I have a G4 Play which I used the @#(* out of for nearly a year, until I dropped it into the toilet for 2 or 3 seconds before getting it out. The battery and SIM fell out and I put it in rice for the night before turning it on, as my whole life was and still is on there.
When I started using it again, it would die as soon as it reach around 45% battery, which was really frustrating. I don't remember the other details like if it was very slow or difficult to charge, or if it would ever reach 100%, but it would also drain much faster than it would usually take to go through that span of percentages.
I ordered a replacement battery from ReplaceBASE which they claimed was OEM Motorola, but it was even worse than the apparently water damaged one. They gladly sent me another replacement which appeared to work okay along with the original one, but it would be excruciatingly long to charge any of the batteries and extremely quick to plow through the charge as soon as I turned the phone on. That is like an entire weekend of 60-72 hours to charge the battery up, and maybe 2 hours to drain, and we're talking about a 2800 mAH battery. And now it varies, from dying when it reaches 25% to actually going down to 0%. Sometimes I can turn it back on and it will keep going to 0% if it dies at a higher percentage.
So my question now would be which of the parts are tainted - the phone, the removable battery, the charger, or any combination of the above? What's going on, and is there any risk in testing the parts with others? If I have a Moto E4 to test the batteries with, would I risk damaging the E4, or if I used the E4's factory supplied battery with the G4 Play, would that risk being messed up totally if it's the G4 Play itself's circuitry that is screwed up by the water damage?
What other suggestions can you think of in this situation?
There are very good chances that the internals of the phone are damaged as i have seen in some cases and you cant know the extent of damage without showing it to a repair expert
Coming to DIY If batteries of both device are physically same than you might try replacing with each other
Tempting with a working E4 would mean chances of damaging a properly working phone which should overall cost you more compared to getting your g4 properly repaired at a repair shop
But, if you are experienced in such tasks you might go ahead and try it yourself but still consider you may end up having 2 damaged phones
And needless to say have backups of both just in case of damage