Alright folks, I'm going to suggest, if you feel like the battery life on your 3ds or 3ds xl is shitty (because it super is) this replacement battery. It is pricey (100 dollars, BUT thats with FREE shipping from Hong Kong which costs about 30 bucks so that helped me justify the purchase) but it is a killer replacement battery.
[HLI-3DSLL] Buy Mugen Power 5800mAh Extended Battery for Nintendo 3DS LL and Nintendo 3DS XL with Battery Door
They're in extremely high demand, but if you order now it'll ship on the 23rd of this month. They say 10 to 15 days shipping, I got mine in about 8 days.
Now, yes, its pricey. But the regular 3dsxl battery lasts a whole measely 6 hours before it has to be recharged, thats with the screen light turned way down and 3d turned off, and it can only be recharged about 500 times before its worn out. This Mugen battery last between 12 and 16 hours, 10.5 hours if you have literally everything turned on, wifi, 3d and screen brightness maxed. You can go from recharging twice a day to once a week with this sucker. The Mugen batteries aren't some cheap cop out, either. They have the lowest return rate in the industry, they produce 3rd party batteries for all sorts of portable devices.
Here's some review action on the thing.
Hardware Review: Mugen 3DS XL Battery Replacement Grants 20 Hours Of Staying Power - 3DS News @ Nintendo Life
The one I'm linking is for the 3ds xl, but they also have one for the 3ds.
Now, the other minor downside besides price is that it does make the system a bit thicker. I don't personally think it makes it unweildy or no longer portable, but it is a bit thicker than stock. Probably twice as thick. The battery comes with a special back to replace your regular 3dsxl back, and you can pick the color so it matches your 3dsxl.
I bought one to try out because I'm buying my kids 3ds' for Christmas and they will fucking annihilate that tiny little battery that comes with the system in no time. They'll play that shit obsessively and recharge it two and three times a day on the weekends and so I wanted an alternative. So, I bought one of these batteries to test out how the shipping worked, how the batteries performed, etc. before going all in on two more for the kids.
I got the battery Friday in the mail (took about 8 days total once shipped from Hong Kong to my front door). It was about halfway charged out of the package. I plugged it in and charged it up overnight (because the battery is much larger, it does take more time to fully charge, between 8 and 12 hours the first time but, after the first 4-5 cycles probably closer to 8) and then I had the system turned on literally all day yesterday, from about 1100 am all the way to midnight before the system started flashing warning me the battery was low. That's how good these batteries are compared to the stock battery that comes with the system. It basically turns your 3ds battery life into a battery life very similar to an Ipad, which has an amazingly long battery life.
So anyway, if you're looking for a good 3rd party battery solution from your 3ds or 3ds xl, this Mugen battery is the way to go. High quality, long life, few downsides.
added: one quick point. The thicker 3dsxl case with the extended battery goes a long way to relieve that claw grip cramping you get in your hands trying to hold onto that tiny sliver of a game system. May not be a big problem to some, but playing the 3dsxl cramps the shit out of my hand after a bit, but the extended battery size gives the system a bit of a meatier feel, a heft to it, and it rests much more nicely in the hand, and no cramping. Really a big plus.
Added2: Just one more thing. From that video link above, some battery life test results (this is with EVERYTHING turned on and maxed, screen brightness, wifi and 3d)
We test the 3DS XL Mugen Battery expansion in a live test with other DS, 3DS, and 3DS, XL consoles. Review of the Mugen 3DS and 3DS XL battery:
The results of the battery test are:
3DS XL (Mugen Battery) 10hrs 32mins
3DS (Mugen Battery) 10hrs 15mins
DS XL 4hrs 55mins
DS Lite 3hrs 45mins
3DS XL 3hrs 40mins
DSi 3hrs 10mins
3DS 2hrs 45mins
Most people don't play with 3d on at all, which will significant extend the battery life either way, but as you can see, even with it turned on maxed out, the Mugen battery gets 7 HOURS more life than the 3ds xl stock batteries will. Its a big difference. Especially if you're playing all those JRPGs like SMT4, Pokemon, etc.