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How often do you use whatsapp, by the I mean messaging your contacts on it? As extensive use of any application such as whatsapp will drain battery on most phones. This is due to the device having to constantly push and recieve messages through you network and also to keep the application running in the background or foreground.
If you contact the whatsapp developers, they should give you more information as to the battery usage of Whatsapp.
Let us know how you get on.
I am having trouble with my N8 and Whatsapp. I have been using Whatsapp now for about 2 years without any problems. I have upgraded to Nokia Belle a while ago as well as the latest version of Whatsapp. Still no problems. Suddenly 2 days ago, I noticed that my battery drains from 100% to switched off in about 6 hours. This is in standby and screen locked. I haven't used the phone at all during this time. I installed Battery Monitor 3 on Nokia store. This showed that Whatsapp was to blame for 80% of that battery drain. I uninstalled Whatsapp, fully charged it again, and it was fine. Then read on some forums that Nokia or Whatsapp fixed the issue, so I installed the latest version. Now I am back to square one. Battery draining rapidly. This is not due to data connectivity. The reason I know is, because I switched off WiFi, Mobile Data, bluetooth etc. Whatsapp is still running in the background, although it won't work. Still draining the battery. If I uninstall Whatsapp, I can have the phone set to 3G on, Wifi on, and Bluetooth on, with my 3 mail accounts checking mail on "soonest" setting, and still my battery lasts a whole day including making and receiving phonecalls. Seems the problem is with Whatsapp, and not the phone at all.
Instant Messaging apps (Nokia Chat, Whatsapp, Skype, Slick, Nimbuzz, IMO, IM+, et al - I've used them all!) are all battery killers. The most efficient ones use server-side connection pooling (e.g. IMO) or a single protocol (Whatsapp). Then take into consideration a black background (Whatsapp, IMO, Nimbuzz, Skype) which is far more battery-friendly on AMOLED screens vs the white background apps (Nokia Chat), then you can see that IMO (which I loved but is no longer supported and has glitches with Swype) and Whatsapp are the best IM options.
Tbh though, I've uninstalled Whatsapp due to lack of friends using it and now I use Nimbuzz all the time.
Finally, bearing in mind that your N8 is two years old it probably needs a new battery.
Hope this helps!