Unbranded HK Batteries tend to be overrated, missold and regularly underperform in my experience. Unless you happen to know which OEM sells to Nokia so you can purchase them directly at cost - I wouldn't bother.

One method they used to advertise over-inflated performance ratings is to specify mAh at a lower voltage and one method of misselling is to use the key words "up to". So for example - they'd sell a 3.7V battery and tell you it has a capacity up to 8000mAh [reading the small print, usually not on the website selling the batteries - the rating of 8000mAh is taken at 1.2V under critical temperatures and the highest density sample - the actual mAh is way lower at 3.7V say for example 400mAh! I'm making up the figures obviously to illustrate.]

At the end of the day I'd stick to official batteries if I needed a critical second battery. Personally I carry around a tiny retractable USB charging cable and keep the IT topped up wherever I can [I even have a small Car Adapter -> USB charging socket.] If I _had_ to be out and about for several hours (and I don't have my car) it's easy enough to get hold of a small "USB" personal charger.