3G uses more battery than GSM usually and using WLAN could also add to the drain.
I have a battery pack that I use to re-charge my phone at times, it works for the N95 -- I got it from Aldi, in fact I picked up a bunch of them so that I can help others with the same situation, but I am in Australia. The E90 isn't listed, but it probably uses the same standard new style Nokia charger. Perhaps you can see if someone in your area has the same type of battery pack.
You can also look at adjusting wireless power potentially and if you don't require bluetooth, then turn it off as it is also a battery drain.
See here for the portable battery:
New Standard battery chargers: Portable battery charger has slim lightweight design