I've also seen flip out solar versions of these devices .. and that's what I was referring too with the term battery bank.
The battery bank works quite well .. my brother has one(he took on travels) and it doubled the life of his laptop(watching videos) from 4hrs to 8hrs ..and it more than tripled the life of his tablet to over 15 hours or more(he lost interest he said!) .. so it may have lasted longer than 15 hours.
Can't remember exactly but his is a 12000mAh version and with a solar doodad, that just does nothing.
he'd left it out all day, once and then for about 2 days another time, and it barely registers a micro volt of power.
It charges ok in the car tho, off a 12 volt source.
Note that the 12000mAh is rated for a 5v load tho, so if your battery is a 7.2v, then you need to account for this.
But even with that accounted for, 8amps pumped into a 1500mAh 7.2v battery is still at least 3 or 4 charges worth of power for 'ya.
Manufacturer claims will always be on the optimistic side of marketing guff .. I can't see 2W of charging power in them solar panels ... maybe for 1.8 volts!
But these battery banks are a brilliant idea for travellers.
You could hook up your own solar panel to keep a battery bank continuously topped up as conditions allowed, and then the battery bank would get a small enough boost to give you maybe a charge or two more for your camera battery .. and any other device that required it.
this reminds me! .... I should chase down the brother and seconder the use of his battery bank for a more thorough testing session.