How to select the right mAh portable battery charger?
I am wondering if someone can help explain portable battery chargers and how to select an appropriate sized one. I realize how many charges you want to get out of it will affect what size you want, but I'm not sure I understand how to relate mAh ratings to how many charges you can expect to get out of it.
For example, I have the New Trent IMP70D 7000mAh which says on their website "Capacity 7000mAh@5V, 3.5 times of iphone 4 battery life." I believe an iphone 4 has a 3.7V 1420 mAh Li-Polymer battery so my untrained brain thinks 7000 ÷ 1420 = 4.9 not 3.5. But okay, you're not going to bring the portable charger down to 0 so maybe that's why it's 3.5? The remaining mAh is just a reserve you can't tap?
So then I'm wondering how many charge times should I expect from it with my 1780 mAh battery - maybe 2.5 (7000 ÷ 1780 = 3.9, minus the reserve)? But since I don't run my phone down to 0 either, then I should really get maybe 5 "top offs" if I only let it get to 50%? Or doesn't it work like that?
As you can see I'm struggling with some of the basic concepts here. I know there's been some threads about which portable charger to buy and I thought that others might also find a little primer helpful as they evaluate different mAh options. I've just gotten the New Trent and only had the chance to fully discharge it (or at least discharge it so the lights don't come on anymore) only once, but I did not get 5 top offs of my Razr. I'm wondering if that will improve over time or if my expectation is wrong.
Thanks to anyone who can shed a little light.