This is a merged topic combining a number of threads about the V’s compatibility with portable battery banks.
The answer to this popular question is: Any charger or battery bank following the USB-PD standard should work, but only use of the supplied Eve power adapter is officially supported and covered under warranty.
Portable battery charger for v by @Jeffrey_Newton
Portable battery chargers for the v that actually work by @Jeffrey_Newton
Using power bank to charge Eve V by @Taa_Ynwa
Which Power Bank for V? by @ethercoin
Post your battery bank/car charger by @Adic27
A rich discussion about the USB-PD standard has taken place in this related thread: