For comparison: to upgrade the memory on my New 3DS XL, I paid $18 USD for a Sandisk 32GB SD card in Nov '15 and $21 USD for a Samsung 64GB MicoSD card in June '16, both bought through Amazon (US).
Also for comparison: I take my 3DS to work everyday and almost never open it. By the time I get home it always has lost a bar. I meant to check it every hour today, but couldn't. Sometime between hour 4 and hour 7 it lost a bar. I'm a little extra about not running out of power so I have 2 of those portable power banks. There are myCharge AmpMax and AmpPlus. The AmpMax is supposed to be 4X a regular battery and the AmpPlus 2x. My memory is that I needed the AmpMax to get through a 10 hour flight that included a lot of game time. I may have needed the AmpMax to get it fully charged from almost no power, but my memory isn't clear. However, if my memory is correct, that would mean a typical power bank probably wouldn't be sufficient.
The more I thought about this, the more likely I felt it was that I continued to game while the 3DS was plugged in and charging. So I tested it. I was able to get two full charges (I let the 3DS run until it shut down) out of the AmpMax and still have a one of four bars left on the power bank.
From the side of the AmpMax:
Model: AMU60KG-A
Capacity: 22.2Wh
Input: 5VDC, 2.1A
Output: 5VDC, 2.4A Max
From the myCharge website:
Rechargeable 6700mAh portable charger
The AmpMax 6700mAh portable charger delivers instant power in a compact, ergonomic design. With 2 built-in USB ports and and 2.4A output this powerbank is perfect for charging two devices simultaneously and up to 65% faster. The 6700mAh portable battery provides an extra 45 hours of power and comes complete with an integrated USB recharge cable. Recharging your powerbank just became a whole lot easier!
Battery Capacity: 6700 mAh portable charger
Recharges Via: Integrated USB cable
Compatibility: Smartphones, Tablets & USB Devices