I must say that I bought a Mugen battery for my E75 and was hugely dissappointed, it had half the battery life of the original and would just go from full charge to empty and shut the phone down. I am not buying another product from them.

I have been using a ZAGGSparq which has charged the phone three times with power to spare. What I do is just leave the battery in my shoulder bag and run usb cable to the N900 to charge the device. Not the pretties thing but it works and the phone is still its usual form factor. The cable has to be hacked (data wires soldered together on phone side, left open on battery side) in order to charge phone. There is a thread about that somewhere on the site.

Fred