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Expensive 18V Festool Powerpack Saved From The Dump

By on March 30, 2024
powerpack festool repair

Fixing dead battery packs often is cumbersome and not without risk. Because explosion or fires are the result when cells are handled wrong. But when my nephew who is a professional kitchen installer recently rescued a defect and powerful Festool 18V battery powerpack from the bin that only was charged about 82 times, I gave it a go.

The last time that I successfully was able to fix such a portable tool battery pack it was easy because those new replacement charger boards for Makita were plenty available on Aliexpress. Complete with temp sensor and new safety fuses and brandnew controller chip. Without those it is quite impossible!

festool repair

However these bigger and more powerful 18V 72Wh Airstream Festool packs were a complete new design that included a controller that even supported a bluetooth checking option.

festool spec

Which enables the user to check the energy level of the Li-ion cells with a bluetooth app on a smartphone, without having to attach a cable to smartphone or powerpack.

The cells used in these Festool battery packs are bigger (longer and thicker) than the commonly used 18650 Li-ion cells. And these bigger 21700 cells are according to the text message on this pack without overcharge protection unlike the mostly protected 18650 cells that do have a small integrated round electronics board with limiting safety parts. These Samsung 21700 cells normally can be re-charged at least 1000 times. And when short circuited a current up to about 40 Ampere will be burning through any low resistant connection. After only been charged 82 times this already defect pack is very uncommon and worth a try to fix. Why one cell afterwards only measured 0V is still a mystery. From the 4 other cells 3 measured almost completely charged at over 4V and only one cell measured a lower voltage of 3.46V. We here will only replace the 0V cell for a brandnew 21700 cell that without shipping cost was sold for about 10 euro. And we’ll see if the result is a new working power pack.

power pack repair festool

This pack had on the outside 2 x 2 special torx screws with pins in the middle, why I needed the special CR-VT10 size bit as shown on the next photo to open it. And I was barely able to fit inside the screw holes to reach for the top of those screw heads to unscrew them. A special screwdriver with longer thinner shaft would have been better but those aren’t cheap.

After those 4 screws were removed the case can be opened by removing the left and right case side from the pack. And the pack itself was also screwed into the left side with another 2 of those same torx screws.

torx screw

Next photo shows the defect now removed 21700 cell with safety instruction.

Removing the bad 0V cell was only possible by cutting through the + and – connected metal strip connections on each side and by sliding it out the plastic holder through the on the + side made opening. By removing some of the plastic edge that was in the way.

samsung battery

Which was difficult because of the white glue on each side of the cell that kept the temperature sensors in place and those may not be damaged for obvious safety reasons! What happened was that at first only the cell was pushed out of its blue cover leaving the now empty blue 21700 cover sticked to the glue in place. But it afterwards was carefully and without any problem removed from the glue and slid back onto the naked defect cell.

And the new replacement 21700 Li-ion cell was easily pushed into the holder with its – pole side first through the + opening. And when in position the + pole side of the cell is like before on the + side where the plastic edge previously was partly removed to be able to shift out the old defect battery.

Now it was time to welder the new cell back to the old strips which will be done with my new Dh20 portable pulse spot welder. Because in the past my expensive 709A spot welder uses way too much power from the AC mains wall outlet that a risk exists that the main power fuse in my home burns through.

Leaving me with a large repair bill from the energy company that is not cheap when they sent over their service man to fix my only 25AT main fuse!

And the active welding time with my mains operated spot welder is only about 35 seconds even when I use a inrush safety limiter.

After which I have to wait at least another 30 to 45 minutes to let cool down the limiter before I can use the 709A again. Which is why I bought this battery operated spot welder. It is shown on next photo. Although it has about 9 different pulse settings, I only used the highest welding setting that was the most powerful. And I used the included 0.15 mm thick nickel strip to weld on to the new cell.

portable welder

Below photo on the left shows the re-connected + pole and photo on the right the – pole.


The short welded on nickel strips were as above photos showed soldered to the old strips to reconnect to the battery circuit of the 5 in series placed cells. Next photos show the front and the back of the used battery operated spot welder. It also can be used as 5V 2.4A output battery pack. According to the specs the output current is 650A max, and the design current is 1200A max.

