What are some of the most important attributes of spacecraft battery and how are these attributes realized?
user
Some of the most important attributes are reliability, low mass.  Reliability is often achieved through qualification testing, demonstrated performance in a flight environment and configuration management.  Mass can be minimized by using high-performance Li-ion batteries that are optimized for the anticipated use case and by right-sizing the battery to meet end of mission power requirements using calibrated battery life and performance models.
assistant