Garmin GPS 12 battery usage/power consumption

I have an older model GPS 12 that I have not used for a few years. I would like to use it for some exploration on my dirt bike in unfamiliar territory. The unit would not power up, so I removed the old batteries, inserted new ones and turned the unit on. Although it lost all the prior waypoints (I assume the internal battery died), it did power up and I left it recharge for 24 hrs. It did recharge the internal battery b/c I am able to put in waypoints, remove the AA batteries, reinsert them, and repower w/o losing data. However, I noticed that even fresh AA batteries only last ~2 weeks before the unit will not power up (even if I do not use the GPS). I also noticed that only 1 (sometimes 2) of the AA's is discharged. I can replace the 1 (or 2) battery and the unit shows a full charge. This is especially evident if I use rechargeable batteries (they will only last a few days before I have to replace 1). I also have a 120vc charger for the unit; even with it plugged in, the batteries still consume power. Can anyone tell me if this is an indication that the internal battery is bad; or offer reasons why the GPS is doing this? Thank you.

 

Thanks for your response. I found that video before and assumed I would follow it if I had to open the case. I automatically assumed it was an issue w/ the internal battery since the unit sat unused for so long and the AAs were dead. However, the unit came back to life after a couple of hours and I am able to remove the AAs without losing waypoints, so I'm not 100% sure it actually is a problem w/ the internal battery. At this point, though, I think it is relatively inexpensive for me to open the case and replace the internal battery and see what happens. The good thing is that I have the MapSource software to back up waypoints and transfer to/from the Garmin unit.