So I got one of these "6500 mAh" batteries off of ebay. I didn't expect much and it's still a bit of a let down. After testing it a few times, the best capacity I got out of it was 3500mAh at 12v. I haven't tested the other voltages yet, I'm not sure I'll bother. I also attached a graph of the voltage over time for my test.
I can't really say how accurate the test is. I used 15 100ohm resistors, each 5 in parallel and those groups in series. I measured just about exactly 60ohms with my crappy multimeter. Then I connected an analog input of my arduino between group 2 and 3 to divide the voltage by 3, giving me a range of 0-4 volts for the arduino to measure. The arduino took a reading every second, and averaged every ten readings to record to an SD card every 10 seconds. I verified my calculations were correct with a multimeter before starting the tests. Voltage wasn't perfect but it was easily within 5% and even better when i tweaked the math a little...
So even though it only delivers just over half of it's claimed capacity, my main problem with it is the charging. It takes between 24 and 30 hours to get it fully charged. It took about a week before I realized it actually has a full charge indicator because I had never seen it. The charger has two lights, one indicates power to the charger, the other indicated that it is delivering power to a device. On the device itself there is a red light that seems to turn green when it is sufficiently drained and also snaps green for a moment when you turn it off, seemed like a weird system to me and unfortunately the seller was no help.
This is the exact one I got (http://www.ebay.com/itm/360507624892), I figure they're all different so maybe if you want to try one of these, don't try this one.
Wishful advertising. Don't believe everything you read?
Well, like I said, I didn't expect much. The real disappointment wasn't even the capacity.