Speculate what happened to my cells

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Sunder
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Speculate what happened to my cells

Post by Sunder » Wed Nov 08, 2017 1:03 am

Alright, so I have no idea what happened to my pack. It's a massive LiFePo4 8S23P battery, charged by a solar charger, and discharged by a 3000W inverter. I have several theories, one of which is a prime candidate, but I don't know how to test for it. They are:

1. The pack was over-charged. The BMS was correctly cutting off the charge at 30v, (24v) battery, so this seems unlikely, but if it wasn't balanced, possible
2. The pack was over-discharged. Again, the BMS does correctly cut power - but again, badly unbalanced, it's a possibility.
3. The BMS was continually bleeding cell row 1, causing it to run flat, while the other 7 rows were fine. This is my favourite.
4. Single possible single cell short, causing the destruction of every other cell in that group?
5. Something else?

Here's a photo of the pack:
Pack-small.jpg
Pack-small.jpg (193.86 KiB) Viewed 106 times
The cells look like this:
Cells-small.jpg
Cells-small.jpg (127.72 KiB) Viewed 106 times
The rest of the pack looks like this:
Nextrow-small.jpg
Nextrow-small.jpg (168.52 KiB) Viewed 106 times
The facts from testing are:

1. The puffed cells are at 30% capacity, and terrible internal resistance.
2. The non-puffed cells have 100% original capacity. I can't load test such a large pack for internal resistance.
3. The gaffer's tape holding every other string together have pulled off, showing that at one point they too puffed, but deflated back. This doesn't fit with my favourite theory.

I'm fairly certain I'll never find out for sure what happened, but I'm keen to know what you think might have happened.
eBike: Q100H on 16S with Phaserunner FOC Controller
eMotorscooter: Vectrix VX-1 on 36S
eCar: Mitsubishi Outlander PHEV... Waiting for warranty to expire
eHouse: Still on grid, but with LTO batteries and 3kw LF inverter...

After 5 builds, the best advice I can give, is start with high quality products. I prefer http://www.ebikes.ca

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liveforphysics
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Re: Speculate what happened to my cells

Post by liveforphysics » Wed Nov 08, 2017 1:10 am

Some events produce gas in a pouch. Overdischarge means the SEI layer gets de-lithiated to mechanical contraction induced fractures, followed by eating the copper metal current collection foil next if it's out of Lithium.

Overcharge breaks the carbonates in the solvent into CO2 and a handful of other gasses in half reactions.

Mechanically damaging the pouch layers through mis-handling may also result in excess gas production.
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Sunder
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Re: Speculate what happened to my cells

Post by Sunder » Wed Nov 08, 2017 1:13 am

Wonder if I can puncture one and see what gas comes out then ;) What's the gas from overdischarge?
eBike: Q100H on 16S with Phaserunner FOC Controller
eMotorscooter: Vectrix VX-1 on 36S
eCar: Mitsubishi Outlander PHEV... Waiting for warranty to expire
eHouse: Still on grid, but with LTO batteries and 3kw LF inverter...

After 5 builds, the best advice I can give, is start with high quality products. I prefer http://www.ebikes.ca

User avatar
liveforphysics
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Posts: 14410
Joined: Wed Oct 29, 2008 1:48 am
Location: Santa Cruz, CA, USA

Re: Speculate what happened to my cells

Post by liveforphysics » Wed Nov 08, 2017 2:31 am

Sunder wrote:Wonder if I can puncture one and see what gas comes out then ;) What's the gas from overdischarge?
Anything that disturbs the SEI will require re-passivating the surfaces exposed by the cracks. As it undergoes new formation/passivation, it makes CO2 and H2 dominately, and I don't know precisely how many sides reaction gasses.
Each carcinogen vapor exposure includes a dice roll for cancer.

Each mutagen vapor exposure includes a dice roll for reproductive genetic defects in your children.

Each engine start sprays them into a shared atmosphere which includes beings not offered an opportunity to consent accepting these cancer experiences and defective genetics life experiences.

Every post is a free gift to the collective of minds composing the living bleeding edge of LEV development on our spaceship.

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