I might be naïve or uninformed, but I don’t know of any other smartphone vendor to sell a dock charger for its batteries, so that when you own a second battery, you can charge it by using the micro-USB cable that came with your phone, thus being able to swap batteries at need without any fuss!
I just ran over the set “spare BM40 battery + dock charger” for Xiaomi Mi-2A aka M2A at iBuygou ($29 with battery). On Xiaomi’s site, the battery is here (99 RMB, about $16), and the dock charger is here (49 RMB, about $8). Brilliant idea.
I always wondered why are people buying several batteries if the only place they can charge them is inside their smartphone! How do they keep both batteries charged without such a device?!
LATE EDIT: Oops, there is also a dock charger for UMI’s X1S, and a dock & wall charger for UMI X2′s spare battery, which can also charge the phone at the same time!