Is my Nokia N8 dead or just the battery?

I have an old Nokia N8 that I’ve been trying to revive. It hasn’t powered on in years. I first tried charging it using the original Nokia AC-3U charger via the barrel port — nothing happened. Then I tried charging it via MicroUSB and left it plugged in overnight (around 8–9 hours), but still there was no response at all — no lights, no vibration, and the screen remained off.

I opened the phone carefully and disconnected the BL-4D battery. I measured its voltage with a multimeter and saw it was extremely low — around 2.7–2.9V after attempting a jumpstart, but within minutes it drops back down to nearly 0V. I’ve attempted jumpstarting it a few times but the battery won’t hold any charge. I also tried checking if the phone would power on when plugged in with no battery, but nothing happens — no indication of power.

I have another battery from a different phone (an LG GS190), but it's wider and the contact pins don’t match, so I can't use it. Given all this, is there anything else I can do to test whether the phone is still functional?