Since there are now many flavors of NANO's out there but the ones that I have with the 4 red LED's on the side indicate the state of the battery charge. While charging all 4 will be on with one of them flashing and once fully charged it stays on. While running on battery the 1st one goes out after a few minutes than after some time 2nd goes off than a bit later the 3rd will go out. the last led will stay on a few hours and once the battery voltage reaches its full discharge it turns off and so does the NANO. The 2 little blue leds and just my guess, 1 might be something like watchdog or every time it starts a sweep, have no clue about the 2nd one.
