UPDATED 2/28/12 - I just wanted to add that soon after I emailed the supplier (BTO Inc) I received a response from Cindi, one of their Customer Service representatives. Cindi offered to replace both batteries or issue a complete refund. Long story short I am very pleased with BTO's handling of this and I would not hesitate for a second to buy from them in the future. Considering this I have changed the rating from 2 to 4 stars.I am not sure how the exact error message is worded but if I turn my phone off and then back on again and it's not plugged into the charger I get a "Incompatible Battery" error message and the phone just shuts off. If the phone is plugged into the car or wall x-former I still get the message but the phone boots up and stays on. Also battery life is about half of what I would expect that of a new one. I am going to cut and paste this review and forward it to the supplier but to be honest over the little more than three bucks I spent for TWO of these I'm not going to lose any sleep. Should also add that I ordered a new phone so this one is being demoted to the role of spare anyway.