Upgrading Cordless Drill Batteries To Lithium

Cordless power tool battery replacements are expensive: you can easily spend $100 for a NiCd pack. [henal] decided to skip nickle-based cells and cut out the middleman by converting his old cordless battery packs to inexpensive hobby lithium cells. These batteries appear to be Turnigy 3S 1300mAh’s from Hobbyking, which for around $10 is a great bargain. As we’ve explained before, lithium batteries offer several advantages over NiMH and NiCd cells, but such a high energy density has drawbacks that should be feared and respected, despite some dismissive commenters. Please educate yourself if you’ve never worked with lithium cells.

[henal] gutted his dead battery packs and then proceeded to prepare the lithium replacements by soldering them to the cordless pack’s power connectors. To keep charging simple, he also branched off a deans connector from power and ground. After cutting some holes in the pack for access to the balancing connector and deans connector, [helan] went the extra mile by soldering on a DIN connector to the balancing wires, which he then securely glued to the side of the case.

We’ve featured lithium power tool replacements before, and these Turnigy packs pose the same problem: they don’t appear to have any low voltage cut-off protection. Check out some of the comments for a good solution.

27 thoughts on “Upgrading Cordless Drill Batteries To Lithium

  1. I did this once, but keeping an eye on the voltage became too much of a hassle.

    You really need some kind of cut-off circuit that cuts the power when the battery voltage goes low, the voltage drops quickly near the end.

    The cheap commercial low voltage alarms for RC toys could be a solution, but they draw a lot of power when turned on so they can’t be left plugged in like a normal protection circuit.

    Normal generic laptop protection circuits might also be used, but you would need to beef up the cut-off transistors since the built in ones wouldn’t handle the starting current of a cordless drill, also the overcurrent protection would have to be removed or it would trip every time the drill started.

          1. Speed controllers in what? Cordless drills? If that’s what you’re saying, then you haven’t owned many cordless drills… I have, and when speaking of NiCd->Lithium conversions, you are most certainly incorrect ;)

          2. That would be true if you started out with a Li* powered drill, however i have never seen a cheap Ni* battery drill with undervoltage cutoff, they run until the battery is dead.

            The “speed controller” is a simple PWM circuit in these cheap drills, and the maximum speed mode is often just a mechanical switch at the end of the linear resistor track, bypassing the PWM circuit completely.

            Now Li* powered drills have undervoltage cutoff, current limiting and more but that’s not what this article is talking about.

          3. sven is absolutely correct – This is exactly what I was referring to. You need to add the UV cutoff circuitry or risk a dangerously discharged battery pack. You should also install a thermistor and add overtemp shutdown. Otherwise, ALWAYS charge these batteries under close supervision, and NEVER store them near anything that they could ignite if/when they catch fire. Put them in a fire safe, ha ha ha

            OR just buy a new NiCd battery pack – They are cheap nowadays, and you can even send your old NiCd packs in to be rebuilt for much less than the cost of a new one.

            The only way I see this “hack” as being worthwile is if your time is absolutely worthless, and you don’t mind the risk of burning your parents’ house down.

  2. Let me explain my “dismissive comment”.
    Of course lithium cells should be handled with care, but they also exist in less dangerous forms, such as 18650 cells with built-in protection.

    Also, there’s LiFePo which is much safer, although with a bit lower energy density.

    1. The problem here is that the properties we are looking for are the exact properties that make RC LiPo packs dangerous. To replace the Ni* cells in an old cordless drill we need a high current battery pack, this means we can’t use the usual protection circuits (they would just trip due to overcurrent or fry their mosfets) and we can’t use regular safe(r) consumer cells since they can’t handle the full load current of a cordless drill like this one.

      If you have ever used a Li* powered cordless drill you will probably have noticed that it limits the maximum current and ramps up the speed rather than starting as sharply as the old cheap Ni* drills. This is to protect the Li* cells since the manufacturers use slightly safer slightly lower current cells than RC packs. The undervoltage and overcurrent protection is also built into the drill itself, locking out the PWM controller when the current or voltage goes out of bounds.

