Battery Recognised & Status Good, but won't charge

Have a nice MacbookPro9,2 i7 that I bought new - never been dropped, damaged, exposed to liquid, etc.

Recently changed out the existing internal HD for an SSD. I’m very careful in disassembly/assembly - use an ESD mat and strap, etc.

Upon reassembly the machine will only run when connected to a power supply- remove the supply and it shuts off instantly.

Passes all AHT diagnostics.

CoconutBattery recognises the battery, lists the status as Good, failure as None, Cycle count as 921 (close to EOL but was charging fine before). Current charge and capacity are all 0. Charging wattage lists as 0 W, although every now and then it will jump up to a very low value like 0.08 W.

Magsafe light is always on green (never orange). When computer is off but connected to power the Magsafe light is green but flickers ever so slightly. Battery charge indicator lights always show full power (even when Magsafe disconnected - indicating either battery is actually charged or perhaps there’s a capacitor in there).

Tried resetting SMC and PRAM, along with every permutation of disconnecting battery, disconnecting charge indicator lights and resetting. Cleaned Magsafe connector, tried several different power supplies.

Inspection of battery and exposed parts of logic board appear fine - have not pulled it down further.