Like all Li-Ion batteries have nominal voltage about 3.6V-3.7V, and full charge at about 4.25V, depending on how risky you are today (yes, there are some minor variations in rated voltages, but not like 10x). Just because chemistry. For lower current (heat losses) you just connect the cells in series, so you got multiplies 2x, 3x, ... of the same voltages (but never fractional, like 1.5x). They typically have more wires to balance the charges between the cells (so the total wires count is #cells+1).
There are also "protected" batteries, which means there's an electronic chip that protects battery from over/under charging, but authors' doesn't seem to have that. It's just a regular unprotected 1-cell Li-Ion cell, and the device provides the protection.
That means author could as well connect to any battery of 3.5V -- 4.2V, not going through the hoops of looking for their particular reader, as long as it's thin enough to fit. Smaller batteries would just have less charge.
PS: I would double-check it's really unprotected tho.