You can only store a certain amount of energy in limited space.
Unless your battery is worn/old/broken there is not much you can do but decrease brightness to the minimum and play DS software instead of 3DS software.
Batteries sold with unreasonable (inflated) capacity rating like… let's say 2500mAh… for replacing the original CTR-003, which has 1300mAh, are just a scam and may perform much worse than the normal battery. Printing on a big number doesn't make a high capacity battery.
Old 2DS lasts about 4 to 5 hours on lowest brightness setting. Should be enough for most gaming sessions. You could connect a powerbank (which certainly has some nostalgic value thinking of the rechargeable NiCd battery pack for the original Game Boy).
Or you could retrofit a bigger Li-Ion cell. I guess any Li-ion with Thermistor and properly connected will power up a 2|3DS. ← Okay, I said it, but the most important thing:↓↓↓↓
Do not attempt to connect different kinds of batteries if you don't know what you are doing. Shorting a battery is dangerous. Connecting a battery with the wrong polarity, even for a split second, is known to cause severe damage on 2|3DS console