This.

There are cellphone chargers on ebay that cost $1 and take AA batteries, which are quite capable of charging any phone that supports the USB charging interface. I bought one to use when I travel abroad and it works very well.

Please do not plug your $600~ smartphone into a $1~ charger. That's just asking for trouble.

Why? Do you think they'll fight because they don't make the same amount of money?

But fine, get a $5 model from a reputable brand.

You can also get an entire phone for $10. It will even have a screen!

Don't spend $50 on a piece of garbage.

You're assuming that supplying 5v to a cellphone through a 5v USB charger is something that damages the phone.

One of my favorite features of my olde Palm IIIxe PDA was that it ran on batteries you could buy at the drugstore, and I wanted that for my smartphone, so I have an AA-to-micro-usb device ("AA power bank charger") that get me there. Ten bucks on Amazon, probably you can do better if you shop around.

An AA is 1.5V (rechargeable AAs are 1.2V).

Once that is established, it all comes down to the mAh.

Most AAs sold these days are listed as 2500mAh, multiply that with 1.5V and you get 3.750 Watts.

Note that this is very close to what you get from a 1000mAh li-ion battery found in many phones (3.7*1000=3700).

Now with a external li-ion emergency battery you will get conversion losses, as it has to go up to 5V for the USB and then back down again at the other end.

Considering that different phones consume different amounts of energy, and that different AA batteries have different capacities, that's hard to say.

If you carry a Wiko Lubi like me, you can probably talk more than 10h with a single battery. If you carry an energy-hungry smartphone, you'll probably get an order of magnitude less.