I tried to replace old 500mAh NiMH batteries into my old siemens C35 by cutting off the 3 original batteries inside their plastic box (there were 3 cells in series), and wiring in place 3 new AAA NiMH batteries. I kept the small chip welded between the "-" contact and the central contact, thus just replacing the cells. Eventually a left the original battery pack in place, just removed old cells and rewired the new AAA outside the cell to try if works.
But the phone does not work: it lights up for just seven seconds the turns off, like with discarged batteries. It however does charge the new AAA batteries such wired, and i think it really recharges them fully at least (it needs 4 hours)
There is only one difference between new and old cells (alongside the shape of course :) the new are 750mAh and not 500mAh.
What it's wrong?