Great, I think I'm almost there. Thanks for your patience. More questions:
If the POWER_LATCH level is low, UPS3 5Volt output is disabled.
1) In this case, shouldn't setting POWER_LATCH=0 actually have interrupted the board power?
If the period of power failure is too short, you have to turn the ODROID on manually.
2) I understand the "short power failure issue", because the system will enter in halt mode, then, as a result of race condition, the AC will start charging the battery and the board will never get the red led off. But.... isn't the latch pin responsible for solving that? It's being hard to understand it's purpose, it seems simple, but still I don't get it!
3) When the system is halted with LATCH=0, the board keeps rebooting. Won't it damage the SD card?
4) After all, what I need to understand is how to setup the system (afaik, the latch pin) to achieve the following:
Premise: The board won't be powered off unless battery is low.
- The board must shutdown gracefully when AC is gone but only if the battery is low (red led must be off).
- The board must boot up automatically when AC comes again (hence the need of red led off).