Weird: Battery won’t charge

So this story begins with me cracking the screen on my MacBook Pro. Being a handy engineering student on a budget, i sourced a used OEM screen off ebay for the replacement. When i removed the back cover, the first thing i did was remove the little battery ribbon cable, and then the main power connector from the battery.

I proceeded with the screen replacement with no issues. I reconnected the power cables, and put the cover back on.

Immediately i knew something was wrong because the machine would not turn on at all. I connected it to a charger, and it booted up and showed a battery error. It is reading battery percentage, and battery temperature, but refuses to charge the battery or run off the battery. Disconnecting the charger kills it instantly.

I took the cover back off to check the connections and also probed the main power connector and got 0.0 V. Absolutely nothing. Now here’s my question: does this observation pretty much ensure that the issue lies either in the battery or the battery daughterboard? I have heard that there is some internal circuitry in the batteries that prevent them from working under certain circumstances (i.e. if a fault is detected).

I decided to take the machine to a computer shop and get it looked at and just pay the diagnostic fee. The guy there told me he took the motherboard out and found no physical damage, so it must be the Power Management Unit, which requires the board to be sent off to get the chip replaced totaling around $320. This feels like a shot in the dark to me.

Upon sending it in to the shop, i felt positive he was just going to tell me it needs a new battery since it’s not putting out any voltage at all. Any advice for what to do?