Thanks for the swift reply, Halk. And thanks for the term "memory effect." I hadn't heard it before.
With that new information, my now extensive knowledge (ie. Google) tells me that there are no known memory effect problems with Lithium ion cells, but there are with Nickel Metal Hydride batteries (found on older laptops).
The PRS-650 User Manual says it has a "Built-in rechargeable battery: 3.7 V DC, 940 mA."