Hi there Tom and welcome
The batteries are internal so switching them out is not feasible, but what you can use is a seperate battery pack with an AC outlet.
This way you can plug the adapter cable into it and charge it just like charging from the all.
You would need to have at least 20,000mAH but I recommend higher to give a full charge twice to the unit and so advise on this product which is a 27,000mAH battery pack which you can get here
This would charge it with about 2 full chargers from the standard charger. It is very portable and allows you to connect any type of device to it.
If you would like any additional information or assistance, please do not hesitate to let me know.
Else if you can take a second to rate my service by clicking one of the stars at the top of the screen then submit, that would be greatly appreciated.