Bought a 3rd party replacement battery for my Lenovo T420. Powered down, removed AC adapter, installed the new battery, plugged AC adapter back in, powered up. Waited 10 minutes, unplugged AC adapter, PC shut down. Plugged adapter back in, powered PC back up, left up for 10 hours to ensure full charge. Unplugged AC, PC shut down. Old battery still works but only holds 15-20 minute charge. New battery shows 91% charge. Appears as if PC acknowledges but doesn't use replacement battery. Also tried power down, remove AC, remove battery, hold power button down for 30 seconds, put battery and AC adapter back, still no luck with the battery.