BQ acts like its shorted, but its not

Oddly enough I have two boards with this exact condition that i’ve been poking at here and there over the past few weeks. Both boards are fully functional in every aspect BUT charging. When hooked up to a battery and plugged in, the BQ ic sky rockets to ~100C temps. Initially it looks like normally boot sequence as I get the 15v, .48a power draw. However, just moments after plugging in the amp draw slowly drops from .48 and settles around .28 or so. Even though I am showing an amp draw, the battery is not charging (probed with multimeter and battery voltage never moves). I am assuming the drop in amp draw is related to some protection mode due to the temperature of the chip.

There are no shorts of any type on anything surrounding BQ. All resistances and diode readings seem normal when compared to a fully working board. I’ve replaced the BQ chip with new IC’s from china, as well as using a known working chip as well, no changes there. Done the same with the inductor as well. Also replaced the fuel gauge with brand new chips. On the one board that is unpatched I was able to get into hekate and took a look at the battery info, and all of that looked fine minus the very weak charging amps.

I also do notice that this chip and surrounding area does heat up significantly with no battery plugged in, was registering around 38C on just USB-C power. Probing the board with a thermal camera shows no evident hot spots / shorts upstream of the inductor. On one of them PI3 was heating up after a while, removed that with no change.

I also did a battery / no battery test with the inductor removed. As expected the chip did not heat up, so I am assuming the problem is upstream of this. ive spent some hours tracing the rail and probing some passives / chips I thought might have gone bad but have not found anything of significance.

Any ideas? If I had an internal layer issue, would this not show up on diode / resistance readings? I would figure if i had an internal trace short to ground, then the related passives that sit on that line would also show the same low resistance to ground? This is a weird one to me as literally everything else on these boards is 100% functional, its just the insane temp and no charge that’s the only thing non functional.

Thanks for any help, I’m truly lost in the sauce at this point, tracing things on what I assume is the sys rail is quite difficult lol

I’d say first thing is ensure this is board only with nothing else connected and see if the symptoms change.if no then> Checking within the joycon connector, BL CN, LCD CN etc ect and ensuring no bent pins, liquid damage/corrosion.

If no change still then I’d double check the resistors surrounding the BQ IC (and M92 while your at it) are all the correct values, countless times I’ve had cases where people have swapped in random values and I’ve observed similar behaviour, so it’s worth checking.

If still no change then I’m going to make the assumption here that this is an active short case (though this is a guess and it could be well something else)

So in this case you’d first need to determine which line is the [primary] active shorted line, best guess you’ve got at the minute is your SYS rail, in which case you should measure resistance to ground (which I understand you’ve done / hopefully in both polarities) and make sure it compares with a known good, if yes, and if this is indeed an active short, then you’d want to monitor the voltage on said rail to see if the voltage is being pulled low. In the case of some active shorts this could be getting pulled low all, some, intermittently (pulsing) over time. in the latter case you’d want to log that - idealy with a scope but you can do it with your meter if it’s fast enough and has a log mode and sometimes with the min/max mode (depending)

If you identify that the voltage is indeed being pulled low then you’ve successfully identified the SYS rail as being the most likely candidate for an active short case (or another BQ line is and SYS is shutting down as a result of short circuit protection… though I haven’t checked the datasheet on this) and can further narrow down… If on the otherhand you don’t see any devation in SYS rail voltage over time, then likely not the line/rail your interested in and need to look elsewhere.

thank you so much for the advice. Next chance i get to look at these boards I will test what you suggested and report back.

I’ll double check all the FPC connectors to just remove the variable, but both boards did boot, display images, and react to touch with zero issues. I am 99% sure the joycons worked as well, but its been a few weeks since ive had them in a case (i noodle on them here and there when i have some time, so I keep them out of their cases) that I am struggling to remember if i tested this. I do vaguely remember checking the resistors around BQ on at least one of the boards and comparing it to known good values, but I will also double back on this and verify.

If you wouldnt mind though, i do have a few questions. What is an “active short”? This is the first time I’ve come across this term, does this simply mean the fault does not display until the line / circuit is active (EG powered on) via battery / USB? I also assume I should be measuring the voltages on USB power only? I should be expecting to see just north of 4 volts on the sys rail, correct? So I am assuming anything under 4v should be considered being pulled low?

Appreciate your advice and all ya do, ive learned a lot from your posts and find myself not needing to post my own help me threads because of them. Cheers :slight_smile:

Correct. I forget if the SYS rail is active without the battery connected so I’d double check this yourself by measuring the voltage at the 2R2 coil ona known good to verify.

Active Shorts are usually as a result of a component/IC failing when they actually get power. For example a hairline crack in a capacitor, partially working at room temp but when power is applied, crack expands as a result, non active short changes into an active short, if the rail/line is protected it will go into short circuit protect, cool down, active short clears and clean rinse and repeat etc etc. (this is just one example mind you)

No, you should be measuring the voltage on whatever line/rail you suspect is the culprit. So in this case you should be measuring the voltage on your SYS rail with the easiest place of picking this up being at the 2R2 coil :slight_smile:

Depends on the normal operating load of the console :thinking: If I remember right I’ve seen SYS drop as low as 3.6V normally. Anyway in order to generate the heat your talking about (and assuming this line is actually the culprit) the voltage will drop significantly lower than even this, Anything under 3V would be red flags (though of course this all depends on how your measuring it, for example the update rate of your meter etc which could potentially miss it)

Haha, good to hear :+1:

apologies, what i meant by this question was should i have the battery connected to the board when measuring the voltages on the sys rail or should I use power from the USB cable only? I know that leaving the battery connected can either throw off readings, cause damage to the board, or both. Just wanted to be sure I was executing a proper test to avoid any false positives or unnecessary damage.

Thanks for helping, I will report back with what I find as soon as time allows me to do so.

Whatever method simulates the original fault, if it does it with the USB on it’s own, then troubleshoot with that and vice versa with the battery. Of course, you’ll probably want to do this intemittently if things are getting toasty and unplug everything and let things cool down a bit before carrying on.

Only with your meter in resistance/diode mode etc, you can safely measure voltage on live circuits whatever the weather :slight_smile: