Homemade Cordless Drill Battery Charger





Introduction: 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.

Step 1: 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

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.

Step 3: Add Copper Contacts to Your Battery Holder

The battery holder needs a pair of copper contacts. I used #10 solid copper wire from a piece of type NM cable commonly used for residential wiring. Mark a pair of holes at the top and bottom edge of your battery contacts for the positive and negative battery terminals. Next, strip all of the insulation off the wires. Then I bent it into a U shape so that the U was inside of the holder and the wire stuck out. Insert it into the pair of holes. The wires should touch the terminals on the battery. A volt-meter can be used to check that this is working.

Step 4: Screw on Both Sides of the Battery Holder.

The battery holder should sandwich the battery pack stem. The contacts should touch both of the battery pack terminals. My pack was tapered, and so the blocks are not square with the ends. I clean up the ends when I finished with the table saw.

Step 5: Build the Circuit.

A diode, an LED, and a few resistors are required to make sure the battery pack is charged at a safe rate. If you have any doubts on how to do this, consult an expert on the topic or quit. MISTAKES IN SELECTING COMPONENTS CAN START A FIRE. THIS COULD BURN DOWN YOUR HOUSE, PERHAPS SETTING OFF A CHAIN REACTION OF FIRES THAT BURNS DOWN YOUR WHOLE TOWN. BATTERY PACKS WILL ACCEPT EXCESS CHARGE CURRENTS WHILE REMAINING COOL UNTIL THEY ARE FULL--AT THAT POINT THEY WILL START TO OVERHEAT AND MAY CATCH FIRE OR EXPLODE. I used an AC output pack, so the charger outputs effectively half that of a DC wall transformer. I aimed for a nice, slow 1/16th C charge rate (capacity divided by 16). Because mine is an AC output transformer, I had to double that to 1/8th. Use 1/16 in the math for DC chargers, or it will kill your packs or maybe start your house on fire.

1.6Amp hour capacity / 8 = 0.2A charge rate.
The nominal voltage of a fully charged NiCd cell is 1.2V.  So for 12V my pack needs 10 cells.  (12V/1.2 = 10 cells).
The fully discharged voltage of a NiCd cell is 0.8V.  10 cells * 0.8V per cell = 8V.
20V charger - 8V battery = 12V difference.
12V/0.2A charge rate =  60 Ohms.
12V * 0.2A = 2.4Watt of heat that will be generated.
2.4Watts / 2 (because of AC tranformer ) = 1.6Watts.

I used a set of 6 resistors to get near my 60 ohms. This gave me about 3.0Watts of power dissipation using 0.5W resistors--Because I used an AC charger, the 3.0W capacity is more than enough, because the resistors get to rest during the negative AC half cycle. They are really only exposed to 1.6 watts.

For a DC 20V charger, the desired charge rate would have worked out to 1/16 = 0.1A and 120 Ohms. 12V * 0.1A = 1.2W. Six 1/2W resistors would probably be okay for this too, but the values would be different (120 ohms instead of 60). BE SURE TO DO YOUR OWN CALCULATIONS -- MINE ARE ONLY AN EXAMPLE and might burn your house down.

The charge indication LED should get about 0.02A max.
(12V-1.7V led forward voltage drop)=10.3V. 10.3V/0.02A = 515 Ohms. A 680 Ohm resistor would be fine. This limits the current into the LED to keep it from dying.

Step 6: Test the Charging Circuit.

Make a cover plate to cover the electronics. Cut the wood to the same size as the sides of the battery holder. Pull the cord from the wall transformer through it to finish building your circuit. Make a knot in the cord to keep it from tugging on the circuit parts when it is being used. Place the battery into the holder and power up the circuit to test it The LED should illuminate. Take this opportunity to mark a hole for the LED in another thin piece of wood that will cover the circuit.

Step 7: Add a Pair of Spacers.

Assuming that everything has worked thus far, it is time to close it up. Cut two spacer strips to keep the cover from crushing the electronics. I used a thin piece of hardboard. Drill holes through the hard board and the spacers at the same time.

Step 8: Put in the Last Screws.

Put in the last screws and try it out. I added a strip of wood to the back of mine to allow it to be screwed to the wall next to the charger that came with my drill years ago. Now BOTH batteries will be ready when I need them.

