Cordless Drill - Improving the Battery




I hate it when my cordless drill gives up in the middle of a job. It seems that the supplied battery packs just don't cut it. So I opened up the battery pack and replaced the NiCads with much longer lasting NiMH batteries. Now I get a much longer use between charges.

Step 1: Disassemble the Battery Pack

My battery pack had 4 Torx screws (I later replaced with regular Phillips screws for easier access). Inside you'll find 10 NiCad batteries. They appear to be just a bit smaller than 'C' cells, and all attached together in series. 1.2 volts X 10 cells = 12 volts. The cells are marked as 1300 mAH which is why they don't last very long. You will probably want to photograph the arrangement of batteries since you will need to make a new pack look just like this. Also notice the thin insulator that keeps the top battery from shorting against the batteries on the bottom.

Step 2: Assemble the New Pack of Batteries

Aquire a new set of identically sized batteries. In my case the size is called 'Sub-C'. I got them at for $24.00. They come with tabs attached. Carefully assemble the new cells into the same arrangement as the original, and solder the tabs appropriately. Be careful not to short out a cell while assembling it - even briefly! You will also need to scratch the surface of the leads before soldering and put a thin coat of solder on each tab before assembly (called tinning). Re-assemble and test. Be sure to re-use the fusable link that came with the original battery pack

Step 3: Notes

Capacity:The new batteries are rated at 3000 mAH, so expect more than twice the run time than the original pack. You may wonder -- will the higher capacity hurt the tool? No: the important thing here is that the voltage is the same, but you may end up working the tool harder with the increased capacity, so don't over-do it. I've been using it for about a year and a half, and it's been excellent.

Charging: Since my old charger is expecting to charge 1300MAH NiCads, it will require a much longer charging time. The spec sheet that comes with your new batteries should help you here. Also checkout for more important detailed info on charging different types of batteries. A better charging method may increase your battery life.

Why do the batter packs die? I've been analyzing the old cells, and I've noticed that several of the cells are bad, but many of them are ok - it only takes one dud to render a battery pack useless.
Since the drill came with two battery packs, I can combine the good ones to make a second working pack. The old cells can be tested as shown below - the important part here is the load. I've used a flashlight bulb as a load which also server as a visual indicator of the battery's charge. Testing without the load will give useless results. I've seen some cells read 1.2volts without a load -- which seems good, but drop to 0.3 volts when the load is added -- clearly a dud.

Economics: A good mod should be economical too! Sears wants $40.00 for a new battery pack - if you can find it (I paid about $75 for the tool with two batteries - so why is a replacement so expensive?) So with this mod, I get a better battery for $24.00

Safety notes: Like many of the projects here on instructables, there are potential safety problems if you don't know what you are doing -- so if you don't know your series circuits from your parallel circuits STOP! don't attempt to perform this mod - it can be potentially dangerous if the cells are mis-wired or short circuited. Also beware any leaking chemicals from bad cells. Also make sure your soldering skills are sufficient before working on this project. A reader from Makezine's blog noted that one of the solder tabs is thinner than the others - this acts as a fuse - make sure you include the fuse in your completed pack.

Environmental issues: dispose used NiCad cells properly. Your new HiMh batteries are more environmentally friendly though some still recommend re-cycling.

Step 4: Soldering to a Battery

Update May 2011:
It's been five years since I posted this instructable - and I have another battery pack to work on. This one is a 19.2 volt pack, but still uses the Sub-C Ni-Cd batteries that my old drill used. Once again, I found that there were a few cells that were bad - making the entire battery pack useless. This time I'm just going to replace the bad cells with identical cells.

By the way... be careful not to short circuit any of the cells while soldering.



  • Arduino Contest 2019

    Arduino Contest 2019
  • Gardening Contest

    Gardening Contest
  • Trash to Treasure

    Trash to Treasure

178 Discussions


3 years ago on Introduction

Lots of good info/experience here. Ultimately, the fix for a "dead" NiCad seems to be the following gradation/choices: 1) Try the voltage ZAP with a larger voltage to "melt" the crystallization. Works for awhile. 2) Disassemble, test and replace the faulty cells in the pack, either with new NiCads or NiMHDs. 3) Try to make up a new battery pack with LiIons. This means balancing the cells, finding a new way to charge them and being really careful in soldering them in series in groups. Or 4) see new DiY Instructable for an Adapter for your NiCad tool. This one is for B&D Firestorm tool 18V sliding batteries, but could be adapted (adapt the adapter?) for your Dewlt or Sears, or... to run on Ryobi batteries...

