The included LiPo pack sustains the very HEAVY draw of the EDF just fine.

I put a 1800mAh 18C pack into the plane and found that the EDF ran poorly because it was drawing more than this battery could provide.

You can fit a thin 2100mAh 20-25C pack in or a 1800mAh 20-25C pack, but they are longer than the included battery packs.

Initially you'll believe that these batteries WILL NOT fit... but you CAN make them fit.

The "bottom" of the plastic battery cage can be cut out.

If you do this and then file the edges of the plastic smooth, you can insert a longer pack into the space.

Part of the pack will protrude into the area in the fuselage, but there is no problem with this.

And the pack is held snugly by the sides of the battery "cage".

I found that an 1800mAh pack was about perfect for the balance and weight of the plane. It increases the flight times to about 10 minutes while not degrading flight performance.