Originally Posted by Tzul
Lithium-ion cells are typically charged to 4.20V. The Nook HD+ uses that value, too. Nominal voltage is 3.7V. I forgot the cut-off voltage that the HD+ uses to decide that the battery is empty, but I think it was 3.4V or 3.5V.
To add to that, make sure you put a load on the battery if you are measuring the voltage of the battery disconnected.
Something like a single 330 ohm resistor would be good.
Bad lithium batteries sometimes will appear charged until you place the load on it then it falls to 2 or lower volts.
Another quick check is to make sure the battery casing is completely flat, if the battery has bulges of more than a few millimeters the battery is about to fail and should be discarded immediately. Another trick you can use is smell the battery, I know sounds weird , but failing lithium batteries have a very unique odor , almost sweet, reminds me of the smell of bananas.