fake imax B6 repair

[Matthew98]

Hello everyone. Let me introduce myself to this forum by asking for help.
I bought a fake imax b6 charger half year ago, or so. Everything was OK, but it recently went horribly wrong in front of my eyes. One day i was measuring Lion cell capacity by discharging it through the unit, as many times before. The cell had big capacity. At least thats what the unit displayed. When i smelt a burn from the unit, i disconnected everything and started diagnostic.
The interface is all right, but some dumb electronic must have failed, because there is stable 12V at output !  When i want to select for example Lion battery with nothing connected, it shows me over voltage error.  Im feeding it 12V and enough amps from a Pc supply, but the output 12V have only around 250 uA, so its not directly connected. I checked only the n mosfet near output joins and as part its ok. I didnt saw any burnt parts, or bulbed electrolytic capacitors. I will check the other parts from under the unit, when the equipment for this will arrive
Any idea what could that be ? I was thing of dickhead move of manufacturer, that it was supose fail after amount of time... it already have time built in
including few photos for identifying the unit version.
Thanks for any idea Matthew

[james_s]

First thing I'd check is the mosfet, measure and see if it's shorted.

I wouldn't spend too much time trying to fix a $20 charger though.

[Armadillo]

The VT13 P-mosfet should be defective. Take it out of the circuit and check it. It is not shorted, but offered high resistance from Vin to the output hence your low current.

[Matthew98]

Thanks for fast replies.
I checked both N-mosfets. One is on photo, another is near output pins and is different brand. It is ok, when i apply VGS around 3.5 or 4 V, but according to datasheet VGS(th) should be MIN 2V MAX 4V. Is that a problem ? At 2V i get resistance 1600 ...
Well, I spent a LOT, like really LOT of money this year, so im trying to save every dollar i can And fixing is fun, right ?
Im sorry. I really couldnt find the VT13 P-mosfet no metter how hard i try looking. I guess i never even saw P-mosfet before 
* With the two N-mosfets desoldered i still get 12V at output.

[sainter]

I would advice against repairing something that you charge a highly flammable lipo batteries with. Fire extinguishers   cost more than a new IMAX B6. Be safe and practice your knowledge safely. 

[Matthew98]

I probably will buy the genuine imax charger, but for now I have no money to spend. So i have to get this charger fixed for half a year, or better full year. Untill i had this charger, i charged the batteries through power supply, so its nice stepup, i guess And as the charger is smart, it shouldnt let me overcharge lipo s and lions so easily...
I was never much about safety, though...
I know, when i get burnt (literally) Its just my fault

[Armadillo]

Im sorry. I really couldnt find the VT13 P-mosfet no metter how hard i try looking. I guess i never even saw P-mosfet before 

Hello, it looks like attached.

[Matthew98]

Thanks a lot ! Im on it
The package really confused me 
Sometimes it scares me how much i will have to know to actually do something... Im turning 19 this month, studying electronic at school for 4 years now and beying interested in electronics from earliest memories I have, I cant still do much...
I hope High school will do little better

[Matthew98]

Armadillo : You were right.
The P-mosfet is busted, but he doesnt have high resictance, instead he is shorted.

* With P-mosfet desoldered and N-mosfets back in place i get 20mV at output with no current when shorted. I just wonder if that could help find the problem.

[james_s]

Thanks a lot ! Im on it
The package really confused me 
Sometimes it scares me how much i will have to know to actually do something... Im turning 19 this month, studying electronic at school for 4 years now and beying interested in electronics from earliest memories I have, I cant still do much...
I hope High school will do little better

There is a learning curve, I was about your age when I finally started to get fairly good at repairing things. To be really good at repair, you have to have a significant amount of engineering knowledge as well as the troubleshooting and repair knowledge. That's one reason so few people repair things anymore, if you can repair things well you can probably make more money designing things. Keep at it though, it's a useful skill and it's enjoyable, fixing something is like solving a puzzle. Once you get good at it you hardly ever have to buy anything, I've never paid a dime for a TV for example and I've had more of them than I can count.

[Armadillo]

Armadillo : You were right.
The P-mosfet is busted, but he doesnt have high resictance, instead he is shorted.

* With P-mosfet desoldered and N-mosfets back in place i get 20mV at output with no current when shorted. I just wonder if that could help find the problem.

There is a internal diode across the P-mosfet. If in fact p-mosfet is shorted then whatever V+ input you connect, you will have V+ - 0.6v strong output. Then it will be not the problem you described e.g. low current output. P-mosfet serves as a buck converter, there is another N-mosfet that serves as a boost converter... Anyway you should measure both direction again. Use ohm range and not diode range to measure both direction. Take it out of the circuit. I suspect the P-mosfet no longer switch. Also check the 2 diodes near the inductor.

Imagine a buck converter, if the switch is shorted what will happen? and when is open [resistance] what will happen? And when the switch is removed out of the circuit, what will happen...

