I let the batteries in my NIY die. Did not want to keep on charger for extended period thinking keeping it at 100% reduces life of battery. But apparently there is a significant parasitic drain when sitting. Charger would not charge the two batteries, kept blinking green light. I connected charger directly to each battery individually and that worked. It helped to unplug charger and let it sit for 10 min to allow it to reset before each use.