JonCondiff :
I got this 3DS XL one summer right before the new 3DS was announced and/or released.(2-3 years ago) Now I believe it's battery is going. A while back I could use it a entire day or two without even being warned about low battery; now It can't even go 5 hours without dying. Does this mean I have to replace the battery?
The short answer is...probably.
The 3DS uses a Lithium-Ionen battery, typically they last a set number of charges. They lose their ability to hold a charge after 2 or 3 years or 400-500-ish charges. If you played your 3DS a lot you'd need to charge it more often so in any case you are probably due a replacement.
There is nothing wrong with the battery, its performing as it should after 2-3 years. If you want a longer play time you can easily pick up a replacement online, just remember to buy the proper screw driver to open your 3DS, I'm pretty sure nintendo use the smallest phillips head available. phillips #00? Its absolutely tiny!
Hope that helps!