have been caught on a advanced visual inspection, but otherwise is most likely very rare. I've only ever seen it one other time.

Second cell was dented - I am not really sure why they would have sent a replacement like that, it wasn't damaged during delivery.

This 3rd one, I am very surprised as it would have easily shown up on a IR or capacity check, it's also a different batch to the first cells. I would guess that this may indicate issues with this later batch.

Little bit annoying as I'll have to disassemble the whole case to swap it, but this is better service than other vendors really. Plus the price per ah is unbeatable.

 

[swwils]

Replacement cell has the same behaviour!

Is it possible cells form a newer batch will never match my originals? Or that the BMS is wrong on one cell?

 

[kommando]

Or that the BMS is wrong on one cell?

At the danger of messing up another cell swap with another cell and see if the issue follows the cell or stays in same position ie its the BMS.

 

[swwils]

At the danger of messing up another cell swap with another cell and see if the issue follows the cell or stays in same position ie its the BMS.

I have been checking with my multimeter occasionally and it all seemed normal - i.e. the cell voltage on the meter was what the BMS saw - so it's a long shot.

Will try a swap monitor lead later on because dissembling isn't that easy, you have to take the whole loom off to get to the pressure plate! If that doesn't work I'll swap the bad cell to own of the top positions to make this easier.

Can't think of any failure mode that would over report then under report other than these replacements being wildy different IR.

 

[wpns]

Can't think of any failure mode that would over report then under report other than these replacements being wildy different IR.

I'm sure they have a pile of random cells, and they just pull one out and ship it to you when you have a problem with a single cell. Matches your experience... Otherwise they've got to break a case or 4-pack or something and have the rest of them sitting around waiting for someone else to have an individual cell problem.

 

[swwils]

After checking the BMS wiring looms, they seemed OK. I checked the cell again and BMS was reading the voltage wrong compare to my mustimeter on the terminals! Just on zone 9. Must be a bad calibration of the BMS!

I will re-top balance and then try to find some details on how to voltage calibrate the Seplos V3.

 

[swwils]

Ok the voltage calibration for the Seplos V3 is easy enough

You discharge the pack, charge cells to 3.3v, let it stabilise, measure it, then hit calibrate. What could go wrong there!

 

[swwils]

Voltage calibration completed, I've cycled the pack a couple of times now and all cells within 0.003v .

So I guess the story is to trust your own equipment not reading of the BMS.

 

[GhettoSuperstar]

I know this is off-topic but what software or setup are you all using to test?

 

[swwils]

I know this is off-topic but what software or setup are you all using to test?

Its Home Assistant and a ESP32 running https://github.com/syssi/esphome-seplos-bms connected to the BMS RS485.

This will only give you the BMS reported data so an external multimeter will give accurate cell voltages. (I have a Rigol DM858)