Replacing a single cell from the middle of a Lipo pack

I was asked in the Zippy/ Flightmax / Turnigy/ Blue lipo testing (pt 2) thread to post some pics of how I replaced dead cells in some of my LiPo packs. Since this was a bit off topic, I created this thread.

If you have puffed cells and are willing to try and de-puff them, see AussieJesters video here: http://endless-sphere.com/forums/viewtopic.php?f=14&t=22650

And now the usual disclamer: This is a delicate task that you can easily mess up. Only do this if you have the right tools and skills. You are doing this at your own risks.

OK, now the candidate pack. This is a Turnigy 6s 5000mAh 25A pack. I removed the blue shrink wrap and some tape and plastic at the ends. As you can see, the third cell from the bottom is busted. This is the one I'm going to replace.



The cells are individually wrapped in clear plastic and glued together. To separate them, I use a thin plastic card that I slowly push down between the cells. To help it out, I put a bit of rubbing alcool on the glue using a Q-Tip. This is easy to do, you don't have to force it hard. The card I used is my MEC membership card. Its thinner than a credit card and doesn't have embossed characters. I think some have used a plastic spatula as well.



Next step is to remove the existing solder. To do this you need a BIG iron and a BIG tip. I'm using a large copper tip initially intended for soldering nicd/nimh cells end-to-end. Its got a lot of thermal mass. Of course you also need a pump or wick to remove the solder.



Before you start getting anywhere near those cell tabs, make sure to mask all the terminals with several layers of tape. Kapton would be ideal, but since I didn't have any I put 4 or 5 layers of vinyl.

The cell tabs are bended at a 90 degree angle on the terminal. After you've removed most solder with the pump, heat the tab a bit and gently pry it up with a small flat screw driver.

In this picture the solder has been removed and the tabs are up.



After the tabs have been released, just pull the cell out.


(notice how I marked the polarity on all cells. You really don't want to mix this up!)

Now carefully slide a new cell in. TRIPLE CHECK THE POLARITY BEFORE YOU DO IT. All you have to do now is reverse the process and resolder the tabs.

Here's the negative (looks better than the original IMO)



And the positive (don't forget to solder back that balance wire)



Final check-up



All you have to do now is re-wrap the pack properly and test your pack. I'll post a discharge curve of this pack when I get a chance.

Nice work.
How long did the whole process take you ?
I'm not sure I could be bothered these days when you have to canabalise another pack but if you've got a few dead cell packs laying around it's a good way to salvage what you've got.

what would be good is to have a parts pile list in the used for sale section where people could sell individual used pouches. i have some ping packs i am trying to salvage the same way by taking out individual puffers and broken tabs so i have been trying to find loose lifepo4 pouches too.

also for the packs that get taken apart, you can just cut out the section of the pcb connector on top for that pouch, then heat the solder more easily, and whack it off rather than use the solder sucker. that makes it easier to reach the tabs to unbend the end.

but maybe just not unsolder it from that section of pcb, but cut out the pouch that has to be removed by cutting the section of the pcb that holds it and then solder the replacement back into place in the pcb with short wire jumpers connecting them in parallel.

Theoretically, could you just convert such a pack to one with less cells? I'm thinking you cut out the old puffer, and then bridge the gap to reconnect the remaining cells in series. For guys like me, who can't solder for beans, or don't have the tools, at least you'd still have a lower voltage pack that's good.

Could you just jumper the pack and leave the puffer in place? Or would that short it, and make a puffer into hazard.

Thank you !!!

I have 6 packs that need work like this, .. yes.... 6...... grrrrrrr.....

Hyena said:

How long did the whole process take you ?
I'm not sure I could be bothered these days when you have to canabalise another pack but if you've got a few dead cell packs laying around it's a good way to salvage what you've got.

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It took me a bit over one hour. I had removed some cells before, but it was the first time I did a replacement.

Next one should be quicker.

Hyena said:

I'm not sure I could be bothered these days when you have to canabalise another pack but if you've got a few dead cell packs laying around it's a good way to salvage what you've got.

