Although with some trickery we can get a partially booting system without the BQ24193 chip working there really is not much point. The Switch will not be able to charge its battery and it would limit further accurate checking of the rest of the system voltages if the BQ is not properly working.
Confirming the BQ chip is working is simple as we have the datasheet.
The only pins we care about are the VBUS (power in), VBAT (battery input) and SYS (power out). The SPI interface and other pins all have passives on their lines so it's as simple as checking the resistors and capacitors have the correct values.
Pay attention to the new pins. Our VSYS for testing was the SW pin, not the SYS pin, and this was in order to test at the same time that the inductor was working as all power passes through this inductor. We limited the current to 1A for the tests and the inductor can handle 4A+ so no harm would come when testing for a short, but it would also help prove the inductor is not at fault by performing the previous test.
Feel free to also check the resistor values using a multimeter. Most are just pull up or pull down resistors and are of little consequence so do not be overly concerned if a 10k resistor measures 6k or a 100k measures 150k for example, the circuit will still function.
Capacitors need to be removed to test and I would only do this under rare circumstances. Instead for the capacitors simply do a diode test on them with your multimeter in diode mode, red probe lead touching USB-C ground, and the black probe touch over both sides of each capacitor.
All capacitors around the BQ should short beep both sides or long beep on one side only. If you get a capacitor that continuously beeps on both sides of the capacitor then you have a short. Remove the capacitor first and see if the pads on the PCB where the capacitor was now beep in the same way (continuous both sides). If they do, your BQ is bad, if they do not and they go back to short or no beeps, your capacitor is bad.
If you have the bypass resistor installed over the two test points of the battery area, remove that now.
Leave the bench power turned off or disconnected, then connect up the battery to the Switch.
Measure the battery voltage pins while connected. You should get a voltage of 2.8V to 4.2V. Being totally flat to fully charged. Touch the gold connectors on the top with your multimeter probes. Red lead on red wire and black on black.
If you get 0V then your battery could be in protection mode where it cuts off external power until it is charged. Continue on with the test even if so, and only if we see problems after the test can we suspect a bad battery. Once a 0V cell gets power it re-enables output and starts charging normally.
As you can see here I have a mid-charged battery sitting around 3.77V.
Wait 10 seconds and make sure the voltage does not drop more than 0.02V, if it does it is a sign something is discharging the battery and so disconnect it and re-check your previous dead-short testing we performed.
The first thing you can try is to simply charge the battery from the USB port. This will require that the USB connector, fuse, MT92T36 and all FETs going to the BQ chip are all fully working, there are no shorts on the board, and nothing pulling MT92T36 rails low.
If you have issues with this test, do not presume the BQ chip is bad, instead continue on with the Bench Setup test for absolute confirmation that does not require any components to work on the board at all except itself.
The test is simple, grab yourself a USB-C Tester and connect the official Nintendo USB-C charger (or a known working 5V or 15V negotiated power supply and cable) to the tester.
Connect the battery to the console, then the USB-C charger with the tester on the end into the console.
Your tester should light up right away and display a voltage. If the MT92T36 is fully working and all of the other circuit going to the BQ is fully operational you should see 15V show up and the current draw should be between 0.1A and 0.5A depending on the charge level of the battery.
Even if you see 5V it is ok (it just means the MT92T36 is likely faulty which we can fix later), so long as your current draw is between 0.1A and 0.5A.
To confirm your battery is receiving the power and its not simply current being sunk at another part of the system, put your multimeter on the battery and measure its voltage before you connect the charger.
Then connect the USB charger and you should see the battery voltage jump on by at least 0.1V and slowly increase over time.
If you got the same readings as above, take your multimeter and measure the VSYS pin output (top of inductor). It should say between 3.5V and 4.2V depending on your current battery charge level, but should always be the same or higher than the battery voltage. If it is significantly lower than the actual battery voltage (0.2V or more) it could indicate an issue with the BQ chip or its surrounding components (presuming you have checked there are no shorts on the VSYS rail from the previous tests).
If it does, your BQ chip is fully operational and working and you can move on to checking the next part of the system.
If you do not see this behaviour continue on to the Bech Setup below to confirm if your BQ chip is faulty or whether it is another part of the system.
Again there are ways to use an electronic load to not have to use a battery for this test, but as we have checked for shorts on the system already, and a failed BQ chip is designed to fail in a way that would not harm a battery (excessive voltage), and over-current is handled internally by the battery package itself, we do not need to worry to much at this point of connecting the battery to the system.
Solder your bench power wires to the ground shield of the USB-C connector for ground, and the top pad of the 1uF capacitor on the bottom right connected to the VBUS pin.
Set your bench voltage to 5V. Although the datasheet mentions the VBUS can be as low as 3.9V it does not work in this setup and configuration, it will need to see at least 5V to start charging.
Now we have the battery connected to the console remember the motherboard is live and juiced up, so be careful not to leave tweezers lying around, solder wire or anything else that could short things out or damage the circuit.
Connect your bench power supply to the ground and VBUS wires we attached, making sure your bench is set to 5V. You can set the current limit to whatever you like, either 0.1A to start with, or 1A if you want to just do the full test.
When you connect the bench power to the wires, you should see instantly the current draw on the bench power rise. Depending on the level of your battery voltage the current will vary.
If your battery is less than 3.0V you will likely see a charge rate of around 40 to 100mA (0.1A) (soft start ramp up).
Once above 3.0V the current should jump to around 470mA (0.47A) which is the max current limit set on the configuration. At this stage your battery voltage should measure about 0.1V above the actual battery voltage. So if the battery was 3.77V like mine above, once I connect the power to charge it, and we measure the battery again will connected and being charged you can observe the voltage rise by around 0.1A and slowly rise up more and more as its charging.
The battery will take an insanely long time to charge at 5V (9 hours) so this is only a test.
If you are feeling brave and want to fully test the BQ chip, you can set your bench power supply to 15V instead of 5V. This will give the BQ much more wattage throughput as 15V 0.5A is three times more than 5V 0.5A.
Be sure before you do this that you are definitely wired up correctly as sending 15V to some other pin than the VBUS of the BQ chip can permanently damage your console (if for example it was on the VSYS inductor pin instead).
If you have done all the dead-short tests from previously, then there are only a few things that can be wrong at this point.
If you have 0V on your battery before connecting it to the console, and it stays at 0V when connecting the bench to charge the battery, you have two options:
If you have a battery with voltage (2.8V or higher) then you should see some increase in current when you connect the battery. You can connect the bench power up first without a battery, then while it's powered on, connect the battery. If you see the current draw jump up by at least 100mA on your bench, you have a good indication the battery is charging.
If the current is simply not high enough (your battery is over 3V and current is less than 0.45A) then double check your soldered wires, the bench leads are connected well and to the right place, your current limit on the bench is high enough, and re-check your battery voltage is definitely above 3V. If all that is correct, then check the current limit resistor firstly it should be less than 250R.