I bought something similar from Dealextreme and get the same results. It's probably because of the lower tension (1.2v) of a rechargeable battery than the 1.5v of a regular AA battery.
I ended buying this and I'm now really happy.
Charge nicely my 770, even quicker than the original charger or than a regular USB port (claimed to deliver 1000mA, and the real figure must be close to this).
The included 18650 battery is rated as a 2200 mhA, but must be around 1800 mhA, and can nearly top up an empty tablet battery. As it's a 3.7v battery, it's like having a three AA pack.
And having a full size USB port is really handy to recharge all your gadgets. Can also be used with a Y cable to power your USB devices connected to your tablet.

Build quality is OK, and additional 18650s can be bought for a few $.

A must have for your tablet.