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I agree. I have some healthy loose cells from another pack I took apart this summer.

dnmun said:

heat the solder more easily, and whack it off rather than use the solder sucker

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I like the idea of a swift "swing" of the pack to throw away the molten solder instead of slowly removing it with the solder sucker. As long as everything else is well covered and protected on the pack, this could be a good technique to remove most if not all the solder on a terminal in a single "heating" event. I'll try it on my next pack.

I plan to hit the liquid solder with compressed air !!

Ypedal said:

I plan to hit the liquid solder with compressed air !!

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Good idea !

Have you had problems with any cells puffing after sitting a while without the heat shrink on them? I have had one 5Ah Turnigy pack where a cell puffed after sitting over night with the shrink wrap off, so I've been nervous about removing the shrink on any other packs. Note that this was a brand new pack and I didn't charge/discharge it while it was unwrapped!

you need to account for the solder SPLATTER after swinging it or blowing it off with air (bill tried that once too).

if married, solder on the ceiling or gramma's dining room table in the dining room will also involve cost of hiring a lawyer for your divorce.

Any work of this type should be done outside or in my case in the garage with the big door open for an "Emergency Maneuver" so blowing the liquid solder away from the pack oposite directionof the other tabs should work out fine.. i've done this on circuitboards with a straw and lung power but i figure air compressor and needle valve keeps my face further away lol..

I,m a solder slinger, but will like to try compressed air. Shoot it at some plywood to be thrown away.

cord said:

Have you had problems with any cells puffing after sitting a while without the heat shrink on them? I have had one 5Ah Turnigy pack where a cell puffed after sitting over night with the shrink wrap off, so I've been nervous about removing the shrink on any other packs. Note that this was a brand new pack and I didn't charge/discharge it while it was unwrapped!

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Nop, my loose cells have been sitting in an ammo box for 6+ months at 3.75V with no puffing. I also charged and discharged them once or twice while unwrapped to test their capacity and no puffing occured. (charging + discharging at 1C and only down to 3.5V)

From my experience, a puffy cell puffs, shrink wrap or not. Nothing stops it. The shrink wrap does seem effective on the normal cells tendency to swell some, but not a puffer. A really puffed pack blows the shrink wrap apart.

double down on dogman's comment. i found that puffers would have other puffers tied in parallel. when they swell in the duct tape, all the pouches on the outside get totally deformed by the puffer pushing them out.

when you cut the duct tape, the lifepo4 packs pop the tape at the last part of the cut and swell so much they will pull the tabs off the pcb subconnector for the pouches on the outside.

i think the reason the puffers are together is that they are all subjected to the same over charging for long periods, so they are all overcharged to the same level. the other pouches which did not spend a long time being overcharged all remain slim and flat, in the same pack, adjacent to a group of puffers.

i recommend turning down the charging voltage on the ping packs to the point where you can just get the pack to balance, then take it off the charger. i think that will reduce the incidence of puffers showing up, but the worst cause of puffers is still over discharging, imho.

best is to be able to charge it up to balance right before you ride, and not keep the pack fully charged until needed.

when i get money to waste, i wanna get some cell logs too.

Yeah, overdischarge is how I puffed the lipo pack that blew it's shrink wrap. It got disconnected from a paralell pack, and run to 0v. So it got unbelievably puffed.

I have some other lipos, that arrived a bit puffy. In use, they seem to just swell a little, and shrink a little as they discharge. One still has the shrink wrap, the other is just taped. Not many cycles, just a few runs on the race bike but they still charge and discharge normal.

Iv'e not had any puffy pings, but I do about half my charging with lower voltage, not able to balance sla chargers. Hmmm. Low amp rates surely help too, and the bms's lvc works.

you won't see the puffy pouches in the ping until you cut the tape off. i wonder if the puffing is caused by the high internal resistance as the cell is damaged is causing heating at the electrode surfaces during discharge and that causes the electrolyte to break down and produce CO2 or maybe just CH4.

i have cut some open to vent the gas and then resealed them but they continue to puff once they start swelling, but the swelling pulls on the tabs of the outer pouches in the 2S group tied through the pcb subconnector. this causes the end tabs to be pulled back over the fold made in the tab at original assembly and that is where i find the tabs are breaking off, and electrolyte/lithium hydroxide released too.

dnmun said:

... and electrolyte/lithium hydroxide released too.