Thank you for the time you spent looking over my instructable. If you have read this far and are thinking, "This was great and totally worth a buck" You can help me continue to make more instructables by making a donation using this link to my ebay store. Thanks, and keep building! -- Yeltrow



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    We have a be nice policy.
    Please be positive and constructive.



    what voltage ac transformer would i use for a 7.2v battery?


    I moved to Philippines and plugged my 120v M12 Milwaulky Li-ion charger into wall outlet. They are 220v here. It blew the charger. Can I use a trickle charger I bought for my car to charge battery. They are 12 v dc batteries. Blown charger is input 120vac 60 hz 750mA output 12vac 3A. Trickle charger is automotive type input 220vac output 6 or 12 vdc capacity 6 amps. in the back it has a 0.8 screw in old inline style auto fuse. I am retired and it costs alot to get a new charger here. Maybe I could repair old M12 charger, what might have blown? TV's, PC's, rechargeable shaver, almost everything was dual voltage so I was caught off guard. They use 220 here, 2 hots and no common. Any help?

    2 replies

    DO NOT.

    Charge /discharge characteristics for li-ion vs lead acid are so radically different that the battery and charger will probably destroy each other and catch fire.

    JonathanD188 is absolutely correct: Do not attempt to charge lithium-ion or lithium polymer batteries using the techniques in this instructable. A fire is likely to result.

    need help... i have a Ozito brushless cordless drill driver. the problem is, even with a full battery, it sometimes wont work.. or sometimes after i stop from running the drill and right after i pull the trigger, it wont start or work again... what may be the problem.. pls help... thank you...

    Hello, I am surprised and delighted to find a forum where
    cordless drill chargers (which have stopped working ), are discussed and
    possibly resurrected! I have such a problem with a DRAPER REDLINE 18-volt cordless
    drill driver. I have tried to get information from Draper’s technical team but
    they were totally useless.

    The Transformer pack (working) says it delivers 17watts, up
    to 400mA DC at 23Volts. (But my open- circuit EMF measurement was 29.5V). The
    Transformer output mini jack plug connects to the Battery through a 17watt Base
    Unit, which fastens onto the top of the battery during charging.

    The Base Unit has a small printed circuit board containing a
    Rectifying Diode (my assumed resistance for this was 13Ω) and a Main Resistor (BURNED
    OUT) in series. The resistor is a now dead short and the colour coding cannot
    be distinguished, presumably due to its having been “fried”. All the colour
    bands look black).In parallel with the
    Main Resistor is a small resistor (measured at 157Ω, in series with a red LED, for
    which my assumed resistance is 13 Ω, although that intuitively sounds high, to
    me). The Battery unit appears almost completely ‘flat’, with only a few volts EMF.
    According to wording on the Base Unit
    its output voltage (to battery) should be 23volts, rated at 17watts.

    I have been trying (by means of an Excel spreadsheet) to investigate
    and estimate the value of the Main Resistor, by assuming the battery EMF of 18V
    opposes the Input EMF of 30V, leaving a net EMF of 12V, to achieve the 23V output
    (to battery) and a charging current of less than 400mA. But without any
    realistic knowledge of the Battery internal resistance, the LED resistance, and
    the Diode resistance, I am rather in the dark. My simple Ohms Law calculations,
    lead me to a Main current –limiting Resistor of 100Ω , with a battery internal
    resistance of 50Ω (which sounds too high to me) and a charging current of only

    Does anyone have knowledge of this type of charger circuit and
    the resistance (and wattage) to use to replace the burned out main resistor? I
    could send an email with photos of the circuit board and my derived circuit
    diagram, to anyone who might be able to help but will try to attach these here.

    RESISTOR drill charger.JPG

    This is not a nonsense like ΧωρίςΌ want us to think... Overnight charging is the cheapest way to charge a nickel cadmium battery (YES ONLY Ni-Cd like the author say), and is performed charging at C/10 (10% of the rated capacity per hour) for 16 hours.. So a 1000 mAH battery would be charged at 100 mA for 16 hours. This method does not require an end-of-charge sensor and ensures a full charge. Cells can be charged at this rate no matter what the initial state of charge is.

    Also another fact, the volts the author use are also safe as the minimum voltage you need to get a full charge varies with temperature (at least 1.41 volts per cell at 20 degrees C).

