This original Bosch operating instructions [PDF, en] shows you how to connect the charger of your hire bike directly to the bike.
If there is no power socket nearby, you can remove the battery from the bike frame and charge it at another location.
The video above (ad-free) shows you in detail how to remove and reinsert the battery.
This original Bosch operating instructions [PDF, en] shows you how to determine the charge status of the battery.
Your rental bike has a modern rechargeable battery. The "memory effect" known from older batteries no longer occurs here. It is therefore better to load "too often" than "too seldom".
Safety instructions: