Bought 2 sets of these batteries for 2 separate robots on August 15th. One set works perfectly, no signs of malfunction. The other set had a noticeably shorter run time (60+ minutes vs. 45), but it was enough to finish that part at home, so I didn't care. Today, that second set lasted about 5 minutes, even though this robot had been charged for several days since it was last started. I should have returned them sooner as the return period has expired and there doesn't seem to be a way to contact the seller or Amazon about this.