Had this mower for a while, so could just be old battery(2014 date), was working fine last week, used it today, seemed like life was a bit short, put it on charger and only shows single flashing red light in 25% section.
Single flashing red on charger indicates defective battery pack. However, the cause could be the battery or charger. You have a 3 year warranty on both battery and charger. Even if you are past the warranty period give EGO customer service a call. They can be reached at 1-855-EGO-5656 (1-855-346-5656) from 8:30AM - 8:00PM EST (Monday - Friday) & 10:00AM - 6:00PM EST (Saturday & Sunday). Call early in the day to avoid hold times.