Homemade Cordless Drill Battery Charger

Picture of Homemade Cordless Drill Battery Charger
In this article you will find step-by-step instructions on how to build a battery charger from wood scraps that will allow you to charge a NiCd (ni'-cad) cordless drill battery.
** WARNING ** only NiCd (Nickel Cadmium) batteries may be charged using this method. The pack type is usually written on them. Many newer drills use other types of batteries (Li-Ion, NiMh) that WILL EXPLODE IF YOU USE THIS PROCEDURE. If you are unsure, do not attempt this instructable. Also, improper construction or calculation of component values can cause the batteries to CATCH FIRE or EXPLODE.
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Step 1: Cut two wooden blocks to hold the battery pack in place.

Picture of Cut two wooden blocks to hold the battery pack in place.
First measure the thickness of the stem on the battery pack and rip two wooden blocks to the same thickness. Then, make a v shaped groove to accept the rounded part of the battery pack. Make the second block with grooves to accept any keying bosses (ribs) the manufacturer may have added to the pack. I found it was easy to press the wood hard against the battery pack and use the dents as a guide for putting the slots in with my table saw. The two block can then be held in place and traced on a thin piece of wood that will become the side. Screw the side to the two blocks and test fit them.

Step 2: Cut a second side for your battery holder

Picture of Cut a second side for your battery holder
Cut a second side from a thin piece of wood. The thinner, the better. It will be flexed when the battery pack is inserted. Cut a relief or leave out two of the screws so that the wood will bend a little when the contacts slide in. The puzzle piece shape isn't required, but it makes it easier to cut on a bandsaw. It does allow prongs of material to come from both sides that can flex out of the way as the pack is inserted.
ijourneaux4 months ago
I really appreciated your posting. I was trying to repair a 19V hand held vac. I had replaced the batteries in the unit (15 C cells) After replacing the batteries, I noticed that one of the resisters had overheated and the charging LED had burned out. Using your description I installed a 800ohm resistor to protect a new LED and an 82ohm 2 watt resister to limit the charging current. This wasn't exactly what your calculations came up with but seemed to be close enough. I am using a 120Vac to 24Vdc 20mA wall charger.
Take Care
anglica861 year ago
this is really nice...but what to do if the battery goes down..then how to replace it. and if the battery is down then what should we do?? we use the cordless for our The Trendy Style Dress Shop
etcmn2 years ago
I know I can build this if someone can help me with the math. I need to build a charger for my 18V Skil ni-cad battery packs. I've had a couple of battery packs die and wanted to take the opportunity to upgrade. The orignal packs were 1300 ah but I plan to buy 1800 ah cells to rebuild them. Would also be nice to build the charger so it can recharge two batteries at the same time.
Thaikarl2 years ago
i tried to figure this out on my own, but i just couldn't get an answer that made sense, so i'm just going to ask. i brought an 18V Royobi tool set home to thailand last year. i bought a 220V --> 110V transformer (stepdown) to plug the charger into. my wife, helpfully plugged the charger into the wall, without the step down transformer. POOF! blew the transformer in the charger. i looked up the transformer in the unit. it specs out as 20.5 VAC 1900 mA on the secondary side. but up here in farm country, i could only find a multi-tap transformer (220V on the primary side of course) that has 9, 12, 18 and 24 V taps on the secondary side. bigger than the Royobi transformer, so i think it will handle the power, but what can i do about the 3.5 Volts higher output than the original one supplied?

is there a way to "dump" or use up the extra voltage so i can use it in my charger? like with some resistors or something? if i can't get it fixed, i have to buy another drill, and i really liked my Roybi cordless. thanks anyone!
milesius2 years ago
I have a 24v 6amp saw that lost its charger, so I definitely need to try this project.
Thank for your instructable.
I figure I can just double your numbers to achieve a safe charge?
Is there any truth to the idea that slower charging will extend the life of Nicad batteries over speed charging them? This has been my experience, and I would like some feedback.
yeltrow (author)  milesius2 years ago
Just doubling the number might not be safe, so be sure to check out the battery specs and run your own calculations. Pick a charger that is 36V or higher and then run the calculations. Also, it will matter if you have an AC or a DC output AC adapter. The power dissipation on the resistors and the maximum charge rate are the most important numbers. Too much power eaten up across too few resistors = fire. Too much charge current on the pack = fire.
Please be careful and test your circuit on a non-flammable surface for while.
msubzwari3 years ago
Thankyou @yeltrow

Your instructable helped me find out a charred resistor value on my B&D Firestorm Drill 14.4v battery charger. The battery is Black & Decker FSB14 FireStorm 14.4-Volt NiCad Slide Style Battery.