1200amp welder

This welder only welds nickel strips with at maximum 0.15mm thickness. Afterwards I was a bit disappointed to see that the copper ‘probes’ can’t easily be replaced like is the case with other similar welders that are sold on Banggood. And this one sold on Aliexpress sadly only worked at best at the strongest 9 setting apparently. But luckily it did its job satisfactory! And helped to fix a defect but expensive at least 140 euro costing battery pack!

Here follow a few more taken photos from this battery pack. With in sequence the front, the top, the bottom, and the re-attached side, with a photo of the pack with replaced and connected new cell.

And a photo that shows the battery charged level on the pack with green leds, with the activated bluetooth connection shown with a blue lit up led.

how to fix festool

festool fix

how to fix festool powerpack

festool powerpack fix

Below photo shows the repaired pack with tape added to prevent the new welded on nickel strips from short circuiting before they were soldered onto the old strips. (The red tape marks also the – pole of the entire pack, and the circle with + sign on the outer right in the photo below marks the plus of the entire pack).

The two white wires go to one temp sensor that is glued into the white glue. And the two red wires go to the second temp sensor also glued to white glue that is mostly glued to the last cell. The green tape marks the new cell. The white sticker on each side originally covered the battery poles probably from getting dirty. And maybe the contact strips from any moisture.

battery of festool

Following picture shows how the bluetooth connected app looks when it is showing the battery energy level of the battery pack on your smartphone.

how to fix festool power pack

Next last photo shows my nephew after successfully testing the now repaired Festool battery pack. A thumbs up is all I needed to know!


albert from netherlands

Albert van Bemmelen, Weert, The Netherlands.

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Note: You can read his previous article on Brandnew Defect VGA to HDMi Video Adapter



  1. Waleed Rishmawi

    March 30, 2024 at 4:18 pm

    a well written articlW full of a lot of information and details. yes totally agree about the danger or replacing battering without having no idea what is going on. you have done an amazing job. thanks for sharing and have a blessed day

  2. Corriette

    March 30, 2024 at 5:41 pm

    Well saved by you

  3. Tayo

    March 30, 2024 at 6:09 pm

    Excellent article Albert!
    I have resurrected a similar battery pack with nearly 21 of those 21700 cells (dual 20v and 60v capabilities).
    I would solder the connectors which is never good for the battery (heat degrades the cell). Your spot welder recommendation will serve me well. Once you go down the rabbit hole of these batteries you will find that it is not as easy as simply replacing a dead cell but rather there are things like conditioning and matching the characteristics of the batteries. As these batteries and devices are becoming more common place it's good to start dipping your toe into this arena. Well done.

  4. Albert van Bemmelen

    March 30, 2024 at 7:56 pm

    These powerful li-ion cells are indeed something to be extra careful with! I do not think that I would risk repairing such large battery packs as you managed to fix Tayo.
    And that is also the main reason why no one would want to risk parking a new electric Tesla or Chinese made car right next to his house. Once they start burning it will take at least a whole day to cool them down and stop the completely burning out battery cells. And riding any electric bicycle is almost as riding on a dangerous bomb if something goes wrong!

  5. Parasuraman S

    March 30, 2024 at 10:03 pm

    Vow! Another excellent informative 'class' for me! Handling such battery cases is very tricky and sometimes very risky too. But you handled it like an expert surgeon. Many thanks for sharing, dear!

    • Albert van Bemmelen

      March 31, 2024 at 12:16 am

      The last photo originally showed my nephew successfully testing the battery pack on his Festool charger with a thumbs up but somehow got accidentally removed from the article by Jestine?
      Instead it now sadly shows a duplicate photo with Bluetooth App connection to a smartphone.

      • Jestine Yong

        March 31, 2024 at 8:34 am

        Hi Albert,

        I'm sorry for that mistake. I have just included that photo of your nephew with a thumbs up.


        • Albert van Bemmelen

          March 31, 2024 at 4:24 pm

          Thanks for the fast correction by adding that last important photo Jestine!

          • Jestine Yong

            March 31, 2024 at 5:07 pm

            Hi Albert,

            No problem.


  6. Yogesh Panchal

    April 3, 2024 at 2:52 pm

    Good JOB!Albert
    you are right Albert these are risky job if you do not understand the requirement.


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