How to Fix a Broken Cordless Drill Charger

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About: I am a journeyman of projects - when I see something that I want to accomplish it, I do so with attention to detail and quality. I have a background in software and hardware, but also excel in DIY home impr...


Step 1: Verify What the Problem Is

This Instructable assumes that you identify that the problem is the charger itself.  In my case, a 15v power supply to charge 12v batteries was pumping out 3v.  Get a simple multimeter and measure the voltage at the terminals of the charger.   If the voltage is less than the rated voltage, then this Instructable might be for you!

Step 2: Find a Donor Power Supply

Since my 15v power supply is pumping out 3v, I need something else that has similar power ratings.

The original:
15v, 200mA

The new supply
15v, 800mA

Well, the replacement is a bit overpowered, so there is a little danger here.  If possible, find one that is a closer match than this one.

Step 3: Swap Power Supplies

I cut the wires to the original power supply, and tested the performance of using the new power supply before making it more permanent.

It worked as it should - actually, much faster than it should.   I need to be careful not to overcharge.

Step 4: Make the Wiring More Permanent

I soldered the wires to the original connection points and even made a pull-stop so pulling the cord wouldn't yank the wires out of the circuit board.


Step 5: Reassemble

Simple, put the thing back together

Step 6: Results

Oops, it looks like I need to test my batteries with the multimeter every time I charge for more than 5 minutes now, because the new charger is a bit overpowered...  Once again finding a more closely matched power supply would have helped.

Step 7: Final Test

There you go, the light comes on and charging commences.  It looks like this project is complete.

Again, I put more details on my blog post , but the steps are the same.

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    7 Discussions

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    bostonyeti136

    4 days ago

    My 15.6 drill charging unit went duff and burnt out the circuit board.oopsy.its 6yr old unit.drill worked ok.battery pack is holding charge.well it was working till it blew battery charging unit board.can I just find positive and negative wire to battery's and use directly from bared wires from original charger unit.or is that too dangerous.and risk of fire or blowing up.please advise so I can hotwire the batteries which are good and be able to plug charger unit to charge them.thankyou for any help you can give.im a Tecno Numpty.ie no good at electrics so simple instructions would be fantastic.the YETI.,

    15575970142251674863030.jpg
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    AZ_Makerbostonyeti136

    Reply 1 day ago

    If the circuit board is fried, it's not guaranteed that the power supply is good (I repurposed another one). If you don't have a meter it's hard to measure and check.

    In theory connecting the power supply (with similar voltage to the battery) to the battery will charge it. Without the protective circuitry though it can charge too fast (heating the battery up and shortening its life) and you have to unplug it at an appropriate time because it won't stop charging.

    Since I didn't try to protect the charging rate (as another commenter posted about) I took the risk of the batteries failing, and they eventually did. I got some extra life out of it this way though.

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    ironsmiter

    7 years ago on Introduction

    Since this is a VERY dumb charger...

    You'll want to drop that current to around 1/10C(putting in 1/10th of the total charge per hour)
    Probably the simplist way to do it, without changing out your powersupply, is to solder in a current limiting resistor.

    Not SURE, since the picture is a bit small, but there APPEARS to be one already on that tiny board. you SHOULD just need to upgrade it, to handle the 800mA powerbrick, vs the old 200mA brick

    Or am I completely off base, and that's the resistor for the LED light?
    If so, slap the current limiting resistor right on the power line coming frim the wallwart.

    12 volt NiCd... should be around 2000mAh pack when new. especially judging by the original power supply.
    Someone double check my math, but i think a 15Ohm, 1 watt resistor should do nicely. even a 1/2 watt SHOULD work. but calculated "need" is for a 0.6 watt resistor.

    4 replies
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    bostonyeti136ironsmiter

    Reply 1 day ago

    You lost me after the first dozen words.i have no electrical skills in the slightest.but thankyou for your advice.if I send another pic.is it possible to identify what electrical components I need to replace if I send the new photo on the board.there is two which have destroyed themselves.where can I buy them.and get replacements from.i appreciate your help so much.if I can replace two components I'm sure it will work again as it should.once again thank you.

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    ironsmiterbostonyeti136

    Reply 1 day ago


    r1 and r2 are likely 1/4 watt 100 ohm (5% tolerance) (brown black brown gold)
    d1 diode is..? and leds are normal cheapies. all are likely fine.

    r3 and r4 are too burned to see the color codes.
    they are likely 1 watt.

    If you tell us the power supply voltage/amperage and the battery voltage, we can calculate what should be there. (just read the stickers). The copper traces on the back side of that board are likely burned too, so you may need to solder in jumper wires to repair it.

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    I'm hoping that these are more helpful pics.two components blown.if you can tell me what they are and wattage or voltage and were to order.i will buy and see if it's savable or scrap.thanks once again the yeti.any advice greatly received

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    AZ_Makerironsmiter

    Reply 6 years ago on Introduction

    Yes, this really is the brute force method.

    I'm actually lucky that the existing board could handle the higher current, since I didn't really investigate the components too closely. I was being a little lazy, and stubborn because I didn't think it should have to be a difficult job.

    All the same, the charger is still good for light use if I'm careful to not overcharge the batteries.

    If the existing charger board wasn't still functional, or if I wasn't willing to destroy the battery and charger by experimenting, it would be essential to add a little circuitry as you are describing.

    Thanks for doing the math - I'll take a look again if I want to redo this the right way...