I have a mid -2012 MacBook Pro that was experiencing the same problem I.e. not starting even though it had been charged.
I pressed the charge indicator and all lights flashed 5 times.
It WOULD start with the charger attached.
I reset SMC and PRAM with the charger attached and still would not start on the battery alone.
Having read the info above I decided to disconnect the battery completely.
With the charger plugged back in, the laptop started straight away. I shut it down and reset the SMC with the battery still disconnected.
I reconnected the battery and plugged the charger in and after showing green for a second the charger light displayed orange, with the battery lights all on bar the last one flashing.
This told me that the battery was working again and i was able to start it up without the charger plugged in.
I checked the battery status in the system information and it tells me the cycle count is on 701. With Apple stating that this models cycle count is around 1000 I figured this is a good way of telling me I’m due a replacement battery.
Thanks for the help up top even though it’s old. I thought I would put this here in case people people are still having issues.
Obviously it may not work for everyone but it’s another way to check.