The Garmin GPS-25 manual indicates the internal battery is a rechargeable Lithium with a capacity of about 180 mAh. It also says the load on the battery when the board is unpowered is 50 microamps. There is no way to see what the state of charge is on that little battery. Or how long it takes to charge it up when the board is powered. A fully charged battery is good for about 5 months. If you discharge the battery completely, the memory and real time clock will quit and you are back to 19.5 years ago again with no way to fix it other than sending it back again. So don't go too far between the new battery install and using the gps again.

My 302 went back to 1995. My GPS-NAV clock had drifted off by 3+ minutes, it did a "search the sky" when I powered it up last week and came up with the correct time.

-Dave