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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.
Since this is a VERY dumb charger...<br><br>You'll want to drop that current to around 1/10C(putting in 1/10th of the total charge per hour)<br>Probably the simplist way to do it, without changing out your powersupply, is to solder in a current limiting resistor.<br><br>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<br><br>Or am I completely off base, and that's the resistor for the LED light?<br>If so, slap the current limiting resistor right on the power line coming frim the wallwart.<br><br>12 volt NiCd... should be around 2000mAh pack when new. especially judging by the original power supply.<br>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 &quot;need&quot; is for a 0.6 watt resistor.
Yes, this really is the brute force method. <br> <br>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. <br> <br>All the same, the charger is still good for light use if I'm careful to not overcharge the batteries. <br> <br>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. <br> <br>Thanks for doing the math - I'll take a look again if I want to redo this the right way...

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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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