You would not get double the run-time if the external batteries were the same rating as the internal batteries!
You have to remember two things. First is plain \'loss\'. You not only go through various connections (each connection has a resistance loss), but you have to go through the charging/power circuit of the Z itself. This eats some battery power.
Second, if you have batteries iin the Z, and who wouldn\'t?, the power from the external batteries would be used to charge them. Charging batteries is always at a power loss. It takes more power to charge than the power return.
I haven\'t done battery electronics in years, so I can\'t come up with any numbers, but there\'s no way that an equal external battery supply can double the run-time of the internal batteries.
Luckily, you can buy batteries with considerable more power than the internals and may be able to approach doubling the life. Maybe there\'s a battery expert out there who could figure it out? Too much trouble for me ;-)
But the Semson\'s units are nice and well worth the money. Even though I built my own for a couple of dollars, I just received the simple $6 Semson\'s unit. I liked the idea of an on/off switch and it turned out to be a bit smaller than the one I made.
I also bought their \"Pocket Power Bank\" with the buolt-in AC charger. This comes with a set of 1600mah Ni-mh batteries and is quite a bit bigger than the simple battery pack. But it has the advantage of being able to plug directly into any AC socket to recharge. That seemed like quite a convenience to me. I didn\'t realize though that it used an external usb connector wire to plug itno the Z. That\'s not bad, but it\'s an extra piece to get lost....
