Ryobi 14.4-18V ONE+ DUAL CHEMISTRY FAST CHARGER - BCL14181H

[TheJay]

My charger didn't appear to be working. The LED indicator lights were both flashing really dim when I attached a battery and even if left on for a long time, the battery would still be flat.

I decided to take apart the charger today and got a bit of a surprise. It looks like it overheated at some point, for a reason I don't know. There are black areas on the PCB and some of the copper tracing has split/come away from the PCB.

My only thought is that some moisture may have got inside and shorted between the two traces?

Any ideas on the cause and whether this can be repaired?

The components on the other side of the PCB visually look fine to me.

 

[TheJay]

What are your thoughts on this everyone else? I don't know enough to comment.

 

[Tony Stewart]

I already commented on the thermal problem and how to fix it. You can buy a bunch , test them in series at max current and measure the voltage drop for each and choose two that are identical within +/2 mV as Nigel hinted at (without specs). That might be equivalent to a temperature difference of 1 'C OR as NTC= -4'mV/'C you can use my method to strap 3 diodes together with tie-wrap as I suggested so they may not be perfectly ESR matched but may be thermally matched at the case temperatures which spreads the heat and also reduces the risk of thermal runaway which increases with temperature.

https://www.electro-tech-online.com/threads/ryobi-14-4-18v-one-dual-chemistry-fast-charger-bcl14181h.164870/post-1437860

 

[TheJay]

Does anyone disagree or agree with my thermal solution?

I was asking everyone else to acknowledge and respond to your post.

 

[TheJay]

Also, after asking for schematics, has anyone looked at them to determine the problem? Whilst I'm confident enough with a soldering iron, I don't understand anything technical and it needs to be explained in basic terms/with diagrams.

 

[Tony Stewart]

Also, after asking for schematics, has anyone looked at them to determine the problem? Whilst I'm confident enough with a soldering iron, I don't understand anything technical and it needs to be explained in basic terms/with diagrams.

I guess you need to understand the thermal runaway problem with parallel semiconductors. Since they have a negative tempco, -4mV/'C when one gets hotter than the other, the voltage drops which causes it to hog more current, then lower its voltage and repeat until it takes almost all of the shared current. This is why Nigel Goodwin said in https://www.electro-tech-online.com...ry-fast-charger-bcl14181h.164870/post-1437319 #2 it was a bad design (within an explanation) and why I explained why they had to be matched and thermally bonded together.

So

root cause: mismatched diodes with no thermal coupling

got it?

 

[TheJay]

Thanks. The diodes have been replaced now with a matching pair, so what's the problem now?

 

[Tony Stewart]

If cool, then no problem.

 

[TheJay]

I don't know whether it's cool, it's the fact the lights flash on it when they shouldn't. It should only flash the way it does when the battery is beyond use. However, it's happening with them all (they work fine in the other charger).

Initially I thought that the charger wasn't charging, now I am not sure. It's possible it is and it's flashing when charging is complete, when it should be solid green.

Why were schematics requested if they haven't been used in response to my request for help?

 

[Nigel Goodwin]

I don't know whether it's cool, it's the fact the lights flash on it when they shouldn't. It should only flash the way it does when the battery is beyond use. However, it's happening with them all (they work fine in the other charger).

Initially I thought that the charger wasn't charging, now I am not sure. It's possible it is and it's flashing when charging is complete, when it should be solid green.

Why were schematics requested if they haven't been used in response to my request for help?

To be honest, I'd struggled spotting any schematics on those links

However, after a lot of searching I managed to find it.

For a start, what voltage is on point '+U' (the positive ends of the diodes you've replaced) - I'm presuming it should be about 32V or so.