Introduction: How to Fix a Broken Cordless Drill Charger

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Step 1: Verify What the Problem Is

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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

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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

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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

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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

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Simple, put the thing back together

Step 6: Results

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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

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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.

Comments

ironsmiter (author)2012-02-05

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.

AZ_Maker (author)ironsmiter2012-09-20

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...

About This Instructable

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Bio: 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 ... More »
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