Phone turns on, won't charge, random battery %, reboots after 2 mins.

Alisha, Thank you so much for your advice. I didn't realise the distinction between Tigris and Tristar. I noticed on some video repair manuals that technicians were able to check fault locations using a voltmeter, but I presume that you can only narrow the fault location down to the board itself rather than to individual components? Is there anyway to isolate the fault to either being in the Tigris or the Tristar chip, or do you just have to go ahead and replace them one by one testing after each replacement?

@El Grande Just as a note, both of these chips are underneath a soldered on shield on the back side of the logic board. So getting a visual on them can be tricky. Before attempting anything too drastic I would remove the display and have a look at the internals. That alone can give you an idea of what the issue is even if you cannot see the components to blame.

Using a multimeter, which can measure several different sorts of electrical things (not just voltage) is typical for troubleshooting board issues. Often people will measure voltage drop (using Diode mode) or check the electrical resistance of a particular line to find faults. You could use on in the battery connector to try and determine if there is a fault with Tigris. You can also check the connector for the dock assembly (the charge port) to find potential faults with Tristar. Neither of these methods account for all the possible issues with either but it’s a good place to start.

Alisha,

Thank you again for your help. I did open the phone up immediately to dry it out, and I have had it open multiple times since then. I have to say that everything looks perfectly fine, so I'm not sure what I'd be looking for to 'an idea of what the issue is'. There isn't a single visible sign of corrosion. Which is why I thought it would be necessary to get the shield off the logic board to have a proper look and to clean regardless. From the videos I saw, it looked like at least the Tristar chip is sealed under some kind of thin plastic layer; but presumably even if the chip itself didn't get wet, any of its exposed connectors on the board could have done.

I will look up some guides for how to check the individual chips using a multimeter, since I do not know which specific bits of the connectors to touch with the meter, or what values I am looking for.

@El Grande The shop which worked on it may have cleaned it up, so it may be difficult to see any damage. But in most cases that means anything under the shields is likely untouched. A multimeter will help check for faults without needing to do that, but that bottom shield is pretty easy to remove with the correct tools. I use my hot air rework station for that since it gets up to 500 degrees Celsius and has a much more controllable focus point to direct the heat. It could also be done by snipping it off, but I don't recommend thins since it's very easy to inadvertently do a whole lot of damage this way.

It may also be worth checking under the sticker which covers the components on the top half of the logic board's back side. It looks like another metal shield but is actually just a woven sticker.

Alisha,

Good point about the shop having removed signs of water damage. But to be honest, I doubt that they did much more than change the screen and battery, since the level of service in Albania is low. I am actually nervous about whether replacement Tigris or Tristar chips are even available. I will get hold of my own multimeter to perform some checks (I guess guides will be available for where to put the contacts and what values you can expect to read). Thank you for your advice on removing the bottom shield; again I'll have to procure a heat gun.