AA battery powered USB charger?

[MAT]

I always carry spare batteries for my phone, camera, ipod when backpacking. Has anyone used one of these rechargers? The advantage is everything else I use takes AA (lithium) so I am already carrying spares, and I figure this will save weight and room if I only need this and a 4 more lithium batteries. My question is how much charge can I get from 4 batteries? I think AA batteries are around 3000 ma/H each. If my cell phone has a 2000 ma/H battery does that mean approx 6 charges per pack (4 batteries)?

Amazon.com: Verbatim AA Power Bank Charger 97928: Electronics

 

[COlineman78]

Well, the problem with rating things in mAh is that you also need to be concerned with voltage. The more correct way of representing capacity is mWh because it accounts for different voltages between the battery and the thing consuming it. This doesn't matter for things of the same voltage (phones usually use 5V), but a AA battery outputs 1.5V. As such, you should convert both your cell phone capacity and the batteries into milliwatt-hours and then you can do simple addition.

For example, the average cell phone battery is around 3Ah (3000 mAh), so that's 15Wh. Energizer Ultimate Lithium's are about 3400 mAh, which is roughly 5Wh. Therefore, it would take 3 AAs to charge your phone. That being said, there will be power conversion losses so you may only get to 90% or so on 3 batteries.