Thanks for the suggestion but it is simply not practical, the risk of losing the two small screws when removing or refitting the battery in the back of a car in a field is too great. I can power my other speakers from a separate battery pack and inverter so the USP of the S1 is lost.
The battery will "go to sleep" after some time (30 days?) and you will need to plug in with the power cord and switch on momentarily to wake it up.
After that you can then run with just the battery.
Hope this helps, _Roger
So I still need to be close to mains power when I re-commission the S1 in case it has 'gone to sleep'! (where is the 'facepalm' emoji when you need it).....