We have just released a 3600mAh extended battery for Nokia Lumia 820 with backdoor that comes in 4 colors (Yellow, White, Black and Red).
Cell Type: Li-ION
Capacity (mAh): 3600
Battery Door: Included
Original Battery Model: BP-5T (1650 mAh)
The first stock is coming on 29th of March. The product is available for pre-order here: Product Link
The 1800mAh extended battery for Lumia 820 (fits in the original OEM socket) is here: Product link