Hardware Switch Lite no charging only when console fully booted

[saaif88]

Hey everyone,

I am trying to repair a switch lite here that had other previous damage. It seems as though the backlight was shorted which damaged a few things on the console. I replaced the daughterboard, main flex cable, main flex cable connector, and TPS61163. Now the console fully boots and everything else seems to be working just fine, except it will not charge the battery.

I tested the BQ24193 chip and even though it has 15V in and 4.2V out, I replaced it anyways with a known good working chip. This unsurprisingly did not fix the problem, but I am not sure where else to look. There are no shorts on any of the other components surrounding it.

Strangely enough, if I plug the charger in first and then connect the battery, it draws the typical ~0.5A at 15V and charges the battery. If I first plug the battery in and then plug in the charger, it boots the console but does not charge the battery at all, only drawing the usual power to run the switch. This might be typical behaviour, I am not sure, but it is odd to me.

I am not sure the fuel gauge IC is the problem here because if I power it with a bench supply at 4.2V the “battery” reads 100%, and even partially charging the battery on another console and hooking up this motherboard to it reads an expected value, such as 35% at 3.85V.

I am not really sure what to look at next. Any help on where to look next would be greatly appreciated!

 

[thesjaakspoiler]

The BQ chip works autonomous when the operating system is not running.
When the operating system is running, it can be controlled by the Tegra MPU but I have never seen any documents on how that actually works.

Just a wild guess : When the battery is present and you plug in the charger, the BQ can't talk to your charger and thus decides that it is not a good idea to charge the battery.
Maybe when you plug in the battery afterwards, this check is skipped.

Do you have a USB breakout board to see if the communication line (CC pin) from the BQ back to the USB port is connected?
Retrosix noticed that just 5V from a usb charger doesn't start the charging process :

https://retrosix.wiki/wiki/full-boot-from-bench-power

The BQ charges this battery through the VBUS input pin. This pin can handle inputs from 3.9V to 17V..

This means a dumb 5V USB cable without any data pins, or a dumb charger without any actual USB negotiation will not charge the Switch as it has to be correctly negotiated and a FET turned on by the MT92T36 which negotiates over USB-C protocol for power delivery.

The MT92T36 will talk to the USB power input device and negotiate power into the system. It will accept 5V or 15V only. If it gets successful negotiation then the MT92T36 opens a FET that passes this 5V or 15V USB power into the BQ chip VBUS pin.

 

[saaif88]

Hey @thesjaakspoiler thanks so much for the reply! I don't have a USB breakout board but I do have a USB PD meter that shows it is negotiating 15V correctly as expected. I am pretty confident that the negotiation is working as I do have 15V on VBUS at the BQ chip. This is genuinely quite a strange issue.

Any idea what else could have been damaged by the backlight short failure? These issues seem unrelated but I am honestly not sure if that killed something else in the process...

Post automatically merged: May 28, 2026

Well, I fixed it. It was indeed the MAX17050 Fuel Gauge IC. There were no shorts on any of the surrounding components and all of the values measured within spec. I decided to replace it anyways since it was the only other thing I could think of that was battery related and now it charges just fine.

Not sure what failed or how it failed, but even though it was reading the correct battery voltage, it was not charging the battery when the switch was powered on. Hopefully this helps someone else in the future!