Opening it to change the battery could be a very simple task, and it shouldn't void your warranty, so go ahead and order a new battery and get a precision screwdriver set. You may also want to try it with a separate charger and verify if your problem is definitely caused by your battery. Is it charging properly? Your charger port or charging wire can cause this problem as well, you'll know by trial and error. Visibly inspect your charging port, you can safely clean it with a clean paintbrush if you have one around, be sure nothing is in the way to obstruct your DS from receiving a charge.