Medical / Biology has it's own author order convention. First author is generally the person that did the most work/research on the publication. Last author is the person who managed it. Second author is generally the primary grunt of work. After that is anyone else that contributed / edited, often in order of amount contributed.
Source I worked at a research hospital for several years and actually managed to snag myself a second authorship on a project.
IMO the solution is to change it from charging the battery to swapping out the battery. The problem then is having a standardized battery configuration/connection and we all know how well standards go. The station can then charge for the electricity and expenses, including an amount to cover the occasional battery replacement.