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To replace dead cells; charger port reads 0 volt - TGF3 batt pack


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My TGF3 stopped working recently after low battery warning. Then the battery no longer charges (charger light stays green when connected).

I checked the battery pack (260 Wh, 16S2P) and found that 2 cells are dead, they both read 0 volt.
Other cells read 4.0 or 4.1 volts which give total voltage of 58.2 volts at the power port.

I'm thinking to try replacing the dead cells. Any tips on how to do this? What is a good way to break the spot welds?

Another thing is, I don't understand when I measure the charger port, its voltage is 0. Is this because of BMS feature that the circuit locks from charging when there is dead cell? Or is the BMS damaged?



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Your BMS might have a protection diode on the charging circuit so there is no live voltage to the charging port.  A few other people have been able to swap out dead cells.  I think Wakefultraveller was looking at the process, and there are some links in his thread that point to another user's solder repair job.

Found it...


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@stillet, it is not always possible via a photo to be sure what a circuit board is doing but your pictures are nice and clear. As @HunkaHunkaBurningLove Has stated it looks like you have a pair of protection diodes D5 and D6 in the negative line, just to the left of C-. If you check with a meter between C+ and the solder pad right next to the D5 or D6 legend you may find it is 58volts there, Altough there is also a transistor, the black one left of D5  & D6 in the circuit, that connects C- to P-.

To change the cells, assuming you intend soldering them, the important thing is to get exactly the same cells with tags and keep as much of the tag from the next cell as possible. Only solder tag to tag don't solder onto the cell as heat can damage them. You may be able to break the spot weld on the faulty cell with a screwdriver under the tag, but obviously be damn careful about possible short circuits, a non conductive lever would be best.

I have yet to replace any 18650 cells, although of soldered plenty of NiCads in the past. However, with the LiPos I've used for model flying, cells can be seen to fail as they puff up being soft cased. Where one cell has puffed up badly and I've split the pack, for example to get a single good cell to use for onboard glow power for the engine, the cell has always had a very short life, Once I actually found someone selling single cell LiPos, I've never bothered salvaging cells since.

So, yes, it is worth experimenting with changing the faulty cells, but I would consider getting another pack anyway as it may soon have additional failures. If the pack does behave you, at least have the possibility of doubling your capacity.

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Thanks @HunkaHunkaBurningLove, @Keith, happy to know the BMS is still good. I checked voltage as Keith suggested, between C+ and solder pad near D5, yes there is 58.2 volts there.

I've ordered similar cells Samsung ICR18650-22P with U-tag from www.nkon.nl

Next step is to remove the dead cells. Will try to pull out only the dead cells, but if that's not possible looks like I have to peel off all the spot weld tags (32 tags) from BMS and pull out the whole cell pack. I'm pretty worried about doing this :unsure:.

Any more tips will help me, thanks :D


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As far as I can tell from the photos you posted, it appears that the tabs on the cells are slid through that slot or folded over the edge of the BMS board.  It might be a matter of just taking a small pry tool like a jewelry screwdriver to lever the spot-welded tabs off the board allowing the batteries to slide out?  Or if you have access to a trimmer or small cutting tool you could slice the tabs near the spot-welds and take the batteries out leaving a small amount of the old tab in place.  Some of these spot-welds are pretty tenacious as the metal fuses together so it's hard to get apart.

You might then be able to insert the new cells with tabs already attached onto the board, trim off any excess nickel tab material and solder the tabs to the board contacts or spot-weld them back on if you have a spot welder.  If not, you could always take the board with the batteries to some battery shop to see if they can spot-weld them on for you.  We have a place called Battery World that can make custom cells for various applications, and they helped spot weld some end tabs onto a new battery for my Sonicare toothbrush.

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You don't have to peel the other cells. It's easy to remove only the defective ones.
Few months ago, I made the same operation on my TG F3.

1- Insert a screwdriver between the cell you want to remove and the 2 neighboring ones and "sweep it" between them until they are detached from each other.

2- Protect the BMS PCB to avoid short-circuits.

3- With a screwdriver raise the nickel strap to disconnect it from the BMS. You could have to use a plier.


4- Remove the damaged cell
5- Rise the soldering iron temperature


6- Apply some flux on the new cell


7- Apply some Tin on the cell and the strap


8- Solder them together. Both sides.


9- Test the strenght


10- Insert the new cell respecting the polarity
11- Maintain the strap on the BMS and solder/spot weld it
12- If you have a glue gun, use it to fixe the new cell with the others
13- You even could double the battery cells


Since you ordered cells with U-tag, you won't have to solder/spot weld the nickel straps on the new cells ( steps 6 to 9) but it could serve to someone else.
This worked for me and allowed me to double the range of my TG F3 (from 6 to 15km) but, as @Keith said, you'd really consider bying a full pack.


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