two more aspects of the battery behaviour:
- mars is very cold! surviving the night requires power to stay warm, so you will need to finish the day with some power left, and ideally this will stay out of the degradation zone or have a very rough morning
- batteries degrade anyway, no matter how well you take care of them, so every day you'll have a little less capacity even if you stay totally out of the degradation zone :)
re: tech tree and customisation, my design north star is everspace, which has a lot of incremental upgrades as well as some qualitative ones that give you entirely new abilities. my plan is to start with a basic rover (inspired by sojourner, the first mars rover) and then have incremental power, range, comms upgrades and unlock new instruments roughly in-line with the real history of mars rovers. the choices the player makes through upgrading the base rover and the choices the make on payload (many more instruments have been designed for mars than have ever flown to it, payload capacity is limited!) should give this range of experience.
i like multiple rovers as an idea (the latest mars rover had a companion helicopter which i would love to have) but i think one constraint i want to keep for my own sake is a single, linear plan. a real rover would not have to split its time between observing and downlinking, it could do both, or it could downlink and move, but managing multiple timelines in parallel makes the controls a lot more complicated for a game that already has a lot of complexity coming to it.