MacBook Pro not detecting battery (even after logic board replacement)

Recently, I cleaned my MacBook Pro, taking of the base off and using compressed air to clean out the dust - taking reasonable precautions and being careful (Disconnecting battery beforehand and reconnecting afterwards). When going to turn the MB Pro back on...nothing! No response - only a flashing orange light on the MagSafe, which sometimes didn't even come on at all. Battery LED indicator lights were also not working - no lights what so ever.

Following some rather short trouble shooting - and speaking/visiting with Apple, they said there was nothing they could do as I had the 2011 model and that was that.

I tried running the MB directly from AC power by unplugging the battery and plugging in the charger - essentially ignoring the battery altogether. However, as this didn't work I ruled out that it was unlikely to be an isolated battery issue. So, the left me with the logic board OR DC-in MagSafe board - rather odd as everything worked fine before cleaning it.

Following this I replaced my current logic board with one taken out of another MP Pro 2011 (sold as fully working), and I also purchased a new DC-in board, as I figured I would just replace both at the same time. Replacing both boards was rather straight forward - no major issues, except some stubborn stripped screws.

Putting it all back together and...it works! But only on AC power, it wasn't recognising the battery. The 'X' appeared in the top task bar battery icon, and CoconutBattery wasn't recognising anything. Tried the usual SMC reset - unsuccessful.

I tried my battery is my partner’s MP Pro (same year - 2011) and it worked - battery is in great health, with good Coconut Battery readings. I tried her battery (which is fully working) in mine and nothing - it wasn't being recognised - so this confirmed it wasn't down to 'my battery'.

Reading around I found that this lack of battery acknowledgement/SMC issues can be rectified by disconnecting the battery indicator light cable - and that worked! With the indicator cable disconnected, the MB runs from the battery and the battery charges - orange light on the MagSafe - albeit the light sometimes drops out, but taking it out and reinserted normally brings back the orange light. Another 'side-effect' is that the MB Pro doesn't sleep when closing the lid.

My question is what is going on here? Since I've replaced the logic board (may have not have even been the issue in the first place) and the DC-in board, and as disconnecting the BLI cable seems to resolve the battery issue - I wondered where in the loop is damaged? As ideally, I would like to reconnect the BLI cable and not 'make do' - I'd rather get to the cause of the issue.

Thank you in advance for any input :D