Two teardowns for the price of one! We never gave you an original 3DS XL teardown (although we have many lovely guides), so here's the skinny on batteries, at least.
On the left is the New 3DS XL battery, with the "old" 3DS XL battery on the right.
It seems Nintendo didn't bother upgrading the battery in the New 3DS XL. Both models feature a 3.7 V, 1750 mAh battery rated at 6.5 Wh.