I have 20 volt PowerSmart string trimmer, hedge trimmer and blower. I killed several batteries until I learned to NEVER let it keep charging after the RED charging light turns off and the Green DONE light.
This takes 1 hour 45 minutes on a completely dead 2.0 Ah battery that I have charging where I can see it and even touch it. I even set an Alarm on my iPhone with a Snooze, and lay it on my leg as I watch TV for 1:45 from when the charging was started.
In the beginning, I thought it was just like charging an iPhone, so I would start one of my 3 dead PowerSmart batteries charging when I went to bed & change to another out in the garage the next morning. I killed lots of 1.0 Ah & then 2.0 Ah batteries.
I now look forward to getting a 12" PowerSmart chainsaw. I'm sure that the 4.0 Ah battery will take twice as long to recharge, but I'll also use it every week in my string trimmer & then on my blower on my little southern California lawn.
BTW, I always carry a spare, fully charged 2.0 Ah battery in my pocket as I string trim & then blow off the sidewalk & driveway.
I keep a chart on an email that I update at the beginning & end of every recharging of each of my 3 2.0 Ah batteries to record how long it took to recharge them from dead. Now that I'm being so careful with recharging them, none have lost any performance time.