I am interested in buying a portable USB (rechargeable) battery pack to power my Axoloti when not near my laptop. It would need to be able to power both my Axoloti Core and an attached midi keyboard (Keith McMillen K board).
I need to know what specifications to look for as I don't want to fry my Ax.
I am wondering if anyone here has any recommendations... links might be helpful for me.
Thanks!
Well, I don't think there's any danger of frying anything if you buy a USB battery pack, as by definition USB is always 5V.
I often use a few different battery packs, as @SirPrimalform said, don't be afraid on frying the Axoloti board, they all give you 5V, and that what you need.
Things you can look into, is battery capacity (to give you more fun time) and maximum Amp it can deliver. The Axo itself uses very little (around 0.3A) but if you add a USB keyboard, it adds up (very rarely over 1A).
My main one is a 3 USB port, 12800mAh, 2.0A (Tecknet BT1280).
It offers a small info screen with the charge left and Amp's being used. I find it really useful and can run 3 Axo's with 3 different midi controllers at the same time, for about 8 hours.
When I only use one axo and a midi keyboard, I can get more than 12 hours of continuous use.
I use it on all my gigs and only charge it up every other few.
As mentioned here these Usb powerbanks work perfectely fine.
Keep in mind that the capacity in the desctription of these powerbank almost always refers to 3.7V, thats the Voltage of the lithium ion cells these powerbank use.
So if you for example draw 1A and have a Powerbank with 10Ah you will have actually less than 10 hours. (7,4 to be precise 
)
@Blindsmyth
That's interesting about the voltage. I guess they use a charge pump to get the 5V? Or do they use two cells in series and regulate down?
Not sure actually though both would introduce some energy loss I guess, wich would give you even less hours of usage. On the other hands these powerbanks are so cheap these days that you can't really complain
P.S. I think it's always several cells in parallel since there is always this reference to the 3.7V.
I'm pretty sure those use a step-up converter circuit rather than a linear regulator. Linear regulation from two cells in series would be 7.4V nominal, 66% efficiency, step-ups can beat that easily.
As @Blindsmyth says, the capacity rating is indeed of the battery not for the output.
Anyway I believe every functional USB powerbank should do fine.
Thanks everyone, I had never used one before, so I wasn't sure about how they worked.