I’ve tested quite a few portable “top-up” batteries and most of them have a problem. The issue is not with the batteries themselves; it’s a "nut behind the wheel" thing. The problem: Forgetting to recharge the battery after you’ve drained it.
Yep, just when you need the battery you’ll often find it flat as a pancake but I have a solution: The Bracketron Xolar3000, a 3000 mAh battery with a built-in solar panel. While it can be charged via its USB port (when you remember) being able to leave it in the sun or even just ambient light and have the battery top up is great (a full charge requires eight hours of sunlight).
The Bracketron Xolar3000 battery with a built-in solar panel
The Xolar3000 weighs just 4 ounces and measures 2.5" by 4.5" by 0.5" (slightly thicker and wider than an iPhone but an about 0.5" shorter).
The blue LED at the top left indicates when the solar panel is topping up the battery (the panel’s maximum output is 5V at 0.8A) and when you repeatedly press the button on the top edge you sequentially select one of the three outputs: 5V 1A, 5V 2A, or 9V 1A. The four LEDs on the top right also light up when the button is pressed to display the battery level. Also on the top edge are the USB output port and a mini USB port for conventional charging.
We were recently away for the weekend and I took a Xolar3000 with me. When one of our iPhones, which hadn't been fully charged the night before, started to run low on power the next morning we topped it up using the battery. What was cool was because we were in the sun, by the time we got back to the hotel the battery had recharged to almost full capacity.
As an emergency power source the Xolar3000 is a great choice and priced at $49.95 it’s excellent value. And if you can just remember to leave it in the sunlight you'll never have an empty battery again.