.Look for "Black and Decker Adapter for Ryobi Battery"


4 years ago on Introduction

li-ions are a whole new ballgame. they have a circuit board inside that makes them a brick if they are run down to far.. I been working with some for a few days and they don't want to fix. they are DEAD and don't want to be nothing else but dead..


5 years ago on Introduction

Well this is a good "smack it a good one" story;

I have a multi-rotor that I crashed hard into a tree, wall, car and fence. Only my second RC flight, ever. The quad copter then would only give an error message via led's and would not work, I was devastated because I was out $600.00 + I had saved hard for--took me a year. The LED code said "call the company" RIGHT its in China and I gave up after several days of no answer and multiple attempts to decipher Cinglish instructions. After two days of internet searching I found a forum post buried on page 100K of a Google search and it said a company rep for this quad copter advised him (the forum poster) on the same problem recommending the following fix: Remove the "brain" (a DJI NAZA V-2 in this case) and hold it in your right hand about 7" above a ceramic counter top with the writing on top, then rap it moderately hard on the counter top .

I was reluctant but desperate so I tried it. To my total amazement it not only fixed it but it flew better than ever after this "smack it a good one" fix.


5 years ago on Introduction

I'm a bit skeptical but there is an easier way than doing this and doesn't cost anything . batteries built up with crystal like particles (best way I can explain it ) that makes your NiCad batteries lose capacity over time . I've come across a way try and zap them with a battery charger without taking them apart or buying new batteries . I zapped a couple batteries I've had since buying a 4 piece set in the 1990's 18 volt Dewalt .Checking now to see how long it will last just in a dewalt radio . Will post results later as I'm experimenting now .


7 years ago on Introduction

Almost 6 years later, and still good information.

The one thing I took away from this, that i will be grateful for as long as I can remember, is the Soldering Video. That little trick/tip is going into the printed long-term storage

6 replies

Reply 7 years ago on Introduction

Ironsmiter, I need to pick your brain a bit. I have a stack, probably a couple dozen, batteries of various makes that I have "liberated" from a big box stores recycling box. One at a time naturally!! Most are 18 volts. My 18 volt charger is dead. Now to the question.....Would it be possible to use a 120 AC transformer that puts out 24 volts AC? Read on before you shoot me down. Put a bridge rectifier on the output to convert the AC voltage to DC.
The new transformer is a Honeywell 120/208/240V, 60 cy, primary to 24 VAC 40VA secondary. I am sure the 40 VA would take some time to charge a 18 volt drill motor battery. All of the instructavles I have read deal strictly with batteries. None mentions the care and feeding of the chargers. Care to share any helpful hints?
I am not a professional so I needed someone with brains to clarify my idea.


Reply 7 years ago on Introduction

wow, wow WOW... overkill.

Unless you're putting dozens of those cells in parrallel, 40VA is gonna DESTROY them.

I'll give you a summery of my research and calculations, as best I can.

Bear in mind, my power supply is going to be a 30 volt HP printer power-brick.
That brought my current limiting resistor VERY low ohmage, and wattage. 10 Ohm, and 1/4 watt.

Now, since you're starting with 40 volt...

The charging voltage is going to be approximately 1.4 v per cell.
You have what, 15 cells? (15 x 1.2 vdc nominal = 18 vdc)
So you need 1.4 x 15 = 21 vdc to charge.
24 v (supply voltage) - 21 vdc = 3 vdc voltage drop across the resistor.
Your cells are probably 2.5 Ahr. You need to charge C/10 , or 0.25amps.
R = E/I = 3 v / 0.25 = 12 ohms.
3 v / 12 ohms = 0.25 A
3 v x .25 A = 0.75 watts
I would upgrade to a 1w or eve a 10w resistor. they're only a dollar, and the added safety is worth it, imho.

You'll have to check VERY carefully. but if all the packs are NiCd, then 12-14 hours with the charger, and resistor described should give you a full charge. It should also be safe if you forget and leave it on a little too long. NiCd can survive almost indefinite charging at 1/10C rate. The extra energy gets converted to heat, and exits the battery through the cell wall.
Bear in mind, quick chargers like my original, have very sophisticated circuitry... but mine charged at almost 1C. it did shut off after that though, as a safety feature. Charge at 1C for 10 hours, and you better have the fire-department on speed-dial, cause around hour 3, it's gonna go Fwoosch!

