Replacing Solder Tab Batteries

There’s a ton of devices out there that have batteries in them but most people never think about it. That’s because they use rechargeables that are sealed inside and topped off with external chargers. [Todd Harrison] has a couple of them, including a cordless shaver and a Christmas light timer. He’s had these for years and the batteries have gone south. They’re not meant to be consumer-replaceable, but that did stop him from cracking them open and swapping out the solder-tab batteries himself.

The batteries themselves won’t be all that hard to source. The shaver just takes a NiMH AA cell. But since they’re not meant to be replaced [Todd] needed to do some soldering. Here you can see he’s using a solder gun to make the connection between the new battery and one of the solder tabs. He uses the gun instead of an iron because he needs to heat the joint quickly, and must avoid heating the rest of the cell which could rupture. As a safety precaution he’s wearing gloves and a full face-shield.

Check out the video after the break to see this, as well as the coin cell replacement in the lighting controller.


40 thoughts on “Replacing Solder Tab Batteries

  1. I’ve done this, but the results are stochastic; even with a soldering gun you can ruin a fair number of batteries. If you check on EBay, there are a lot of outfits (like “Just Batteries” in Montreal) that sell sets of batteries with the tabs already spotwelded to them, and all you have to do is solder or bolt the tabs together.

  2. I have got to question the safety of this. Most battery sources also sell cells with pre-welded tabs. Far safer!

    Having done the same thing often enough to save my cordless devices from the trash, I applaud the hack, just suggest you try a safer method unless there is no choice.

  3. I use .7 pencil lead and an atx psu or lead acid battery to spot weld tabs onto batteries or other things too large to solder nicely.

    it takes tricky use of flux and the graphite sometimes explodes but it’s very handy.

    With a bit of practice the joints can be totally solid.

      1. It’s pretty easy to predict when it will explode. You can use either a larger lead for lower resistance or a longer one for more resistance. They’re usually glowing too brightly to look at before they explode anyway.

      2. it’s definitely a situation where safety goggles are a must. certain brands of lead are less prone to failure than others, and the length of exposed lead between clip and battery also matters.

        it’s less hazardous than you might think. and like the other guy said, thicker lead also mitigates the risk. a normal wooden pencil lead with a sharp tip works wonderfully.

    1. So… you use the 12 volt DC output on the ATX PSU?
      What polarity?
      Does polarity depend on which whether the anode or cathode is being welded?
      Is the tab “sandwiched” between the battery and the graphite?
      Is the flux between the tab and battery, or between the graphite and (?) or both?
      How do you get a decent “grip”/connection on the graphite with the PSU lead (Lead as in wire, not the element Pb).

      Ink wiring mimes Juan too no…

  4. Not sure if they still do, but Batteries Plus used to weld packs together if you bought the raw batteries from them (I think there was a nominal charge).

    There are also special soldering tips(hammer head)for directly joining positive and negative terminals together (used for hobby RC battery packs).

  5. I tried this with a soldering gun before but had no success. Ended up using a large bank of capacitors to spot weld metal tabs or wires onto the new battery. Worked great for replacing the BIOS battery in a laptop that plugged into the motherboard instead of fitting into a battery holder.

    1. Could someone kindly make an instructable or suggest how to make a battery holder? I bought tons of solar garden lights not realizing the batteries are not replaceable :( They were expensive and only lasted a few months. When I broke them open the batteries are soldered. Could I make some kind of battery holder so that I install regular rechargeable batteries that I could easily remove to replace?

    1. i believe you actually want lots of power. the more power you have the faster you can heat the tabs and the more focused the heated area will be resulting in the overall heat of the battery to be lower

      1. I actually just built a pack from eneloop cells for a radio. Made my own hammerhead tip for a cheepie 40W RS soldering iron from a nut and bolt with the same thread as the normal tip uses. One of the tricks is to carefully make a cold soldering joint. For putting them in series (the pack I made had 4 pairs of batteries in series, similar to a common 9.6V RC car pack) After sanding the ends, I used my soldering station to quickly add solder to the ends of the batteries, then let the cheepie soldering iron heat up, lined up the batteries in some angle aluminum, heat the solder between the batteries, rapidly pull out the soldering iron, and slam the batteries together. As soon as the solder solidifies, I cooled the batteries in the freezer for a few moments.

