There's a more interesting question: What's the voltage output ?
4 non-rechargeable batteries make for 6 V, 4 rechargeable ones make for 4.8 V.
So if the voltage is 6 V, we know for more or less sure that the Reader withstands 6 V input voltage. If it's 4.8, we know we could use rechargeable batteries as a recharging power source.
If so, it'll be a breeze just getting hold of a 4-batteries holder and simply solder a piece of wire and a plug to it and we're done.
If voltage is regulated down to 5.2 Volts, we'll need a 5-batteries holder and a voltage regulator if we're into DIY.
Estimated costs of a DIY mobile charger/backup battery: 3 Euros plus batteries.
Drawback: Some effort.
Benefits: Flexibility and free choice of batteries, usability beyond the magical 1-year-till-dead-barrier of LiIon batteries, and the priceless "I-made-it-myself" feeling.
So it might be a great thing if you hook up your voltage meter and tell us the results.
