Fix LiPo Batteries with Bad Cells

Lipo batteries degrade over time, sometimes this degradation results in dead cells. It seems to be a waste to throw the whole pack away because other cells might still be working. In this post we will show you how to make a working LiPo by combining the good cells from multiple damaged/defective batteries.

Warning: LiPo batteries can be dangerous, please understand the risk before doing this DIY hack. If you are doing this hack, you are doing so at your own risk, the author and OscarLiang.com are not responsible for any fires or damage caused.

The Lipo batteries have to be the same brand, capacity and discharge rate for maximum safety and performance. Do it outdoor if you can, wear a pair of safety glasses just in case :)

Following this guide to dispose LiPo batteries.

What you need

  • Multiple LiPo batteries with dead cells (with enough good cells to make a working battery, for example for a 4S LiPo you need 4 good cells)
  • A multimeter to check battery cell voltages
  • Wire cutter
  • Stanley / Craft Knife
  • Soldering iron (recommendations)
  • Hot Glue or Liquid Tape
  • Electrical tape
  • 80mm PVC heatshrink tube/sleeve
  • A big bucket of sand to put out fires (just in case!)
  • Do this outdoor if possible to avoid burning the house down, or filling your house with toxic gasses from LiPo fire

My recommendation for all sorts of tools for building quads.

Step 1 – Removing packaging

Remove the exterior plastic packaging from the LiPo battery. Make sure you are cutting on the side with the internal protective layer. This thicker flexible layer will help ensure that you do not puncture any of the cells with your knife.

cut-open-package-lipo-bad-cell-combine-fix-battery

Do Not Cut through the Cells!!!! Very Important.

Step 2 – Separating the bad cells from the good

Separate the good cells from the bad. When taking the cells apart, working in a warmer room help separating them more easily because the adhesive is less sticky when warmed up.

Use a multimeter to determine if the cell is bad or not.

separate-cells-lipo-bad-cell-combine-fix-battery sticky-lipo-bad-cell-combine-fix-battery

If the cells next to each other are good, then you don’t need to separate them, just leave them stuck together.

I also mark the good cells with “OK” in case I forget which is which :)

spearate-cell-mark-good-lipo-bad-cell-combine-fix-battery

Step 3 – Putting good cells together

Check with a multimeter which terminals are positive and negative, remember the cells must be wired back together like this: + to – to + to – to + to  – to + to –

Lipo-cells-connection-anode-cathode-terminals

Then I solder the cells side to side in series: + to – to + to –

pack-together-lipo-bad-cell-combine-fix-battery

Step 4 – Soldering wires

When soldering on batteries terminals, you need to be extremely quick, and use lower temperature for soldering. Don’t overheat the battery, otherwise it might cause a fire if you are not careful.

  • Solder the main connector lead (XT60) to the Lipo cells, red to cell 1 positive, and black to cell 4 negative (as shown in Pic 1)
  • Then the balancing lead following the wiring schematics diagram above. Starting with negative wire: first black balance lead connects to the negative of the 4th cell. (Pic 2)
  • 2nd black lead goes to – of the 3rd cell, or the + of the 4th cell (Pic 3)
  • 3rd black lead to – of the 2nd cell, or the + of the 3rd cell (Pic 4)
  • 4th black lead to – of the 1st cell, or the + of the 2nd cell

solder-connector-cable-wires-balance-lead-lipo-bad-cell-combine-fix-battery

Lastly, the red balance lead goes to the + of the 4th cell. Then secure the solder joints with some hot glue.

solder-wires-hot-glue-protect-lipo-bad-cell-combine-fix-battery

Step 5 – Final check and re-packaging

Check the voltage of the discharge connector, as well as the voltage of the balance plug, make sure each cell is reading the correct voltage.

check-voltage-multimeter-lipo-bad-cell-combine-fix-battery

Get some PVC heat shrink sleeve for re-packaging (80mm heat shrink is great).

You can remove and save connectors / balance lead from old or dead 2S or 3S LiPo’s. If you have a 4S with a dead cell or two, you can convert them to a 2S or 3S pack. They are great to be used with your FPV goggles and TX.

Edit History

  • May 2016 – Article created
  • July 2018 – Updated
Credit: Hans Turpyn

11 thoughts on “Fix LiPo Batteries with Bad Cells

  1. Bills

    I have a 6S1P 22.2V 3700mah Lipo battery w/one bad cell.
    The balancer shows two rows of readings (3 over 3). It’s the bottom left reading that is low.
    All the other cells are above 3V.
    The bad cells shows 1.5V at the start of Nihm charging and gradually decreases while the others go up.
    Which is the bad cell??
    How do I revive it??

    Reply
  2. BCSaltchucker

    thanks for the article. I really want to turn two of my 3500mah 3S into one 4S pack. but the more I see what is going on inside these types of lipo packs .. I will just say no.

    Reply
  3. Marcus

    I also must disagree in „lower heat“ better use a big tip on a even bigger iron and as much heat it can give. Most can do 450 deg celsius what’s good cause time is more important. The shorter the better.

    Reply
    1. Oscar Post author

      I use high heat when working with large volume of metal. The contacts on the battery are fairly easy to heat up so i don’t find high heat is necessary.

      Reply
  4. SwissFreek

    I would recommend a larger soldering iron than what we typically use. Your article on irons recommends ones like the Hakko 888, Weller 51, and TS-100. They are all great irons but even with large tips you will struggle to get the solder on the tabs to melt without overheating the battery. I would recommend using a soldering gun as it will be able to provide the heat necessary to quickly solder and desolder the tabs. Yes, it’s less convenient, but like I said, I’ve really struggled with my Hakko 888D and damaged a few cells while trying to put in enough heat to melt the solder, especially since most of it is lead-free.

    Reply
    1. Oscar Post author

      I personally have no problem with the TS100 with the original pencil tip :) It definitely helps a bit when powered by a 6S lipo :)

      Reply
  5. Vince

    Loved this solution as it seems obvious but lacked the confidence before reading this.. I am currently working with a 4s that has a torn positive tab and cannot get balancer to read the 4th cell after resoldering but I’m fairly certain that this should be repairable except i can’t seem to get it on the right part of the tab… Thoughts?

    Reply
  6. Ron

    Immediately after soldering one of the tabs I will pinch it with a gloved finger. It helps wick the heat away faster. Using a pair of needle nose pliers is also a good way to heatsink the tabs but it’s also tricky to keep from shorting to the other terminals.

    Reply
  7. Edward

    Hi Oscar,

    Soldering the tabs on LiPo cells can be difficult. The tabs don’t like to wet out with normal solder. I’ve had good luck using the Harris Al-Solder kit on a number of packs. The kit comes with flux and solder for joining aluminum and other metals. The downsides are that the kit is expensive and you basically need to dedicate a soldering tip for doing batteries.

    Reply

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