Small, practical , compatible with all mobile devices including self-adhesive sucking disks, those are power banks FIXED 4000 mAh. Power banks have a built-in microUSB cable with a reduction for Apple Lightning devices. That way it is easy to charge Android, iOS and Windows devices.
Power bank FIXED 4000 mAh can easily be fitted in your hand, its height is only 12cm and the thickness is less than 1cm. In addition it is thin therefore it can be hidden in a packet of a coat or a jacket.
There will be no trouble with holding the spare battery thanks to additional self-adhesive sucking disks. The sucking disks are attachable to any surface and they assure a firm connection of the power bank and your device.