2 Millisecond short

MadMax03

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Sep 21, 2019
Messages
249
This has probably been asked a thousand times, but I was assembling a LifEpO4 battery pack and accidentally shorted a cell for a split second while attaching terminal leads. The short lasted for as long as it took to make a loud snap and throw a few sparks, sending the cat (who thought I was making him breakfast on the counter, not building a lithium battery) into freakout mode and hauling ass to the living room.

Did I harm the cells?

Attached is just a pic of the battery. I made a 12V, 120ah out of 40ah cells.
 

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MadMax03

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You shouldn’t be charging without a BMS!

Watch you individual cell voltages, YOU are the BMS in this setup.
On that note, I have finished charging, and full pack charge is 14.51V. But here are the individual cell voltages from right to left:

3.35
4.30
3.36
3.43
3.53
3.35
4.19
3.39
3.74
3.33
3.61
3.81

What gives? Are these bad cells?

Here is the link to these cells:

 

MadMax03

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Since there was no BMS or balancer, it looks like some of your cells got overcharged. I was hoping this was an April Fool's thing, but no so sure now.
I haven't put a balancer on it yet. This is not an April Fool's thing. This battery will be going into a car, replacing the under-hood lead acid. So it will be charged with the alternator, and not through a BMS. Alternator charging cable will go straight to the positive post of this battery, the negative post of the battery to the factory ground location.
 

Any name you wish

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I'm all ears if you know of a BMS that can keep up with a 320A alternator. Thanks for your input though.
So, you are putting a LFP battery set, under the hood of a massive heat sink, without a BMS. Straight charging with a 320A alternator. No balancing....because? I get the idea of doing LFP for EVs as they don't have a massive heat box there, but for combustion, why?
 

100 Proof

"Please Lord, don't let me do something stupid."
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MadMax, It sounds like you are setting yourself up for failure. Possibly a spectacular and catastrophic failure.

1- You did not top balance.
2- you are planning to charge and discharge at greater than 2C rates.
3- You do not appreciate the purpose of a BMS.
4- You're exposing your cells to temperatures that may be well above or below their design envelope.

Please reconsider for your own safety.
 

TacomaJoe

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I doubt that the short damaged the cell. Overcharging the cells (above 3.65V) would damage them more. You need to balance the cells manually to get them close enough for any active balancer to be effective. Not sure if an active balancer can keep up with the alternator charge current.
 

MisterSandals

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MadMax, It sounds like you are setting yourself up for failure. Possibly a spectacular and catastrophic failure.
And what ever happened to MadMax01 and MadMax02?
I'm all ears if you know of a BMS that can keep up with a 320A alternator. Thanks for your input though.
I think you are mistaken about the capabilities and limitations of LiFePO4 batteries.
Spend a few minutes watching the videos about LiFePO4 batteries on the sister-site:
 

Horsefly

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Denver, mostly
4.3v is cell bloating territory.
And because he isn't going to use a BMS (and especially since he apparently didn't top balance first), every charge / discharge cycle will have those same cells get over 4V. They will bulge and probably die an untimely death.
 

A.Justice

Swears he didn't start that fire.
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On that note, I have finished charging, and full pack charge is 14.51V. But here are the individual cell voltages from right to left:

3.35
4.30
3.36
3.43
3.53
3.35
4.19
3.39
3.74
3.33
3.61
3.81

What gives? Are these bad cells?

Here is the link to these cells:

You need to discharge those some and then top balance ASAP.

You also NEED a BMS.
 

MadMax03

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Messages
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You need to discharge those some and then top balance ASAP.

You also NEED a BMS.
You are right, I did not top balance. I balanced all at 3.272 then threw it on the charger. My mistake. There still is no solution to run a 300A capable BMS in series with the alternator charging cable. This isn't really an issue. LFP is used pretty extensively, along with LTO and LiMn2O4 Ford C-Max batteries in the car audio world. Passive balancers are used, but never a BMS. No explosions yet.
 

MisterSandals

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Location
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There still is no solution to run a 300A capable BMS in series with the alternator charging cable. This isn't really an issue.
Not an issue? It is nearly 100% of your issue.

The boating folks with bow thruster by LiFePO4 aspirations are dealing with this issue (crazy amps and need for a suitable BMS):

 

MadMax03

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Sep 21, 2019
Messages
249
Not an issue? It is nearly 100% of your issue.

The boating folks with bow thruster by LiFePO4 aspirations are dealing with this issue (crazy amps and need for a suitable BMS):

Nope. Not an issue. Go to an SPL competition someday and see for yourself. Tons of car stereo enthusiasts been running LFP in their cars for years now, including myself.
 
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