Easy Nicad to Li Po Drill Conversion




Introduction: Easy Nicad to Li Po Drill Conversion

About: Was it you or I who stumbled first? It does not matter, the one of us who soonest finds the strength to rise must help the other. - Vera Nazarian, The Perpetual Calendar of Inspiration

I like to pick up early vintage cordless drills at thrift stores, they go for cheap especially if it has no charger and a dead battery, this vintage Sears 7.2 volt nicad tool went for $2 because it was the latter. I had on hand a T-Plug 11.1V 1500mAh 25C 3S VOK Discharge LiPO Battery that runs for about $8 U.S., including shipping, and decided to use it for the hack. I’ve found from past conversions that the voltage disparity isn’t much of an issue, the D.C. motors used in these products are pretty robust and can stand a wide margin of abuse and still live a long life.

The contents of the following Instructable represent the experiences and outcome of the author, no guarantee is made as to suitability of the repetition of the information presented, therefore careful study should be undertaken by those wishing to duplicate these results, including a self- assessment of prerequisite skills, knowledge, and understanding of the subject matter.

Thank you.

Step 1: Tear Down to Build Up

The first step is to remove the defunct NiCad cells and clean up the contacts and the housing that transfer battery power to the drill. I soaked and scrubbed them in vinegar to dissolve acid and corrosion, then rinsed well to neutralize them. A test fit confirmed that little more need be done to incorporate the new battery into the build.

Step 2: Route Power

I soldered a couple of 18 gauge wires to the contacts as down-leads to connect with the Lipo cells via a pair of insulated screw terminals instead of soldering them together, this would allow for easy replacement if it became necessary.

Step 3: Final Thoughts

The only case modification I made was to provide the balance charger connector a feed-through hole to allow hookup to my charger, and by using a salvaged set of contacts, I can easily recharge and balance the battery without any issues whatsoever. The weight decrease is remarkable and the output power is fantastic, so for $10 U.S. it’s a deal for me.



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    35 Discussions

    good instructable, only concern I would have is that you took a 7.2 volt drill and repowered it with a battery that puts out 11.2 volts. Wouldn't this arrangement eventually burn the drill motor up ? You are going roughly 4 volts over rated output. Otherwise it's something I might consider for a backup drill.

    5 replies

    I have been using a 4.8V Bosch drill on 7.6V for several years, and no undue overheating, and on inspection no discoloration of the windings. These animals are fairly tough and forgiving.

    I think it could hurt the trigger/switch, if not the motor.

    No problem, believe me. I have done it several times. You worry too much, give it a try!

    Not worrying. I've done it before too. It worked until it didn't :)

    NO PROBLEM... absolutely. I have used Lithium cells with higher voltage on a Black & Decker drill that used NiMH or Nicad "Versapack" (two of them, so it was 7.2 V), and that works BEAUTIFULLY with the higher voltage of the 3 cell in series Lithium batteries, and also used an old electric R/C car motor supposedly meant for 12V with up to 14.4 V with absolutely no problems, Author has said it perfectly: the small DC motors are rugged and not critical of voltage (up to a point, of course!).

    Your idea is good BUT, you will be causing problems or fire if you don't add a couple of additional details.

    Modern Li ion battery drills have built in protection circuits to cut the drill off when the Li ion batteries drop to a pre-determined voltage level. This is to protect the battery cell voltage from dropping too low and destroying it. Thus you need to add a protection circuit into the mix. A lipo low battery circuit will alert you when the batteries are reaching the critical low voltage level and let you know it's time to stop using the drill and recharge it. Here is an example of what I've used in the past for my conversions...


    A good instructable done by DIY Perks shows how to incorporate this...

    HOWEVER, even he didn't discuss the need to balance charge the batteries once you have added this protection circuit.

    For that, you should look at this youtube video (there are many others) on how to add the balancing circuit to the battery pack to keep you and your drill safe.

    When charging the batteries you need to make sure that you are not over charging them. This is not a good thing! Some Li-ion batteries in a battery pack will charge more quickly than the others. With full power going to all the batteries, this could damage the battery that is already at peak voltage (or could cause a more serious issue, like fire).

    For this reason, you should also build into the battery pack the ability to balance the batteries as they are being charged. The charger you are showing in your example has the capabilities of balance charging and cutting charging power to those batteries in the pack that gets to full capacity. But you must also include balance charging contacts.

    Li-ion batteries are great if used properly but can be quite dangerous if you are not handling them or charging them properly.

    My advice is that for anyone interested in doing this conversion, PLEASE do more research online on the proper way to do the above steps to ensure proper and safe use of Li-ion batteries. This is no joke and should not be taken lightly. Good luck to all.

    4 replies

    BEST RESPONSE AND TUTORIAL HERE!!!!! THANK YOU!!!!! You may have saved a garage or house and lives!

    Thanks Tomjasz for the kind words:

    I wasn't trying to diss the presenter but felt I had to point out the additional steps that a novice should be aware of in order to have a safe conversion.

    I've converted a few of my old drills several years ago and am very happy with how they perform now. Even so, I still don't leave them alone while they charge...just to be cautiously safe.

    In addition to the usual charge protection circuits, I have the practice of charging Lithiums inside a large ceramic pot, and to place this and the charger on the ceramic tile floor of my bathroom, where there is nothing that can catch fire. I've seen a parked car being completely incinerated at an R/C flying field because the owner felt confident and left his Li-Po pack under charge inside the hot car parked in the sun...

    And don't over-dishcharge them! What a weird term.

    Thats good stuff Finding a battery to fit seems to be the difficult bit . After cleaning the contacts I have found it helps to recover them with solder if you can . Including the ones inside the drill head otherwise corrosion comes back quickly.

    2 replies

    You can also coat the contacts with silicone grease, that helps keep corrosion from coming back.

    This is the most dangerous battery in the lithium lineup. undercharge and overcharge can both cause a fire. No alarm or low voltage signal can be a disaster. PLEASE use due diligence with these packs. RC folks are well versed but others may not be. LEARN about Li Po FIRST! 18650 cells would be far better or even repairing with the same cells!

    1 reply

    I agree and I am an RC folk. However, I have this same drill and I don't think the case is large enough for the 18650 batts. I will have to check, as I rebuilt it years ago with the same Sub C batts the factory uses, but now they have already gone bad.

    $2 drill. $8 battery. $60 charger. I think this project is over the cost of a new drill for someone without a balance charger flitting about.

    3 replies

    I just got a balancing charger from Amazon for $8 because I didn't want to wait for the $5 one from Hobby King.

    Boy you SAID it! HobbyKing takes forEVER to ship, won't ever buy from them again.

    Sadly it's about a $1 value. Be careful that you don't start a fire. Charge in an ammo can closed with the watertight seal removed. That will contain any fire. Li Po stinks!