These people eBay link sold me a 2300 mAh BP-5L compatible battery (I know it says "1700 mAh" on eBay, but 2300 mAh is what I got) for a very reasonable price. I can confirm that it fits and charges normally in my 770 and that its runtime is certainly longer than the standard 1500 mAh battery. Maybe the specs are indeed optimistic, but for EUR 9.95 incl shipping I'd still call it a good deal. And before anyone asks: No, I don't have a share in that company.