The solar panel charges the super capacitor which runs the ISS and stores energy for use when there is no solar. With no solar, and no remaining stored solar, the battery is the source of power. You are fortunate to be in a very sunny location, a ISS located in the U.K. for instance will be using the battery a lot more. The ISS power consumption is very low, it just takes a quick little sip of power each 2.5 seconds. You are fortunate to be in a very sunny location, an ISS located in the U.K. for instance will be using the battery a lot more. Having said all that, nothing lasts forever so with the shorter winter days upon us I strongly suggest you replace the battery now before it finally expires. If you have a meter please post the voltage of the old battery 