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The red light on my Black and Decker 18v cup charger doesn't light up when I try to charge a battery. Answered

Is there anyway of repairing the charger or do I just have to buy a new one.????


I opened my good unit to get the values. I measured R7, the read value on the meter is 15.1 ohms. For Q1 the transistor number is SS8550, PNP 25V 1.5A TO-92. Available at Digi-Key part # SS8550DBU-ND. The resistor part # is PPC15BCT-ND. The unit works fine.

How do you know it is done after 2 hours?

I actually fried my resistor after replacing all the subC batteries, seem to be getting average life out of it on my weed trimmer, which is a pretty high drain application.

I have been leaving it to charge overnight.

Thanks for that, ptlander: I went up to a 1watt 50 ohm resistor and it charges a battery in around 2 hours, with no sign of overheating ... doesn't seem that the battery holds the charge very long, though -- maybe a subjective issue :-)

Thanks for your response


8 years ago

My fs18c (black wallwart w/single black cap/cup w/red LED slides on top of the battery) did this too, the led would slightly light when first plugged in, then quickly dim and go out.

As appolo said, it was a resistor, fried to the point of burning off all its paint and charring the circuit board. I replaced it and so far all is good. Note that Im fairly certain the cause was a battery that shorted itself (0v, continuity test beeped with both polarities, where on a good battery it only beeped in correct polarity and read several V), so check that your batteries are good before trying to charge them again.

to fix: crack the wallwart open. Easiest way is with adjustable pliers, place the jaws just under the weld line on the plug side and squeeze, it should start to crack open and eventually pop apart.

Find/remove the charred resistor (R7, see (crappy) pic) and replace it. I found via variable resistors that the charger output was correct around 40-100 Ohms. I used a 1/4watt 100 Ohm and the Red LED came on again when attached to a battery and the voltage was ~19v unloaded, ~15 loaded (battery showed ~14 by itself). If yours is still identifiable, replace with correct rating. I used a 1/4 watt where I think they used a 1/8th judging by size, which hopefully will make it live longer.


I have just done this project too - I can confirm that I shorted out the battery because I had just replaced the battery pack with a set of subC batteries and I noticed a short after my charger failed. I have fixed the short now and replaced the resister with 1/2W x 100R resistor, and the light is on and charging for about an hour. This is my first soldering/desoldering project (both the battery and the charger) so learning a lot....

Something slightly different: I live in Cairns Qld Australia and have a B&D 18v charger, in which the red light wouldn't light. I removed the anti-tamper screws from the charger and split it apart. I too found a fried resistor, detached from the PCB rattling around in the case. The board is charred badly but I reckon I can repair that with a small solder build-up. My charger is B&D P/N 5100235-09 and the resistor appears to be a carbon film job, but no indications of its value anywhere ... any suggestions please? thanks in advance.

I have charged using 39 ohm resistor at 1/2 watt a few times getting good charge overnight and no noticeable heat on the pack. Charges up to 19.5 v on my multi. Don't know if it is the best/fastest charge but it works.

What is the value of Q1 transistor?

tmacko above is wrong in the r7 replacement wattage rating.  it has to be at least a 1 watt power resister.  i used a radio shack 50ohm 1 watt resister which had a 16.83 volt output.  they come in 2's for about 2 dollars  Do not work on on this unit with power on.  Use extreme caution..

I have one of those, burned out resistor. I believe the PNP pass transistor is turned on for full 200 ~ 220 mA charge, then when full charge is sensed by the unidentified IC (probably a Maxim chip), the transistor is turned off, allowing the resistor connected between emitter and collector to trickle charge the battery. So, that value of the burned resistor should be more like 3.3K or so, allowing a few mils of charge current, rather than  40 ohms that will keep charging at 200 mA. Make sense? I also replaced the transistor with a 2n3906 gp PNP, still testing - waiting for full charge.

Has anyone found the exact value of the original resistor yet? As revjimjones says, the resistor is in the 3.3k area. I started higher 4.7k and the light did not come on. Then tried incremental values down to 3.3k. Possibly a 2.7 to 3.3k would be most accurate. But not knowing what value the chip is expecting, the proper operation of the device is in question.
Also the guy saying it has to be a one watt is way off base since the original was a 1/4 watt. Increasing from 1/4 to 1/2 sounds like a good idea since the original 1/4 burned out. If we could find the correct value, using a 1/2 watt would probably allow the charger to last for years.

edit: that was a 2n4401 600mA collector maximum power - a 3906 would probably burn up, handling 200mA constantly. I'm testing now with a 3.5K, 2W resistor, we'll see after 6 hours what happens...


9 years ago

It's a question of being able to dismantle the power unit! many chargers nowadays have thermal sealed housings preventing easy access. Other types have anti tamper screws. I'm not familiar with Black and Decker chargers, it may be a very simple charge circuit of the type used on budget power drills, these usually comprise of a mains transformer a diode and series resistor. Very often I've known the resistor fails and goes open circuit after a time resulting in no current output to the battery. It may be more complicated and use charge control circuits. Here is a bit of a guide... If the charger is very light to handle it could use SMPS and these circuits can be tricky to fix! If it's got some weight it most likely has a transformer inside which may be easer to fix. You will need some basic electrical/electronic knowledge to attempt a repair but be extremely careful if you do get it apart as the circuits within can be lethal!! Finally__remember safety is paramount if you can repair it. As I don't no your level of competance here, be careful, you don't want a fire or risk injury... be safe. Regards