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In my Lipo's the electrolyte/lithium stuff has a very distinctive smell (smells good actually). So far, 3 of my packs contained cells with leaking electrolyte and I could tell even before removing the shrink wrap just by smelling them.

I think when you replace a cell or with ping cell group. You could leave a little z in the tab before the pcb for a little freedom. My problems with a split pack 48v20ah ping both times on the 16th cell. I should had left some room there with the tab right ?

yes, ping must have a special machine that folds the ends of the tabs so that as the pouches swell up over time, the tabs can 'unroll' because the pcb ties the plus side of the lower cell to the negative of the next cell, and the pouches, as they swell, push the tabs of the pouches on opposite ends of that pcb subconnector over the top of the pouch. it unfolds over the mylar and if you look closely, the tab where it comes out of the mylar, bends through 180 degrees so that the first part of the tab sticking out of the mylar folds back over the mylar.

then there is a little squiggle up to the spot where it goes into the slot in the pcb. i think they assemble it by stacking them in groups, with the opposing groups of pouches with their orientation reversed for each cell. then putting the little pcbs on, bend over the ends and whack it with a buncha solder

i am now thinking it may help reduce the tab tearing if we could cut the pcb subconnector at the end of the cell, ie: between the opposing pouches, then reconnect with a short jumper of stranded cable, or even a piece of solid wire, like 12G bare ground wire bent so it can allow the two section to move apart as the pouches in each of the groups that make up each cell then could move independently and not put so much pressure on the tabs of the outer pouches. then release the tape, let it swell without tearing the tabs off. gonna try that next, with the pack confined in a box i can assemble while compressing the ends with some clamps. that will keep the pouches under pressure and make it easier to service again later.

that is why you have the #16 failing, because it is at the end of the pack, so the outer pouch is pushed out and that tears off the tab on the anode. after that the pack doesn't balance and you end up with a dead battery in a few weeks.

almost all the bad pouches i have identified in these dead ping packs are the ones on the end, with anode (+) tab rotted off by electrolyte where it exits the mylar, like a clean break right along the mylar border.

El_Steak said:

the electrolyte/lithium stuff has a very distinctive smell (smells good actually).

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Hah, you'd probably like the smell of anthrax too :lol:
It' a sort of sweet smell isn't it ?
A few times I''ve been out riding and copped a whiff of what I though was that electrolyte. Needless to say with the batteries located between my legs I've quickly dismounted!
I wasn't a cell venting though, it was the pollen of some flower/plant I was riding past!

I'm a solder slinger from way back (my workshop floor looks like I've dispatched a frozen T1000 terminator :lol: ) but I got a new air compressor for christmas so I'll have to try that.

Just a note of caution, I fixed 3 more packs and on one of the cells I had trouble getting the solder to melt. I heated the cell a bit too much and it puffed. Just a slight puff, but still.

I'm not going to keep this pack, I've ordered a few more from HK.

At 60$ shipping included for a new Turnigy 6s 5000mah 20C pack, I'm not sure its worth the time, trouble and risk of replacing individual cells.

I had two 6s packs with a weak cell so when I got my new 2s pack I did some lipo surgery. To my amazement, all went well! I made a little time lapse for Sh*ts and giggles.

And for the record, I duct taped the ends, put the hard plastic back around the outside, then put on more duct tape followed by black shrink wrap. It looks a lot better.

The camera stopped recording half way through the second pack so I cut it short. The damaged cell in the pack was on the middle, cell #4.

[youtube]4OZd7OIuQbo[/youtube]

dnmun said:

whack it off rather than use the solder sucker

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You should ask the pack what technique gets it off best