Help! My cordless mower battery won't charge - flashing red light

Hey folks, having a weird issue with my 40V Greenworks mower. Battery won't charge - charger shows solid green for a sec then flashing red. Tried cleaning the contacts with alcohol, no dice. Battery is only 2 seasons old. Anyone seen this before? Should I just buy a new battery or is this fixable?

Rick - before you spend $150 on a new battery, check the charger output with a multimeter. I've seen bad chargers cause exactly this. Also, those batteries can go into protection mode if they get too hot - leave it unplugged overnight and try again tomorrow morning when it's cool. What's your storage routine? Leaving batteries in a hot garage kills them faster.

Flashing red usually means communication error between battery and charger. Could be: 1) Bad thermistor (temp sensor) in battery pack 2) Corroded balance leads inside 3) One cell group voltage too low. If you're comfortable opening it (wear gloves!), measure each cell group - should be 3.6-4.2V each. If one's below 2.5V, it's probably dead. PS: Don't charge Li-ion batteries on concrete floors - myth but still makes me nervous!

My boss had same issue with his DeWalt stuff last month! We took it to Batteries Plus and they said it was a dead cell. They rebuilt it for like $80 instead of buying new. Maybe check if there's a battery rebuild shop near you? Also dumb question but did you try a different outlet? My garage outlet was wired wrong once and chargers acted weird.

Thanks all! VoltVictor - charger shows 42V output so that's good. SparkySam - not opening a Li-ion pack, nope! Carl - great tip on rebuild shops, found one 20 min away. Gonna try the overnight cool-down first, then maybe the rebuild route. Will update tomorrow. Appreciate the help!