2012 MacBook air, Service battery alert
Some things to watch out for. When removing the original battery the power cable may not flex enough to clear the connection housing on the logic board. No worries, just release it with a slight pull then remove the 5 screws holding the battery assembly and gently pull out while guiding the cable from the housing in the logic board.
When installing , guide the connector into the housing and press gently to engage the pins. You don’t have the fully seat the power connector yet. There is enough flex in the cable to align and seat the battery and insert the securing screws. As detailed before, don’t overtighten theses screws or you’ll break the ears on the battery (like was done at the factory on my MBA!)
once the battery was secure I used a plastic spudger to fully seat the power cable connector.. be patient and keep pushing gently until FULLY seated this way you won’t have to reopen the Mac to seat the connector!
I pressed the power button and , booom! The Air booted right up!