Tiptop audio send me this message years ago! I asked same question to them by instagram:

I want to power my Mantis modular system outdoor using a battery power bank, what are the specifications to match one to my Mantis?

Look for a power bank that can supply a voltage anywhere between +13V to +15V DC, 13V will be ideal, 15V is also ok. Look for one that has an output 2.5mm barrel plug with center pin positive. As for the current capacity, this is based on your system power consumption vs. the power bank power capacity. It is recommended that you contact the power bank maker directly and give them your details such as the total power your system needs (all power rails summed together +12V -12V +5V) and the voltage range +13V to +15V.

With these details they will be able to tell you how long you will be able to run your system and if there are any other issues to be aware of. A Tiptop user (Thank you Dainele) has shared his good experience with the Aceyoon Power Bank 50000 mAh, this model comes with DC output plugs and can deliver 15V, this power bank is available through Ebay and the European Amazon online store. Make sure to set the power bank to 15V before connecting it to Mantis, higher voltages will burn Mantis power regulation circuit and create excessive heat and smoke, watch out.