3DS XL battery drains quickly?

I received a Pokemon X and Y special edition 3DS XL for Christmas a little over a year ago. I didn't use it very much up until a few months ago, but I noticed that the battery drains in an hour and a half to two hours, and only lasts about a day when in standby. It used to last much longer than that. I'm charging it for as long as I'm supposed to and when I'm supposed to do it. I'm extremely gentle with my games and I haven't damaged the system in any way, shape, or form. I have been using the charger from my DSi to charge it, but I didn't realize it up until the other day when I found my 3DS charger in my closet (we moved over the summer and the chargers must have gotten mixed up). Could this be a contributing factor?
If I need to get a new battery, I sadly cannot purchase it from Nintendo, seeing as Nintendo does not support PayPal and that is the only way I can pay for things. Sending the system in for a repair is most certainly out of the question as well, seeing as my warranty is long expired and we've dug ourselves into a financial ditch that we probably won't be getting out of any time soon, if at all.