Consider some googleing on "homemade NiCd chargers.
Tons of info is to be had at RC groups. they use packs much like these to fly their planes(though they're starting to switch over to li-po and li-fe)

Doh, just forgot.. you'll have to measure the ACTUAL voltage, under load, because you're gonna get a voltage drop over the rectifier. You're going to have to recalculate using the lower, measured voltage.


Reply 7 years ago on Introduction

Ironsmiter, you must be an electronics engineer!!
I certainly appreciate your rapid reply.
I am not that much into electronics, and had to re-read your reply several times to boil it down to my level. I am more mechanical and my dabbling into electronics is fixing things broken. Determine what is bad, cut out the old and replace it with a like item. Your reply was a real insight in how to determine (calculate) the needed resistor.

An amazing bit of info can be had on these new-fangled gadgets (computers) if a person knows where to look for it. I wish computers had been available when I was young. I pre-date Television!!
Correct me if I am wrong, but this is what I determined from your reply.

18 volt batteries have 15 1.2 volt cells.
To charge these cells will require 21 volts @ .25 Amps for 12 to 14 hours for a full charge.
To drop the 24 volt output from the transformer, after it has gone thru the rectifier, will require a 12 ohm 1 watt (or better yet a 10 watt) resistor.
Now if I can just figure out the 40VA output of the transformer...Many years ago I have read how the VA is calculated, but it escapes me now.

I REALLY appreciate your thorough explanation of my question.


Reply 7 years ago on Introduction

Nope! I started the same way you did.

Just happens, I post-date color TV.
Though i do remember, vividly, when the Internet switched and we all started using the WOrldWideWeb.
When I started, we Gophered instead of Googled.

If, and that's a pretty big IF, i got my numbers right then...
12 ohm, 1Watt(or higher) resistor should take your power supply, and turn it into a very crude 1/10C "slow charger" for your 18V battery packs.

Pardon me while I plagiarize.
The only difference between W and VA is the power factor. The power factor, unless known, is an approximation. For purposes of our calculations, we use a power factor of .85. The VA value is always higher than the value for W.



Reply 7 years ago on Introduction

oh, and to be VERY CLEAR for the folks trolling in the background, or in the years to come...

that's AFTER the transformer is rectified.

I actually have absolutely NO idea what'd happen if you fed the unrectified AC into the battery(though I have a pretty good guess, and it involves letting out a lot of magic smoke. You know, the stuff that makes electronics work until you let it out?)


Reply 6 years ago on Introduction

The un-rectified AC would just blow the wiskers off and the NiCad would work good again. Just have to limit the AC time to a few seconds...


6 years ago on Step 4

The Dremel tool effectively removes the surface oxidation. I suspect this is why the solder sticks. You could probably do just as well with alcohol and some flux. A rougher surface should also help prevent the solder site from being flaked-off. Dremel tool looks like an easy solution too.


6 years ago on Introduction

Hi i have been reading this interesting section and wondered is it possible to do the same with Li-ion batterys as i have a 21.6v with 2 battery packs and none work . When placed in the charger or drill they put the overheating light on. Anyone think this can be fixed.


7 years ago on Introduction

I'm a newbie on here.... Need a little help is poss
I've taken apart my Makita 18v Cordless drill 2.6ah NiMH battery because it would not charge (flashing red and green). I've tested all the cells and they all read 1.34v so i'm thinking its one of the three components that is stopping the battery from charging (see image). It was hard work getting the cells out of the case and i think this may be an indication that the battery may have been over heated at some point. The charger is working fine with my other battery although that battery is also starting to loose power quickly.

Does anyone know what each of these components do and how I can replace \ bypass one or all three to get the battery to charge again?
Info: the red component was glued to the top of one of the other cells in the pack.

1 reply

Reply 7 years ago on Introduction

I was told that the;

rectangular Klixon component is just a fuse,

The red component is a thermal cut-out\reducer switch used for reducing the power input when on the charger if overheating occurs,

The yellow component is a thermal cut-out\reducer switch used for reducing the power output when on the battery is in the drill but overheating occurs due to heavy usage.

All components worked on this battery, so i guess the batteries can no longer hold a charge.

Hope this info helps some one.


7 years ago on Introduction

Does someone make and sell a converter to actually plug in your 19.2 sears cordless drill and saws? Sometimes at home you don't always need to be cordless.

1 reply

Reply 7 years ago on Introduction

I removed the battery from my drill motor and just connected a wire to each terminal with clips that fit and plugged the other end into my 25 amp switching power supply and it works good. lotsa power.