Here are my calculations which fixed my charger. I used /10 to match original charger circuit. Wall wart is rated 17.4V / 210 mA. I suppose it was designed to fully charge a pack in 10 hours.

2.0 Ah capacity / 10 (dc supply) = 0.2A charge rate
17.4V charger – 14.4V battery = 3V difference
3V/0.2A charge rate = 15 ohms
3V*0.2A = 0.6Watt of heat
0.6 watts / 1 ( dc xformer) = 0.6 watts or 600mW.

Some faulty cells in one of my battery pack caused the charger malfunction. So I am now keeping an eye on battery volts when charging the pack. I disconnect the charger when meter reads about 17.4 volts (1.45 max per cell x 12 = 17.4).
yeltrow (author)  msubzwari3 years ago
You are welcome! I was reviewing your comments and it's a nice case study. I was thinking after reading it that it is also important to consider the case of a fully discharged pack when determining how many resistors to use to handle the heat. The voltage across a fully discharged pack is 0.8V per cell times the number in the pack. In your case, you have 12 cells. 12 x 0.8V per cell is 9.6V. 17.4V - 9.6V = 7.8V. (7.8V of difference)/(0.2A of Capacity/10) = 39 ohms of resistance. 0.2A * 7.8V = 1.56W. To safely dissipate the 1.56W, I would recommend splitting the 39 Ohms across 7 or more Two of the 5 packs of 0.25W resistors all wired in parallel would be 47Ohms, which would extend the charge time a little but have 2.5W of heat dissipation capacity and would be a safe bet. The other standard value that would work okay would be to get a 5 pack of 0.5W 220 Ohm resistors and wire them all in parallel for 44Ohms and also 2.5 of heat dissipating capability. Again, a little slower charging, but capable of safely dissipating the heat. Thank you for the detailed discussion of your application -- It really helped me think through this instructable after not having looked at it for a while.
Using a Makita 6260D (9.6V drill) the voltage generated does not exceed 1 Volt :-( Using another Makita 10.8 Volts (normally powered with Li-ION battery) does not rotate at all even wqhen holding the switch down. It seems it bloks when no power is applied.
Phil B4 years ago
Does the LED stop glowing when the battery is fully charged?
yeltrow (author)  Phil B4 years ago
The LED will stay illuminated. Most of the 1/16th C still will flow through the battery even after being fully charged. It will be reduced slightly because the voltage of the 12V pack will be a little higher than 12V when it is completely topped off; however, it will still not be high enough to prevent all current from flowing. Although it is common practice to trickle charge NiCd batteries forever, it is best to take the battery out after 16 or 20 hours.
will41 yeltrow4 years ago
1/16th C charge rate (capacity divided by 16) when you say capacity are you referring to the battery per say 2000 Ah trying to figure out the math on this why do you divide by 16 thanks
yeltrow (author)  will414 years ago
will41, thank you for your interest in my article. If you look at the datasheet for many NiCd batteries, they suggest that no more than 1/10th of the capacity be charged in an hour if you are going to let it cook on a "dumb" charger (which this instructable is). The most conservative specs recommend 1/16th of the capacity be charged in an hour. 1/16th C is a relatively gentle continuous charge rate, so that is why I selected it. Also, the resistors get hotter the harder you drive the charger, so it keeps those running in a safe range as well. As a side note, the wall transformer for my original Ryobi charger is 20VDC, so it appears my choice of an 18VAC transformer wasn't a bad one at all. You may want to find one with about 8 volts of overhead too.
will41 yeltrow4 years ago
i have the math figured out now,little confused on the wiring i cant makeout the picture which wire is wired to the resitors and which are wired to the led which side is positive and negative can you help me out thanks
yeltrow (author)  will414 years ago

The battery should tell you how many mAh (milliamp hours) the battery has at full capacity. This is how long the battery will run for if you were to discharge it at a given current draw. This is the capacity or "C" I speak of in the article that I divide by 16. For example: 1400mAh means you get 1.4A=1400mA for 1 hour, or about 700mA=0.7A for 2 hours, etc. The mAh are measured under ideal conditions, and generally decrease by as much as 50% or more if you discharge the battery at a high rate. Aslo, while you are reading the battery * MAKE SURE IT IS A NiCd BATTERY * or you may well BLOW IT UP OR BURN DOWN YOUR HOME. You did not state if your wall transformer is 24V AC or DC. It will also say this on the transformer. Please use the schematic to figure out how to wire this up -- the photo is just an example. The symbols can be looked up on the web. You may want to check out the wikipedia pages for Diode, LED, Resistor, and Ohm's law for details on what each of the parts do to get a clearer understanding of how they work together.
Not only a good idea for a battery charger. Also a good idea for a battery holder to power other projects.

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