    Finally, the best charging practice is to use a timer to prevent overcharging to continue past 16 hours, so if anyone has the expertise, can make a circuit to do this (or use arduino). I will try to make a circuit capeable of reading the third connector on the battery pack, that is a thermistor, so if the battery pack gets too hot, the charger can cut-off the charge.

    Also, i will post pictures later, because my fast-charger was water damaged, so i will remove almost all components from it's pcb and replace them with a circuit like the described in the tutorial... also i will keep the fan inside the charger, so i be transforming the fast charger into an overnight charger, but will fit perfectly.

    Hello, I just opened up the charger for my Skil screwdriver, the battery is 14.4V and 1.2 Ah .
    The charger is powered by a 18 V AC 500 mAh transformer ... The big resistance is around 13 ohms, the circuit has the diode and the led and 2 small resistors but also a transistor connected somehow to the led !
    I measured the voltage at the battery terminal and it is only 5 V C what should I replace in order to repair it?

    OK, I know the guidelines about posting say to be nice positive and constructive. But it makes no mention about people who post stupid (to the point of being dangerous) hacks of something that can be made the RIGHT way or even purchased at a comparable cost without risking anybody's safety.

    The giveaway is the beginning sentence: "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".

    So you are making a charger out of wood scraps, right? How idiotic is this: You are using wood scraps making an improper HOLDER for an improperly constructed makeshift charger.

    If you had half the brain of a sheep you'd already know that a charging circuit is a lot more than a few resistors and capacitors thrown together and that a properly build charger must have some sort of overcharging protection in the circuit, either thermal or electrical or, better, both. This means that a circuit must be smart enough to either sense the electricity flowing into the battery and cut off power when due to increasing internal resistance (as battery reaches full capacity) the battery resists more charging, hence the overheating. Or have a thermal sensor which will sense that overheating and cut off power. OR in reality, have both which is how these chargers are built.

    But you also say a lot more (very dangerous) nonsense on which you're basing your calculations, i.e. a fully CHARGED NiCD cell does not have 1.2V NOMINAL charge, but a lot more. Have you ever charged a cell like that and checked its voltage? It is going to be as high as 1.42V for a new cell to as low as in the mid 1.30s V for a more used one. Spent cells (e.g. that do not hold charge anymore barely go over 1.3V usually 1.28 V) Also a completely empty cell that is allowed to reach as low as 0.8V (as you say) will have probably sustained enough damage to not be useable any more. Typically, these cells should NOT be left to go below 1.0V, that is why it is not a good idea to keep using your power drill if the battery shows signs of slowing down. That is why most manufacturers suggest to put the battery back to the charger once you are not able to do the job as you would normally could, using a freshly charged battery.

    So please people DO NOT TRY THIS AT HOME! Ιt may have worked (actually we do not know that 100%) for this guy but it may not work for you especially if you are not very savvy with electricity. Indeed you are risking the danger of fire, which is actually one of the few if not the ONLY correct thing I read in this article! (lol)

    One last note to INSTRUCTABLES: Should n't you guys have some kind of moderation? Someone should verify what every one is posting on your site as "instructable" BEFORE it is actually posted? The usual disclaimer "do it at your own risk" is not good enough, don't you think?

    PS: And before anyone starts complaining, I tried to be as nice as I could. But as you all know stupidity is invincible, right?

    3 replies

    If that's as nice as you could, you really aren't making anything like a reasonable effort.

    billrule: Yes, there was a high "eh" factor on this build. :)

    Yes, wood is a terrible material to build a charger from. I totally agree with that. However, all the whiz bang features you speak of are all comminly missing from all but the most sophisticated of chargers for NiCads. The risk of fire that the thermistors protect against are for rapid chargers, not 1/10th C or equivalent. Lithium ion chargers need that also. This in neither a lithium, nor a high current charger. The voltage cutoff of 0.8V under load is a manufacturer recommended cutoff. The 1.2V is also nominal under load. The tapering effect of the resistor takes this, and the higher voltages into account. Pakcs bursting into flames take more current than 1/10C. A short in the wiring causing the pack to short or the resistor overheating in this design is probably the largest concern. A fuse rated near the continuous charge average current should also probably be added in series with the pack. That fuse would blow if the pack were SEVERELY under voltage or if there was a wiring fault. Thanks for the safety recommendation; however you may wish to spend as much time in circuit analysis and fact checking as polishing your insults.

