HTC MEGA?!?! that is a thin phone... no point making it huge like the HTC BA also it already has good battery life :P

I have tried something like this with my HTC beetles(ipaq hw6515) but only with 2x2200mah cells, it worked about 10 days without charge but after 2-3 cycles I've encountered problems - auto turn-off when there's 40% left. thanks for the idea, I was thinking where to use 6x2200mah cells
P.S. checked in the airport - no one thought that it was a bomb...

I have replaced original BA battery with 5 lithium batteries (3.7V 2400mAh each) according to this thread.
I have connected them in parallel and joined with corresponding wires in battery cover. When I charge them with original BA charger they get to almost 4.2 volts. When BA turns off because of low battery they have 4.1 volts. So they last for 2-4 days of common use. But people in here are talking about two weeks or more.
I don't understand why BA uses only 0.1V if it has another 0.4V available to completely discharge battery.
What can be wrong? Where is the difference between my batteries and yours?

It's easy Your batteries are dead got them out of an old notebook? They got voltage when there is no load on them. But as soon as the phone draws some power - the voltage drops for a split second to below 4V or less.

Give us some more info, I'm probably wrong