Hello,
According to the manual the battery in is 5.5x2.1 barrel center possitive, 4 to 10V DC
As the battery handle is yet to be released and I want something in meanwhile so I can take it on the train I am thinking of getting something like this.
https://es.aliexpress.com/item/32971764791.html
I will have a look at a local electronics store first, and see if they have any available.