    I've got a Mastercraft drill (model 54-2973-4) that uses an 18V battery pack and the charger circuit doesn't work (charge rate controlling resistor blew, R1 in the schematic in the instruction) and I don't know what resistor value I need to put in. The thing is, the battery doesn't say the mAh value so I can't determine the resistor value I need. If it helps, the battery has the number 54-2751-2.

    So my question is: how do I fix the circuit?

    As well, are you doubling the charging rate for AC chargers because no current flows to the battery for half the time (during the negative AC phase)?

    2 replies

    Hi belau,
    You can probably use many internet sources to estimate the capacity of the batteries just by their physical size. When in doubt, underestimate the capacity. If you are in doubt on the battery chemistry (i.e. not sure if it is a NiCad) STOP. Just buy another one. Don't risk it. You are absolutely correct that during 1/2 the AC cycle there is no current flow, and so we are able to use twice as large a resistor compared to DC based chargers.

    and by "large" resistor, I meant current. The resistor is 1/2 the value in ohms, but 2x the current.

    Can you make a battery pack holder that runs a 12v motor?

    I have made a 15v 13ah li-ion battery pack from laptop battery's and I was wondering if it is possible to make a charger for that battery pack. I appreciate any type of information you could give me

    2 replies

    Hi rhman96: Lithium battery packs are NOT SAFE to charge using the method documented here. They should have individual cell over-voltage protection, temperature monitoring, and a crowbar circuit that blows the fuse if a fault is detected during charging. The reason for this is that a single weak cell can look like an undercharged pack, so the charger can keep pushing current in. Excess charge into a lithium pack causes them to catch fire in a spectacular way. With enough persistence, I am sure you could learn about all the chips and calculations required to do it safely, but PLEASE do not use my circuit to do it.

    i have faound a charger that is suposed to work for 14v li ion battery packs. this is what the description says about it

    "Charger for 14.4V and 14,8V, Li-ion battery packs This indoor charger is designed to charge 14,4V and 14,8V, Li-ion battery packs. It works on all power between 85V and 264V (no voltage adjustment is needed). Charging light: 1. The lamp lights green when the charger is ready to use. 2. The lamp lights red when charging is in progress. 3. When your battery pack is almost fully charged, the light is orange. 4. When your battery pack is fully charged, the light turns green. Features: • Automatic voltage adjustment • Constant voltage with constant current Product Specifications: Input: AC 85V-264V, 47Hz-63Hz Output: DC 16,6V-17V Charging current: 1,0A (Max.) Charging temperature: + 5 ° C ~ + 45 ° C Charging Humidity: 30% ~ 90%"
    do you think this will work if i just conect it to my battery pack ?

    thanks yeltrow. i'm considering building a battery charger for a cosplay sword that illuminates. i was debating if making it rechargeable would make it more sales friendly if i decided to make a bunch of them and put them on ebay. this tutorial was a great start for me.

    and thanks again for handling that ΧωρίςΌ situation with such class. why he came in so offended and responded like he was The Superior Spider-man i have no clue. even if he found your tutorial lacking there are so many more positive methods to address it.

    1 reply

    Like the previous post pointed out -- A fuse directly at the battery clips should be added. The biggest concern with this design isn't really the charger putting excessive current INTO the battery, it is the potential for something to touch and short circuit. The current that could come OUT of the battery would be extreme and could cause a fire. Playing with electricity is always taking on risk -- one just needs to understand the risks and make choices on how to mitigate those risks. This class of charger is NOT the best for the long term health of one's batteries. A timer to cut off the current after a full charge (i.e. 24 hours or so) or a some other means of not putting charge in when none is needed will add to the total number of cycles one can get before the batteries wear out. I think I remember seeing that trickle chargers cut the total number of cycles one can get out of a battery by perhaps 30%; however not having a charger of any sort reduces the number one can get by 100% :) Note that the manufacturers don't really care if you get 5 years out of your packs and so they don't bother. They plan on making more money off new batteries or selling more drills. My electric razor has rechargeable NiCad batteries in it, and the stock charger has kept them in good health for 15 years!