Overheating battery charger

[madan1]

Hi everyone.
Just got another "faulty" device for pennies .
This time it is a black & decker drill HP122
After some tests I was able to pinpoint the "fault".
Indeed the problem is that the battery charger is overheating, thus getting in protection mode and refusing to charge the battery after very short time.
The battery worked fine for 20-30 minutes non-stop after just 20 minutes of charging from flat, which leads me to believe that it is still OK.

Aaand back to the problem - the charger is this one https://www.partshopdirect.co.uk/bla...lights-t11441/
5100235-04
input 230V/240V~ 50hz 30W
output 15V = 1A, 15VA
red and black are the power leads for the battery
white and blue are for the thermo sensor

When powered without the battery it reads around 20V.
After about 20 minutes of charging and then kept at -10C for another 10 minutes, the transformer is so hot that it cannot be touched. The radiator with the A1757 was significantly cooler.
The D1-D5 big diodes are not shorted.
The transistors also do not "beep" on the multimeter.
The resistance on the high-voltage side of the transformer is 60Ohm ( measured on the end of the power cable ). On the low-voltage side - less than 1ohm ( measured on the pcb ).
No visual damage to the pcb or the components except for the heat discoloration on the bottom of the board.

So any ideas what could be causing the overheating?

[cpt.charlie]

Re: Overheating battery charger

Does the battery actually takes any charge? what is the voltage of the transformer while charging?

[petehall347]

Re: Overheating battery charger

does it go into thermal shutdown ? if not thermal sensor circuit has a fault .
you might want to monitor current draw of the battery when charging unless transformer gets hot when not charging .if it does transformer might be bad .to check that disconnect secondary of transformer and test again to see if it heats up .

[ReeceyBurger123]

Re: Overheating battery charger

Same with my drill charger and brand new batteries Bosch Pr something I think tranny gets burning hot. Doubt its anything to worry about.

[stj]

Re: Overheating battery charger

how hot does it get if there is no battery?

[petehall347]

Re: Overheating battery charger

if it got way too hot it should burn out an internal fuse in the transformer .that is if of course it has one fitted . most have one .
i find these things do run hot .they are crammed into a small box without much thought of design .
the battery might be taking too much charge because of internal resistance . it may well perform ok when just charged . but likely wont hold a charge as long as it should do .. does the battery get very warm /hot when charging ?

[madan1]

Re: Overheating battery charger

Does the battery actually takes any charge? what is the voltage of the transformer while charging?

Yes, the battery charges.
I will measure the voltages while charging and report.

does it go into thermal shutdown ? if not thermal sensor circuit has a fault .
you might want to monitor current draw of the battery when charging unless transformer gets hot when not charging .if it does transformer might be bad .to check that disconnect secondary of transformer and test again to see if it heats up .

Yes, exactly - it gets too hot and stops charging ( green LED lights up ) due to excess heat measured on the thermal sensor. It "thinks" that the battery is too hot, while the heat is being generated by the PSU itself. If I'm not able to find a faulty component on the PCB, I will disconnect the transformer and will measure it.

Same with my drill charger and brand new batteries Bosch Pr something I think tranny gets burning hot. Doubt its anything to worry about.

Usually I do not worry about hot transformers ( they have thermal fuses and so on ), but this time the heat causes the charging to stop, which makes the usage of the drill kind of inconvenient .

how hot does it get if there is no battery?

Good question. I will test it and report.

if it got way too hot it should burn out an internal fuse in the transformer .that is if of course it has one fitted . most have one .
i find these things do run hot .they are crammed into a small box without much thought of design .
the battery might be taking too much charge because of internal resistance . it may well perform ok when just charged . but likely wont hold a charge as long as it should do .. does the battery get very warm /hot when charging ?

I'm not sure if the battery gets "too" warm. By the time the charger turns off, the battery is still not that hot as the charger itself. Most likely this is just heat transfer from the charger, because only the closest to the charger part gets warm. By the way, there is no charging control circuit in the battery.
Most likely the transformer fuse does not burn, because the thermal sensor switches it off.

