Replacing Dead Battery Pack Cells Saves You Some Coin

DIY Drill Battery Replacement

No one will deny that cordless drills can be super convenient. Sure, they need to be charged once in a while but that’s not a big deal. The big deal is when the batteries no longer hold a charge. Buying a new battery pack from the drill OEM is not cheap. If you need several, it’s almost cheaper to buy a new drill/battery combo.

It is not uncommon for only one cell is bad in the battery pack. Getting a replacement cell makes economic sense. And at about $1 per cell, even replacing all of the cells in the pack is way cheaper than the alternatives. [ksickafus] had a battery pack that did not work and not only did he replace all the cells, he wrote a great instructable about it.

The process started by removing the cells from the plastic container. Since they were soldered together they came out in one unit. The cluster of cells was then laid down on a piece of paper and the perimeter of each cell was marked to document the cell orientation. Next, the leads connecting each cell to its neighbor were noted on the same sketch.

The new cells were then laid out on the template to make sure they were in the same orientation as the originals. [ksickafus] uses braided shielding as his new tabs to connect the cells together and learned from experience that flux is necessary for this type of repair. Once everything is soldered up, it’s time to re-assemble the cells in the plastic case and give it a charge. If you do this at home, make sure you keep an eye on it the first time so nothing goes wrong!

If replacing NiCd’s with NiCd’s isn’t cool enough for you, maybe popping some LiPo’s in your drill would be up your alley.

43 thoughts on “Replacing Dead Battery Pack Cells Saves You Some Coin

    1. In my tools I noticed that NiCd elements were the most reliable ones over time, compared to other technologies. I don’t know if that was the rule, or just pure luck.

      Regarding soldering cells with a soldering iron: NO, NO, NO!
      There are reasons why the manufacturers NEVER use a solder iron for battery packs.

          1. I’ve seen in person someone have a lithium cel (button cel) blow up when they were attempting to solder it and 1/2 way between myself saying “you really shoudn’t d……” *bang*

            I have/use quite a few Bosch cordless Li-ion based tools and have rebuilt the battery packs, have invested ($150AUD) in a small spot welder made for batteries to do the terminals and has worked great. Have done a few battery packs for others and has paid itself over time too.

          2. To bonsaichop : I’ve been soldering button cell once, with an iron around 350°C, no problem. The result was pretty nice. I may have been lucky this time.

        1. I used to rebuild packs with 18 C cells, and on my first two attempts I tried soldering. I found there is a very fine line between getting the joint hot enough that the flux can cut through the oxide and provide a good mechanical/electrical connection, but not so hot that the cell vents. I heard a few do so with a sputtering hiss, but it didn’t seem catastrophic, so I marked those cells and continued. In the next 2-3 months, every marked cell failed without exception, plus a few others. I also had one failure from a separated solder joint that could have used more heat. Not worth it.

          1. I found the Dremel grinding wheel works great in cleaning the surface and increase the surface area when I needed to solder to make battery packs. It cut down the amount of time to do wet the surface and provide a solid joint thus minimizes the damage to the cell.

            I recently soldered a AAA pack for my cordless phone from some old still in plastic pack of cheapo Chinese NiMH that I had sitting around for 10+ years. That pack seems to be working in the phone for the last couple of years. Most of the AAA seems to be fine after that 10+ year in storage, but the AA aren’t.

            I still don’t recommending in doing that. I had nothing to lose as those batteries would have gone for battery recycling/disposal program anyways.

        2. The reason why you never ever solder the cells is that the electrolyte in a NiMH/CD cell is a solution of water and potassium hydroxide. It boils.

          The heat also destroys the plastic seal, which causes the cell to dry out and stop working.

      1. Sometimes there are solder tabs versions of the cells – i.e. they spot welded a metal strip that you can solder to. Since the strips do not make direct contact, it is easier to solder and a bit safer as you are not heating the cells as much.

        FYI Some of the batteries at Digikey has solder tabs…
        >No. J-B Weld is not considered to be a conductor. It is an insulator.

