the battery is to power the memory that your game save is in when your DS is off. like all batteries, it will eventually die. i would hope that the battery doesn't get used if there is no save present, but who knows? replacing it may be as easy as pulling it out and putting another one in, but it could also be soldered in. i haven't looked at it myself, so i'm not sure which is the case. either way, it is do-able.

 

I believe the battery is rechargable. On your G6, just plug it in and leave it for about 4-8 hours and it will give the battery a recharge. I went about 4 months without recharging, but now i go about 2-3 weeks between recharge so i dont lose saves.