Didn't run the drone for several months. Worked fine before that. I'm plugging it into the same DJI charger that I used before. Battery lights come on the first two as though it's charging up to 1/2, but even after several hours they don't go above that. Finally this morning, all 4 battery lights were blinking in repeating sets of 3 blinks. Still won't turn on, pair, etc. Nothing.
I've looked at the troubleshooting. Battery contacts are clean and intact. RC charges with the same charger, and is powering on fine.

Any ideas? Thanks in advance!

have you tried a different battery .... even a mavic mini battery?

Hi, fans8e64ddee. Thank you for reaching out. Please try the following steps below:
1. Please check whether the aircraft is connected to a charger. The aircraft cannot be powered on when it is charging.
2. If a new battery or a battery that has not been used for a long time is to be used, please charge the battery first to wake it up.
3. You can insert the battery into the aircraft or a charging hub and connect it to the charger for charging. Observe the battery level indicator display. If the four LEDs blink at the same time, this indicates that the battery has been damaged and cannot be used.
4. If the above three cases do not apply and your aircraft cannot be powered on by pressing and holding the Power button for more than two seconds, there may be an error with the Power button.

Please keep us posted.