if you are going to extend it through portable charger, why not thinking about solar? Either you attach it to n900 or modifying the portable charger.

Also, check this out: http://www.instructables.com/id/DIY-...adio-for-%245/
If someone up to try this, try to link AAA batteries together (I am not sure about 5800/N900 battery size/space so probably this might or work. Anyway linking 4 of AAA's batteries with each 800mAh (you can find higher) this means 3200mAh.

Now for people who are interested to know the difference this will gave them , simply use the ratio to determine your extra running time.

Now if I remember n900 is having 9 hours as a call time (Forget this , unless you want to compare your gain to it) I will consider 6 hours in extreme using

(3200mAh/1350mAh)* 6 hours = 14 hours and 22 minutes in extreme using conditions
now for the talk time:
(3200mAh/1350mAh)* 9 hours = 21 hours and 20 minutes of continues talking