The specs on the N97 says it takes a
BL-4L 1500 mAh battery.
Just searched
http://www.mugenpowerbatteries.com/ web site and they have a BL-4L replacement that is 3600 mAh! It requires Mugen to make a separate replacement back to the phone and increases its thickness.
I have one of their extra power/extra thick batteries on my HP iPAQ 210 and it is great for movies on long trips.
Of course you wouldn't want to use it for everyday, just too thick, but for special occasions where really long battery life is required I sure hope that Mugen will make a replacement back for the N97 that will accept their BL-4L 3600 mAh battery!
Mugen's specs:
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HLI-E61iXL
Extended-size Hi-Capacity 3600mAh
Battery for Nokia E61i Smartphone.
Li-ion 3.7V, 3600mAh
Colour: Silver-Grey with extra battery cover in Silver-colour
Battery Type : External Battery
Replacement for: BL-4L
(Made by Mugen Power)
PLEASE NOTE THIS BATTERY INCREASES THE THICKNESS OF THE DEVICE.
REPLACEMENT BATTERY COVER IS INCLUDED.
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