  3. Make a replacement pack from Headway 38120S cells (10Ah LiFePO4).

    You will have exceeded the original capacity so greatly that no matter what you do, you won’t drain the pack in the course of a day. So no possibility of undervoltage, assuming you simply charge each time you sleep, and the cells are determined to be in good condition and well-balanced at that time. With a 3C discharge rate, certainly no worries about pulling too much current either.

    I’m only half-serious, of course. It’s a calculated risk to rely on any safety method that relies on human attention. Though more unlikely, should you somehow still manage to undervolt such a large pack, the result could be far worse.

    But the pack itself is doable, assuming it’s acceptable to wear the battery as a fanny pack or similar, and connect it to the tool via an umbilical. Bonus points if it can be easily detached, and auto-retracts into the pack. Having the battery’s weight supported by your body, rather than your hand and arm, might reduce fatigue to less than the original configuration; despite the heavier battery. Some of the cells could be replaced with a beefy adjustable step-up converter, for a pack with less run-time, but with reduced weight/size and more voltage options. Done right, it could be the finest battery you’ll ever own.

  4. “high energy density”

    Few people appear to understand…
    “high energy density”
    …relates to energy per unit of mass.

    NOT energy per unit of volume.

    Modern NiMH have more energy per unit of volume (~30% more) than comparably priced Lithium.

      1. Pedantic detail in disregard of the point ALERT!

        Anyway. It’s a problem. After reading the linked post about salvaging lithium cells, it’s a detail the authors are unaware of.

        Hopefully assdf’s post will educate.

        Maybe the authors Josh Mash and Mike Stacy could correct posts like this….

        “Lithium cells outperform Nickel Cadmium and Nickel Metal Hydride in almost every way.”

        to something more correct, like…

        “Lithium cells have some advantages over Nickel based batteries and sometimes these advantages can outweigh their many drawbacks.”

        Maybe also don’t imply it’s powerful by changing the following line….
        “If you learn the basics it’ll be easy to use these powerful batteries in your projects.”

        to something more suitable…

        “….it’ll be easy to use these batteries in your projects. p.s. Don’t mess it up or you’ll catch fire.”

        1. >>Pedantic detail in disregard of the point ALERT!

          Considering the terms are often used interchangeably, I feel the same can be said for your initial response. I don’t see how either of these articles are being misleading.

        2. “Pedantic detail in disregard of the point….”

          No, its not. He’s right. Your post (5 of 6 lines, anyway) is either very wrong or very poorly phrased – and such terribly phrased posts have the same effect as being totally wrong. This is not a pedantic ‘detail’ – its the whole point of the terms we are using. (And yet you presume to correct and educate others.)

          Energy density can by used for energy/mass or energy/volume. Both are correct.

  5. On all of the protection circuitry stuff: I’ve done this to a few drills so far and all I have to do is put a piece of velcro on the outside of each drill where I can stick my cell checker. You could even use a cell alarm which would let you know if you were nearing low/dangerous voltage under load. I just keep an eye on the cell checker anyway and keep it between 3.7 and 4.2V.

    1. I’ll just take rebuilt NiCd batteries, thanks. I really don’t see how any benefit of switching to Lithium Chemistry in a battery pack like this can outweigh the danger of keeping these fire hazards in your house. I have several friends who have experienced lithium battery fires, all under different circumstances, and I prefer to keep my house from turning into ashes ;)

  6. I’ve made two packs using 18650 cells. They work fine. I have added a cheap undervoltage alarm circuit with a switch to turn it off when not in use to avoid draining the batteries in storage.
    The packs have balance connectors for correct charging with a lithium battery specific charger.

    Agreed with everything said about handling these batteries carefully they have more potential to cause problems than other safer battery technologies, but we all have them in our laptops lying about in various states of charge. This is precisely why this hack is popular,the cells are powerful and readily available.

    Just make sure your cells don’t short, check the insulation and set any wires that might loosen in epoxy resin to reduce the risk.

  7. in fact none of the rc lipo’s have protection except for


    at 5 amp cut off it may not be suitable tools since they can easily pull many more amps at initial startup and when under load however if the load is a radio or laser range finder or a borescope or an led light it should be ok for since the demand is quite low.

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