How to troubleshoot my battery

I have a water damaged iphone 6 and I'm trying to isolate where the problem is.

I've removed the motherboard, cleaned it and all connectors with denatured alcohol, reassembled and ... nothing.

When I plug the phone into a charger with a USB meter in line, the charger is putting out 5.15V but 0 amps, so I know the phone is not taking a charge.

I've tried checking the battery itself with my multimeter, testing at the red and green locations.

No power.

When I tested the red and blue locations I got a brief .27 V reading and drained to 0 in a second or 2. Not sure what that's about. A capacitor charging and draining?

I also plugged the phone into the charger and read where the red and green leads connect on the motherboard.

No power.

This makes me think that, while the battery appears to be dead, the problem is possibly in the charging circuitry as well. Is this a reasonable guess?

Any ideas as to where to go next?

Thanks in advance,

Tom