Chosen Solution
Got a used 15” Macbook Pro 2015 DG. Plugged it into a third-party charger, fans spin up and screen displays a battery icon with a little bit of red and a lightning symbol below it. No light on the magsafe connector. Shuts off after a few seconds. Same behavior when I power it on with just battery with the addition of a plug icon. I disconnected the battery and tried powering on with just the charger, but no change. Also tried resetting SMC. Eventually I replaced the magsafe board and also moved all components (except battery and touchpad) to a new upper case. Magsafe connector now lights up, initially green and then stays on orange, but still shows battery icon and won’t boot. I have a genuine charger on the way for further testing. I’ve not found any information describing the exact same symptoms I’m seeing.
Genuine charger seems to have solved the issue! No longer getting the battery icon and MBP seems to boot and run normally. However, if I disconnect battery and use the charger, I get the Apple logo and progress bar goes for a few seconds before it restarts. I’ve been reading that this is normal behavior if battery is bad or not detected.
