Need help with fried capacitor on cordless battery charger

[yacaph]

A capacitor in the charger base for one of my 18V cordless drills over heated and I can no longer read the color bands. I created this schematic by tracing from the circuit board. It's been so long since I've done any of this I need some experts to help.

Does anyone have any idea as to may have the cap to over heat?

This is a sharp as I can get with my old G3 cellphone

 

[davenn]

the circuit doesnt look quite right

show us so close up SHARP and WELL LIT pix of the board and that so called capacitor

Dave

 

[yacaph]

Thanks for responding.

I've updated the post with the circuit board photos taken with my old G3 cellphone (only camera I've got)

 

[davenn]

that's bad... you need to experiment with your photography a bit

I assume you mean the largest component in the view ....
as I suspected the circuit didnt look right .... it appears to be a resistor
the circuit wouldnt work with a capacitor in that position
you need to unsolder and lift one end of it and measure it with a multimeter on Ohms range and tell us the value you measure .... you may well find that its still OK


Dave

 

[KJ6EAD]

It could be that the large resistor overheated simply because the battery is at end of life or the Zener diode has failed short. Either condition could cause the other as well.

 

[davenn]

we dont even know if it is a zener, it could just be a plain ol' 4001 or similar

yacaph, what are the markings on the diode ?

Dave

 

[(*steve*)]

In addition to everything that everyone has said, that's the worst way to charge batteries and results in their early demise and then you go and buy a new battery pack.

Get a charger for the type of battery it is (NiMH, NiCd, Pb, etc) and charge it directly from the terminals which supply the power.

The charger will probably cost less than a battery pack, and your batteries will last *much* longer.

I got a charger with magnetic leads that can be connected either way around, and which automatically determines the type and voltage of the pack. It cost me well under $100 from memory.

 

[yacaph]

Thanks Dave it is a 5.5 +/- ohm resistor, Does that value sound right?

 

[(*steve*)]

5.6 ohms is a preferred value.

If you have room you could replace it with a 5W wirewound resistor which shouldn't get quite as hot (I'm guessing the original is 2W)

 

[yacaph]

Steve are you saying not to use the base and charge directly from 20 VDC charger (I mistakenly put 18 V on the schematic)?

Joseph

 

[(*steve*)]

No, I'm suggesting you get a proper charger that will take care of the batteries and not overcharge them to death (which is what the simple circuit in these things does.

I'll try to find a link to what I purchased... (as an example)

 

[yacaph]

Thanks Dave, I think I have room.

 

[(*steve*)]

OK, here and here are examples of what I have.

 

[yacaph]

Thanks Steve, I just bought a timer that allows me to set a charge time (I use 4 hrs.) to prevent over charging. I plugged a power strip into it allowing me to charge multiple batteries at the same time. Whats your opinion?

 

[(*steve*)]

That's better than nothing. However you can still overcharge them if they are not fully discharged.

It might be worth taking a discharged one and putting it on charge, then monitoring the temperature of the pack every 20 minutes or so. At some point the temperature will start to rise quickly. This indicates that the batteries are being overcharged. If that's 3 hours, then set your timer to 3 hours in the future.

 

[yacaph]

The diode is OK. It passes one way and blocks the other. It's probably because the battery life is over. I was attempting to charge the battery pack up enough to determine which individual Ni Cad batteries need to be replaced. I guess I'll have to disassemble the battery pack to find out.