      2. My Snap on cordless tools are all NiCd, some of which are knocking on 5 years old, I still use them every day, only one battery (they are all on rotation) doesnt seem to keep a charge and drops a cell over a week or so. I promiced myself last year id upgrade to there Li-ion lineup but given how well these batterys are doing I may just re-cell the packs. Have to say, Ive been impressed with them given how other NiCd tools stuffer premature battery failure.

        1. I have a snap on 1/2 drill and impact. My batteries are about 6 years old, all 4 have failed. They did sit almost a year when I went on vacation traveling and didn’t use them. I’m thinking I’ll recell them, snap on gets alot of $ for the packs. Just have to find a decent, affordable way to get a spot welder going. Wonder if I could use my tig welder, turned down to a few amps?

    2. NiCd can handle trickle charge better than NiMH. Also they were probably old enough before they are manufactured in China i.e. trade lower quality for cheaper price.

      Replacing a cell or two on a pack might cause the rest of the cell to over discharge as the new cells have more capacity – both newer and may be increase capacity. i.e. these new cells would still have juice while the other ones are running low. So while this buys sometime, but you are better off making a new pack in the long run and reusing the old NiCd for something else demanding.

      1. The high current handling capacity of NiCD is due to the electrode geometry more than anything about the chemistry. Regular cylindrical cells are formed with a spindle electrode in the middle surrounded by a cylindrical shell for the other electrode. NiCd cells are formed by wrapping a long sheet in a roll, giving it a very large surface area and therefore high current capacity.

        NiMH batteries have the same feature. Plus, they have significantly lower self-discharge rate, and they have means to suppress hydrogen formation due to overcharging, which leads to oxidation of the cell as the hydrogen slowly diffuses out.

        1. The reason why NiMH appears to take trickle charging worse is because the hydrogen recycling is an exothermic process and results in overheating batteries if you don’t limit the overcharge current.

          You cook your batteries one way or the other in any case.

        2. That and the fact that these cells comes in much higher capacity, so you would increase the charging current for the same charge time. Also charge termination condition as the delta V is easier to read when it is like 1/2C or 1C charge rate.

          Higher current means higher rates of hydrogen recombination when overcharged.

    3. If you read a bit NiCD battery, beside being reliable, they offer you the biggest discharge currents – over anything else in the market. So if your drill is powerful and came with NICD from he factory you will feel the difference when popping a NIMh in instead.

  1. I made this more than ten years ago :) And yes, I replaced all the cells : since they had a greater capacity, I was best than before.
    With modern tools, it’s sometimes more difficult, since the battery packs are no more easy to open (sometimes even sealed), and the main problem is the cheap charger that use only a green led for controling the current (take 3-5 hours to charge, and if you forget it, it ruins the battery…) I use now a intelligent universal charger (from skyRC), and everything goes well.

      1. Imax B6 or their clones. I use one for charging my dewalt 20v lithium packs ever since my 3yr old noticed the LED animation on the charging dock. After turning it on and off many times (to see the animation) the charger died. Tried to repair but wasn’t the usual suspects. I think i could cram the guts of the B6 inside the dewalt charger, but it’s so easy to connect i haven’t bothered yet.

    1. FYI for the DIY: Multiple chip vendors make charger controllers for rechargeable battery.

      Ni based batteries are not exactly easy for determining charge termination (vs Li), so don’t bother to do it without a good controller. I use TI BQ2002 for charging NiCd pack in my home made UPS. It requires some parts + constant current source.

  2. Good solution to those expensive packs and with a hot iron and large tip you can solder quickly with minimal heating to the battery. Anyone thinking of soldering Li-Ion, A123s, etc think twice, they’re much more sensitive and likely to explode. A safer option for Li-Ion is to buy tabbed batteries that you can solder to the tabs, or build a spot welder of some sort and spot weld tabs between the batteries.

