It has a battery that is one of the very few reliably rechargeable watch type cells. As with the Neo carts, the MK2/3 uses SRAM for the save data space since SRAM memory needs a constant current to retain data. Unlike the neo cart, the SRAM and battery combo in the MK2/3 seems much more reliable.
On a full charge these batteries can hold the data in SRAM from 5 days+ (perhaps much much longer, but 5 days was the last I checked). Unless it comes dead from factory it should not need to be replaced any time soon. Due to the nature of rechargeable Li batteries (and I am not certain with manese lithium batteries) it is
possible these ones can become damaged if drained entirely.
I havent used my MK3 64 for a few weeks and it still retains 2.54V (according to the datasheet it should be 3.1V), the data still stays intact after 10 minutes. A DMM and a careful hand could tell you if your battery is bad (less than 2V or so would be needing a charge, less than .4V would be bad), but in this case I would look for an exchange if you have not had the MK for very long.
The battery is a Sanyo ML621 Magnese Lithium rechargeable battery (industrial Sanyo line) (
PDF datasheet can be found here)
