I bought this a month ago thinking I would need 4 batteies but the most I have ever needed has been 2. It has been charged ONCE and used once. Its on the charger now ready for a new owner.
It is the "pearstone" brand they sell at B&H, 100% compatible with the M8 and M8.2. Sells new for $70, buy this one for $40 plus $4 shipping. No paypal fees. BTW, I have another one of these that I use all of the time with no issues.