[Matthew98]

There is a internal diode across the P-mosfet. If in fact p-mosfet is shorted then whatever V+ input you connect, you will have V+ - 0.6v strong output. Then it will be not the problem you described e.g. low current output. P-mosfet serves as a buck converter, there is another N-mosfet that serves as a boost converter... Anyway you should measure both direction again. Use ohm range and not diode range to measure both direction. Take it out of the circuit. I suspect the P-mosfet no longer switch. Also check the 2 diodes near the inductor.

Imagine a buck converter, if the switch is shorted what will happen? and when is open [resistance] what will happen? And when the switch is removed out of the circuit, what will happen...
[/quote]

I used Ohm range this time and i got 5ohms at both direcions whatever i do. I had it out of ciruit before as well. It it kinda hard for me to imagine all of that, but i ll try Checked all diodes i could find. the big one, 2 under tempeture (3pin) connector, one hidden under screen and one between add and entor switches. They all seems fine, i guess.
Lets do your little quiz now.
Shorted switch, input is directly on output. As its DC, inductor wont make any changes. Resistance should limit the measured current and with no switch, i will probably get only what inductor will "read" from near fields. Right ? Well, i struggled there a little, which is really shame ... Anyway i dont know what im doing wrong with measuring the P-mosfet If i am now...

[Armadillo]

The P-mosfet is defective, if you have available, replace the P-mosfet. Because without it, its open circuited. There will be no output. Because even in Boost Mode, the P-mosfet needs to be ON. In buck mode, it will be switching. In off mode, it suppose to be High Impedance. The buck mode circuit is before the boost mode circuit, hence no buck no boost. I think the less than $20 fake should be the same as other fakes, I imagine.

The other note you need to be aware is, finger static on the gate can turn a good mosfet on even when the finger is released. For that you can watch the recommended video below;

The P-mosfet is a only a few cents, but I don't know how much it will cost in your country and whether it will be cost effective for you. Judging from your skills at removing the SMD, I think you are already good in electronics and you know where to get it.

In future, if you need to get a fake charger, get the "Extreme balance charger x 605", the quality is way much better than the photos you showed but its not cheap.

[Armadillo]

I was thinking why low amp if P-mosfet is not high resistance, it dawn upon me that there is another N-mosfet that need to switch the black output to ground through the current sense resistors. Can you check as attached.

[Matthew98]

I buy almost everything through ebay, so it takes about month to arrive, but i will try find some in school.

The N-Mosfet does work, but the gate doesnt hold the voltage. When i was testing it (soldered out of curse), it showed short circuit for few moments (maybe half second, or less) and then it raised up quickly to disconnected circuit. Even my ohm meter couldnt measure anything.   
Left Resistor 0.2 and right one 0.7

[Armadillo]

The N-mosfet is suspected. I would suggest that you replace both the P-mosfet and N-mosfet.

Below link, I have not bought from them before, don't know if its a fake. You should search around.

http://www.ebay.com/itm/4pcs-Original-AO4468-4468-N-Channel-Enhancement-Mode-FET-Alpha-Omega-SOP-8-/261012270586?hash=item3cc58b2dfa:m:mI4KQW4YlRZ-vTvypNSWRWA

http://www.ebay.com/itm/10pcs-New-AO4409-AO-4409-MOSFET-SOP-8-NEW-GOOD-QUALITY-/191674564293?hash=item2ca0b1bec5:g:ZnYAAOSwbqpT5gYn

[Matthew98]

Thank you for all your help
I have aredy learned more, than suspcted from this.
Do you thing, that replacing the P and N mosfets will solve my problem, or it will just guide us towards the core of problem ?

[Armadillo]

Replace the defective components as you have discovered.
In repair, as long as the symptoms are answered by the repair logics, then you should develops confident in your repair. The symptoms are what you have described and yes I think the 2 mosfets should cover the repair.

For me, I would just probe the mosfets drivers transistors and op-amps state changes to confirm they are healthy. Too lengthy to describe here though. Just change the 2 mosfets and cross your fingers.

The rule of repair I learnt from this site.... "if it's working, don't touch it".... meaning.... no need to probe any further unless you have symptoms unanswered. 

[Matthew98]

Today the components arrived. It works again and so far i didnt discovered any faults Thanks !

[Armadillo]

  Well done.

[Matthew98]

Well done.

Thats a little self-praise, dont you think ? its all your work

[Armadillo]

thanks but it's all your efforts and electronics knowledge to troubleshoot based on those little pointers we gave.
Well done! You own it.

[hpapagaj]

Hi, I can't find any answer and I don't want to open a new topic for this one question:

Can I charge NiCd/NiMh 14.4V (3A) battery (from cordless drill) pack with imax b6? If yes, the power supply for B6 must be 12V or greater, because of the higher charging at 14.4V?

[Armadillo]

Hi, I can't find any answer and I don't want to open a new topic for this one question:

Can I charge NiCd/NiMh 14.4V (3A) battery (from cordless drill) pack with imax b6? If yes, the power supply for B6 must be 12V or greater, because of the higher charging at 14.4V?

Of course you can. Its a intelligence charger. Inside the voltage is boost up to match your NiCad battery.

[hpapagaj]

Thanks for the fast answer.

BTW: What a great forum.