No power when battery connected, but starts when battery disconnected

Hi everyone. First post of my own, but I have used this resource often.

Recently my girlfriend’s MacBook Pro shutdown mid-use and would not start again. The model is the late 2016 15” MacBook Pro Retina, with Touch Bar (A1707).

We tried SMC resets, PRAM and every key combination we could find. Nothing worked. So she took it in to one of the local authorized repair centres. She was told that it was the logic board and it will cost more than the price of a new Mac Mini to repair.

She took the laptop back. I’ve opened it up and disconnected the battery. The machine starts up if the battery is not connected. If I shutdown the machine and reconnect the battery, then it is stone dead, despite the power supply being plugged in.

This is repeatable. I’ve being tinkering disconnecting and reconnecting the battery connectors with the same result.

I’m hoping there are some more tweaks that I can try, otherwise I’m going to try find someone who does board level repairs.

Any ideas?

Update: Dan’s suspicion of the battery being faulty was spot-on. I found a battery from a local supplier. I installed it and the machine is up an running again without any issues.