Hi Steve,
Do you have the Ipad docked in the DL1608?
Heat rises and is conducted from the mixer and can cause the Ipad to overheat.
At first it will slow down, then it will hibernate.
Remove the Ipad, place in shade and in a couple minutes it will be ready to go again.
Putting small blocks of wood under the DL1608 can help the flow of cooling air around the mixer.
Active cooling with a small fan is another option.
Cold air falls, heat radiates.
For maximum cooling, wrap the ice pack in a towel and place it on top of the Ipad.
Or you could put the ice pack under the Ipad and just count on conduction.
For charging batteries, generally the bigger and heavier the unit the longer it will last.
Some of the small power supplies are only powerful enough to charge a phone, though the trickle charge from one can extend your Ipad's usefulness a good hour or two.
Here's one of many reviews online -
http://thewirecutter.com/reviews/best-usb-battery-packs/Good health, Weogo