Anyone try to repair a failed 18v battery pack?

[silence2-38554]

This may go beyond the scope of this forum, but I’m curious if anyone in here has experience with building / designing Li-Ion battery packs?  Essentially, one of my 4.0 Ah batteries took a dive recently, as it was slowly self-discharging & wasn’t reading the voltage it should have a full charge.  Festool support took care of it & sent me a replacement, no questions asked!

So, all is well & good with my surplus of batteries, but being the tinkerer I am, I decided to pull apart the pack & do some investigation.  I discovered these batteries use Samsung 21700 cells, five of them.  When measuring the DC voltage of each individual cell, I found that cell #4 was reading a significantly lower voltage than all the others.  In fact, all the others were perfectly balanced!

So, I ordered a replacement cell for a whole $5 & took the time to swap it into the pack in place of the faulty original cell.  I charged it up, everything seemed mostly fine for a day or so of testing with it, then I ran it completely down to zero & when I connected the pack to my charger, I got the dreaded blinking red LED.  Pulled the housing off & re-tested the cells.  To my disappointment, the brand new cell I’d just installed was completely dead, while all the others were reading a fairly strong remaining voltage.  Not only that, but I tested continuity for each cell through the pack & the new cell is actually showing zero resistance / strong continuity between its poles, which no battery should ever do.

And that’s where I’m at now.  I know all of this really isn’t worth it to salvage a battery pack, but I’m just so dang curious!  Mounted atop the cells in the housing is a small BMS board that a bunch of sensor wires attach to.  It seems to me the only question is: Did I just have horrible luck & receive a defective 21700 cell?  Or, the more likely case, does this battery have a defective BMS board?  If so, would anyone have any idea where to source a new BMS board for these batteries?

Thanks for reading!

 

[AstroKeith]

First - do take care! Lithium cells, if faulty can explode and even good ones if over discharged or over charged.

The BMS doesnt charge individual cells, but monitors each one.
Assuming you reconnected the sense lines correctly, then the BMS may have had trouble with balancing the 5 cells, as the new one would have been at a different charge state to the other 4.

But the measurements you have do describe what looks like you have a duff new cell - now at least.

Was it a good make, or a 'rip off'.

 

[peter halle]

Well, as has been noted, lithium battery replacement is not for the faint of heart and has real dangers.  We have had a policy in place since Festool USA took over ownership of this forum to not promote, allow, show dangerous practices and over the years moderation has taken place as needed.  Have there been instances where things got missed?  Of course.  We aren't perfect.  But be advised that this thread has a very distinct possibility of going that way.

Just a heads up.

Peter. Halle - Moderator

 

[Peter Kelly]

How did you attach the nickel connector tab to the replacement? Lithium-ion cells need to be resistance welded not soldered as the heat will damage them. 

 

[silence2-38554]

Thanks for the replies everyone!

[member=72891]AstroKeith[/member]

I'll be taking all of the cells to be recycled today after I strip them from the internal battery frame.  Since two are testing continuity between the poles, they're making me a bit nervous.  I did notice when I received the new cell that it was partially discharged & I'm unable to charge individual cells, so it's possible the BMS just couldn't deal with properly balancing.

[member=1674]Peter Halle[/member]

Thanks for the heads up.  I was unaware & can totally see their caution around the topic.  The further I dive into it, the more I'm realizing how much specialty equipment is necessary to do these types of repairs safely & effectively.

[member=15972]Peter Kelly[/member]

Honestly, this is likely where I veered.  I've now purchased & returned two models of spot welders from Amazon for this project.  Neither could successfully re-weld the factory tabs.  A very powerful welder is necessary if attempting to re-weld the factory tabs, as Festool uses .3mm tabs to connect their batteries, which I've come to learn are fairly thick in the world of battery packs.  In my frustration, I decided to solder them, as I have all other electronic components my entire life.  After watching some thermal imaging videos on soldering vs. welding batteries this morning, I've decided to scrap all of these cells & save the remaining components for another day.