  3. Finding a harbor freight battery pack that is the same or higher voltage is what I have done in the past. Sometimes they won’t be in the same configuration (layout of cells), but you can break them apart and resolder them together. Since they have tabs, that’s pretty easy. The cells might not be as good, but they make up on price and ease.

    — John

  4. If the potential problems (accurately) described above deter you from rebuilding your own pack, there is still an alternative. My rescue squad had two 28V Dewalt packs die. We use them on powered-lift Stryker stretchers. I took them to a local outfit that sells and rebuilds batteries and they rebuilt them for less than 1/2 the best Dewalt price I could find. It’s been a while and it looks like they will last at least as long as the original batteries. I won’t name the company here, but it was one of the national chains.

  5. I made a new battery back for my B&D cordless drill a while ago. I used a battery box from ebay to replace a stack of nicads with 3 18650s. you cant use the original charger any more but I can just swap and go charged 18650s when it dies.

  6. I’ve done this to a bunch of DeWalt batteries just like the one in the picture. At first I took several batteries that were bad and used the good cells from 2 or 3 to make one good battery, Later, when I ended up with a bunch of empty battery cases I bought a hundred Sub-C’s on eBay and started soldering up new packs.

    I did a few for my Dad and he says they work better than the original batteries ever did, even when new. He says they last longer than the OEM cells. He’s been using them for probably a year now and they are still going good. I’m sure he’ll want me to do it again someday when necessary, rather than paying twice as much for a new battery that doesn’t last as long.

  7. As for soldering, here’s the trick I found.

    I bought cells with tabs already welded on. I was about to rip one tab off each end, since you only need one to solder up a chain of cells in series. But I was worried that could damage the cell, so instead I folded the tab back over the top of the cell on one end of each cell, then trimmed it off so it would protrude past the side of the cell. Then just lay the tab from the next cell on top and solder.

    I discovered this makes it much easier to solder since you are not soldering directly to the cell casing, and I’m sure it conducts much less heat into the cell itself.

    The only catch is that it adds a little height to the cells and I had trouble closing up some of the battery packs when done. I left the paper

    1. My comment got cut off.
      I left the paper covers off the bottom of the packs on those to get back some space and be able to close up the packs. The cases are plastic anyway, so I’m not sure why the cover the top and bottom with stick on paper sheets anyway..

  8. Replacing a single cell is kind of a bad idea because the new cell will have different impedance than the other cells. The cells will charge and discharge at rates than each other. They will actually have different voltages in the series cell stack. This causes two issues-

    First, when the pack is at a low charge state and pulling heavy current, the overall pack voltage may appear fine, but the high impedance (old) cells may be below their safe minimum voltage. The low impedance cells can still deliver current though. This can result in the high impedance cells developing a reverse voltage across them. This is called a pack inversion, and can severely damage the cells. (especially for li-ion.)

    Second, during high states of charge, the inequality may result in some cells being charged to a higher voltage than the others. The higher voltage might exceed the maximum charge voltage for the cell, which again leads to severe cell damage.

  9. With NiCd’s on the way out (not green), the ones available now are very questionable if from the usual sources.
    There are lithium retrofits for some NiCd based tools. New charger and pack fits in old tool. I put 2 18650 cells in series in a little Makita 4 cell NiCd drill. It’s not high torque but handy with a Phillips tip in dissasembly etc.

  10. Could you just glue the the contacts to the insulating pads that are above and below the batteries? I believe they would have enough surface contact to provide a good electrical contact. No soldering, no heat, no explosions. If they are not packed in tight enough, a small sheet of foam could take up the slack.

  11. Where can you get 4/5 sub-C cells, tabbed or not, for $1 each? I have a 18V reciprocating saw with a bunch of dead batteries. May as well buy a new saw instead of new batteries – unless I can find new cells dirt cheap.

    1. I never found them for $1. Best I could do was about $1.70 each, from an eBay seller. They were Tenergy brand and were good quality cells. For a dozen cells in a 14.4v battery that comes to about $20.00, instead of $55 for a new battery.

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