I found the optimal solution in speaker TT-SK03 of TaoTronics.
Per
· Wireless Speaker with integrated battery
· Bluethooth connection
· Water-resistant housing with detachable suction cup
· Small, handy, lightweight and shapely designed
· Panels on top are clear and intuitive to use
· Speaker can be used as a speakerphone for Rufaufnahme
· The pairing with the device works Bluethooth simple
· Plastic materials are well made
· Sound quality and volume of the small speakers are good
· Range is 10 m relatively well
· Battery is adequately dimensioned
Contra
The battery can be charged only unfortunately using a barrel connector. Better is a standard USB connector (micro or mini USB) would have been. So always an extra cable must be carried.
No 3.5mm jack plug socket available to just quickly connect something without Bluetooth connection.
Conclusion
I find the Bluetooth speakers very convenient as mobile and versatile. The sound quality and volume are good. The battery is adequately dimensioned. The speaker is easy to use. Here we have a good price-performance ratio. From me a strong buy recommendation and therefore a perfect score.