FPV Repair Shop #5 – Li-po Batteries Cables Replacement

Today we have 4 faulty 4S Li-po batteries on the operation table. Lets see what’s wrong with them and how to fix them.

DISCLAIMER: Operating on live batteries is very dangerous. This post is made for educational purposes only. This is not a tutorial. Do NOT try this. I may NOT be held responsible or accountable for any damage you do to yourself, others, or any property.

Battery 1

This battery was in a quadcopter crash that broke part of it’s balance cable. I’ll need to replace the balance cable for sure, but I want to make sure that no cells were damaged in the crash.
Also, some of the insulation was ripped and wrapped with electrical tape, so I’ll have to replace that too.

I measured the cells’ voltage through the broken balance cable and they seem fine. Voltage is good and pretty uniform and the internal resistance is also fine and uniform. This means that the cells are good and the only problem with the battery is the broken balance connector.

How can this be fixed?

There are a few ways I can fix this depending on the damage level and how much work I want to put into it.

The easiest fix would be to cut the balance cable in the middle and solder a new balance cable and connector as an extension, but I don’t really like this solution. I did that to one of my batteries once, and it makes working with the battery not very comfortable because the solder joint and heat shrink make a rigid section in the middle of the flexible cable and make it hard to tuck it in during flight.

Another solution is replacing only the plastic connector. This means extracting the metal tabs from the plastic connector and inserting them into a new one.
This works great and can save opening the battery if the tabs are in good condition, but it’s fiddly work.

The ultimate solution is to replace the whole cable. This means removing the battery’s insulation and shielding, desoldering the connections from the cells themselves and soldering the new cable directly to the cells.
This is the best solution, but it’s pretty dangerous opening the battery, so it’s not for everyone.

In this specific case, the cables are fine, the connector is broken and the tabs work but don’t look that great. I decided not to take a chance on the tabs and replace the whole cable up to the cells.

Opening the battery

First, I took all the insulation tape off and see if there’s any more damage.

The cells themselves look intact and not swollen. Some of the shrink tube was ripped, so I’ll have to replace that too and make it look better.

Next, I removed all the padding, adhesives, insulation and shielding from the battery to access the solder points of the cells, but I made sure to cut the outer shrink tube neatly on one side so I could put it back.


There are 4 cells in this battery, each cell has 2 contacts and they are all connected in series. The balance leads are connected to all the different solder points.
The main positive and negative leads of the battery are soldered together with the two outer wires of the balance cable. This means that I need to be careful desoldering these wires so that I don’t desolder the main leads with them.

Another thing you have to remember when soldering and desoldering on a live battery – IT’S LIVE and it has A LOT of energy in it. Shorting the leads or the battery pads with the soldering iron, your tweezers or pliers, or even the cable you just desoldered flapping in the wind will cause some sparks if you’re lucky and start a big impossible to stop fire and A LOT of smoke if you’re less lucky.

I bumped the soldering iron’s temperature a bit to help with the big solder joints and to help melt part of the solder fast so I could take the balance wire out and in before the the rest melts and the main leads fall out.

Wrapping it up

I tested the voltages through the balance wire and the main leads to make sure the operation was successful and it seems fine.

I replaced the Kapton tape over the solder joints (the orange tape in the pictures) and put all the padding and shielding back on the battery.
I put the old shrink tube on and put some new clear shrink tube on top of that to hold it all together and make it look good.

Battery 2

I’m not entirely sure what happened to this battery, but the battery voltage tester only detects 3 cells out of 4. This could be a dead cell or a connection problem.

Taking it apart

The balance cable and the connector seem fine and don’t have any obvious signs of damage, so I’ll have to inspect the cells.
In order to test the cells themselves I need to take the battery apart.

After removing all the insulation and protection, I tested the voltage at the cells and they all seem fine, so there’s no dead cell.

The solder joints look fine too, so it’s not a connection problem at the cells. I think that one of the wires in the balance connector got loose from the metal tab and doesn’t make contact or the metal tab was bent and doesn’t make contact with the pins of the male connector.

Either way I’d replace the balance cable, so it’s convenient that I opened the battery.

I replaced the cable, tested the voltage again and wrapped it up with all the original protection and insulation including the outer shrink tube and a new clear shrink tube.

Battery 3

This battery got it’s main leads’ insulation slashed by a quadcopter prop.
It seems to also be missing it’s shrink tube.

How can this be fixed?

These leads are very thick and carry a lot of current. I wouldn’t recommend any fix other than replacing the leads with new ones.

The XT60 connector can be transferred from the old lead to the new ones, but I prefer to just solder a new connector.

Here we go again

I took the battery apart to expose the solder joints. This time I’ll have to be extra careful when I solder and desolder the big leads, because they will require more heat and more time to melt the solder, which means that it would be harder to desolder only the big leads without moving the thinner balance wires.

I replaced the original Kepton tape, added some more fiber reinforced tape and put new shrink tube on the original label.
A quick voltage test and it seems as good as new.

Battery 4

This one has it’s balance cable ripped completely off. luckily, it didn’t short.

I can measure the cells’ voltages from the exposed wires, but it’s pretty scary when all the cables are flapping in the breeze and I would need to take the battery apart to fix it anyway, so I’ll measure the voltages directly on the cells when the battery is apart.

Taking it Apart

In this battery, the sticker is under the shrink tube, so I don’t need to keep the shrink tube.
This battery has some nice shielding, it’s a 0.5mm aluminium plate on each side, rather than the glass(?) fiber plate on the first two batteries (looks like pcb material).

I removed all the adhesive tapes and kept the black one just because it has the branding sticker on in. I’ll throw the fiber reinforced tape away because it’s full of dirt and I have a new roll of similar tape.

Further Inspection

After exposing the solder joints, I can test the cells to see if they are all good.

I first desoldered the balance wires to make it easier to work with and then measured the voltage and internal resistance of the cells. They are all fine.
This means that I just need to replace the balance cable.

I soldered a new balance cable and tested the voltages through it to see that it works.

Closing it Up

The reinforced tape that holds the shielding is not sticking very well because of the dirt it accumulated, but it’s still stuck very well to the shielding and the padding on the solder joints.
Replacing the tape would be hard and might damage the padding and the shielding, so I kept it and wrapped it with more reinforced tape.

I put the original black tape with the sticker back and wrapped it all with new clear shrink tube.

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