you can potentially wire them to the initial cell that is in there, THEN everything routes through the protection on the original pack, and it can be charged (slowly) using the same methods.
i have done that with 2 PDAs that were lacking in total power and space for it.
eees funny, we can get this stuff down to the size of a watch, but we still need a TANK to run it
first choice is to find a beloted battery or use multiples, they have extended batteries for almost everything, even if they have to put a guppy door on the back to fit the extra large cell. of course there is a lot of FAUX china overrated extendeds too :-( so you gotta watch for real verses fake.
I Parellel direct to the inital CELL item, on the battery that is original , that doesnt bypass anything, like the protection or the charger. it is not always possible to connect direct to the cell item, like with most phones you have a plastic wrapped battery.
then i made a CASE for the whole system, so the battery would have physical protection.
the second choice would be to use a (seperated) cell phone charger thing, that ran on li-ion or li-poly, or even ni-mhy. they have a bunch of them things that will charge solar or via a usb connect, that way it would make a standard usb charge port connection, they also arent so huge or bulky when using a li-poly.
i came close to wedging a 5AMP li-poly on one
it just wouldnt FIT, think about li-poly for the job, because it can be nice and flat, fits more things easily. you can get 1400-3000ma ones that will Fit the size of the back of most devices, then you still have to wire it up, strap it in, and make some effort to protect the bagged cell from damage.
Disclaimer: some foresight knowlege and fear (hey fear is good sometimes) , and care would be needed to insure everything is correct.
