bruh

[BurgrKingFootLettuce]

Any clues to what could be causing this? Safe mode hasn't fixed the problem, relatively new S9 (5 months old), and haven't 

installed any new apps recently. Battery just drops from about 35 to 0 in an instant.

[Bing]

it happened to me few times. today the store ran diagnostic and said it told then I was using unofficial charger, but I'm not...

are you using genuine charger?

[BurgrKingFootLettuce]

Yep, using the one that came in my box. Had it plugged into my computer instead of the wall socket, not sure if thats what the problem is?

[Bing]

could be. but many people use unofficial charger (laptop etc) but this problem isn't wide spread.

Samsung last year replaced my charger and they do it for free for once. so I'm out of luck here. you can try asking them to replace yours.

[Bing]

take it to Samsung store for them to run diagnostic

[Bing]

factory reset nor temperature were the issue in my case.

[Wolf1]

Regardless of hype, I put at least a trusted antivirus protection program on any of my device's that can connect to the net. Have seen & read articles that malware/viruses can chew up a healthy battery. Would add that to the list of diagnostic tests before turning to Service Tech

[CHMultimedia]

Ok so this is definitely a faulty battery indicator.

No, not the fault of the charger. Not the fault of an app. (Apps can't drain 1000mAh in a second. The processor isn't powerful enough). The real causes are faulty battery indicator and/or temperature as FragmentedChicken said. But a factory reset won't fix it.

Don't do those "Calibrate the battery" gibberish you find online. It won't work and may damage more the battery because some of the steps are for Nickel/Zinc-Carbon batteries, which are a whole other thing compared to Lithium batteries

Go to a store or service center. Bring the charger with you so they won't say you used non-official chargers and deny the repair

[Bing]

let me know what Samsung tells you. I've the same issue