  6. I also usually don’t recommend soldering to batteries, however… with his preparation he’s doing pretty good (sanding the terminal, cleaning it, fluxing it, and using a high power iron)

    Look here, he solders it in just a couple seconds:

    There’s no way that’s harming the battery, in my opinion.

    Now as some others suggest, using a lower temp iron and leaving it on there longer is A BAD IDEA. You want to heat up just the metal tip quickly and nothing else. The ice water sounds good, but I think that’s just overkill. Freezing the battery might not be a bad idea.

    When replacing one or two batteries in series, this works just fine. Building a whole pack is different, and I would say figure out how to spot weld them, or buy the pre-tabbed batts.

  7. There is a reason that you cannot find batteries with soldered tabs, because it is highly dangerous.
    No one has mentioned ‘out gassing’ of toxic materials.

    The tabs must be spot welded, so that the resistance of the join is low and to ensure the cells are not subjected to excessive heat.

  8. Soldering to batteries is a BAD idea. Just as it is a bad idea to solder thermal fuses!

    You should spot-weld the batteries, as others have said.

    There are many DIY\Open source designs for spot-welders too :)

    And of course, you can use it to weld more than just battery tabs.

  9. Of course it’s not ideal to solder directly to batteries, but some of us only need to do it once or twice, and would rather not build a spot welder only for one such occasion. Also, the additional resistance is negligible for many projects.

    I’ve found that the best/easiest way it to take a bench grinder or dremel and grind a tiny bit off each end of the battery you want to solder. It seems like they add some coating to the metal that makes it very difficult to solder to, or it could just be the oxidization. After grinding a small bit away, the solder always instantly wicks right onto the battery, which lets you solder quickly and keep the cell cool. Using a 100-150w soldering gun is ideal.

  10. Like may others have posted soldering directly to the battery is not ideal. There are rubber components that allow the battery to vent in case of overcharging. If this vent is damage the battery would have a shorter life span since it would not be sealed correctly. If you look at the video closely at the AA battery in the vice you can see that the cover on the battery has split and the insulating ring appears to be loose. While these will not affect operation, they are a sign that the battery got pretty warm when he soldered negative side. I assume he cooled it before doing the other side. He is nice and quick on the positive side though. I am pretty sure the vent is on the positive side.

  11. Using a soldering gun is far from ideal. I’ve been soldering quite a lot of sub-c RC car cells and have bought a 150W iron with a 1/2″ tip about 2″ long. It stores enough heat to heat up just the end of the battery quickly and doesn’t overheat the rest of the battery. If you use a soldering gun with almost no thermal capacitance, the battery cell will end up sucking away all the heat faster than the gun can deliver.
    With proper equipment, soldering cells is a) easy and b) safe. You can also solder LiIon this way.

    Spot welding is also a good idea, but i’d definitely suggest safety equipment for that. I’ve “pushed” those sub-c cells, too, and that’s basically discharging a capacitor bank through the cells, spot welding anything inside the battery that isn’t 100% welded. I wouldn’t recommend using it for soldering tabs on the outside when you’re drawing a high current (RC cells have to deliver around 60A), but it is just fine for razors, cordless phones and the likes.

  12. I’ve recently replaced the battery in my Philips shaver – there are 2 3/4 AA NiMh batteries inside. Strangely enough, they are soldered in, there’s a warning about the absence of user servicable parts in tha manual, but underneath the batteries, there’s a big fat label on the PCB what type of battery should be used as a replacement.

  13. We’ve got a battery shop nearby where they use a pusle welder for these jobs; I’ve had them re-do my dad’s DeWalt 18v battery (using, get this, 4/3 subC cells who ever invented such a funny size) and it came out *great.*

  14. I can’t thank you enough! I’ve been trying to do this for quite some time with no success. Also scary I never considered the button cells to explode as you mentioned. Very lucky it never happened to me. Thanks again from some one who Hates to throw something out that I know can be fixed for next too nothing


  15. If you have to buy batteries why not get them with tabs pre attached like I did for my razor. It saved the scary job of soldering tabs and made the job easier for a novice. Try Amazon.

  16. Can you help me with a tab that has rusted? I have an animated christmas toy that I would like to see work again however I know nothing about doing anything like what you are doing. I just need the tab replaced not the battery itself because it takes regular C batteries.

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