[petehall347]

Re: Overheating battery charger

well it seems like the thermal protection is working as intended . just a matter of finding why its getting hot .. i suspect the battery at fault .. only a new battery will confirm this quickly . or it means putting an ammeter in series with the battery when on charge .
possibly a diode is flaky passing ac so look at that too .

[madan1]

Re: Overheating battery charger

15 minutes on the mains without battery - no heat generated at all.
I will have test it with the battery on.

Hmm you said bad diode and AC.. will I be able to see this on an oscilloscope without the battery or the battery has to be hooked up?

[budm]

Re: Overheating battery charger

well it seems like the thermal protection is working as intended . just a matter of finding why its getting hot .. i suspect the battery at fault .. only a new battery will confirm this quickly . or it means putting an ammeter in series with the battery when on charge .
possibly a diode is flaky passing ac so look at that too .

Yep, check the charging current.

[madan1]

Re: Overheating battery charger

Just tested the charger with the battery on.
I'm not familiar with this kind of equipment so I do not know if this ripple is within the normal operational limits.
Also I noticed that it is not the transformer which is getting hot, but the A1757. After several minutes the transformer did not even get warm, while the radiator of the A1757 was very hot.

[stj]

Re: Overheating battery charger

that waveform, and frequency makes me think the output from the transformer is not being smoothed after the diodes.
and not all the diodes may be good either.

[madan1]

Re: Overheating battery charger

Do not believe what this shows as freq. value. I'm not sure this cheap DIY oscilloscope calculates it properly. With different timeframe the frequency was ~50hz so I guess it lacks cpu power.
Anyway, the freq is N-times-power_network_freq ( 50hz ) and about 1V peak-to-peak. Is this something I should take care of or is normal for this kind of hardware?
Now when I'm reviewing the pcb, there only 10 elements on the power side ( the rest are for the thermal/control circuit I guess ) - 5 diodes, 2 transistors, 1 cap, 2 resistors .. so I just might replace them all .

[stj]

Re: Overheating battery charger

rectified mains would be 100Hz
*but* it should look like a series of humps without spaces between them.
which is why it may have 1 or 2 open-circuit diodes.
i would desolder & meter them all.
then do a recap - all 2 of them!

[budm]

Re: Overheating battery charger

I would check the charging current to the batteries, as you can see that pass power Transisor provides the charging current to the batteries, more current more heat, when over temp is detected, the bias Base drive circuit for the transistor should turn off the power transistor, or when the batteries is fully charged.
What did you use for the scope ground?
The scope does show 100Hz.
https://www.badcaps.net/forum/attach...7&d=1484274177

[madan1]

Re: Overheating battery charger

So we have a verdict
I will replace the diodes and the caps. Most likely I will also replace and the A1757. Should I go all the way and replace the transistors?


edit:

I would check the charging current to the batteries, as you can see that pass power Transisor provides the charging current to the batteries, more current more heat, when over temp is detected, the bias Base drive circuit for the transistor should turn off the power transistor, or when the batteries is fully charged.
What did you use for the scope ground?

I connected the oscilloscope to the battery terminals ( red and black wires )
I do not think the charging is interrupted due to high current. When the charger is cold it works for like 20 minutes, but after that it almost immediately goes to "green LED" mode when the battery is connected... so I guess it is related to the temperature.

by the way what determines/controls the output current?
ps. ok, got it - that transistor just switches on and off the current to the battery. I thought the circuit was smarter and constantly controlls ( increases/decreases ) the current according to the battery state, temp., etc.

[madan1]

Re: Overheating battery charger

I just measured the current - ~3A. The output specs say 15V 1A... what does that mean?

[petehall347]

Re: Overheating battery charger

means the battery is pulling too much current .

[madan1]

Re: Overheating battery charger

but why?

[petehall347]

Re: Overheating battery charger

its probably got an internal problem like the internal resistance because of old age .
i was thinking you could add a 1 amp regulator chip but not too sure what then happens to the other 2 amps . i guess it would still cause an overload . i have never tried it so if anyone else knows better please advise ..
i was looking at something like an lm317 .