Everything that draws power has equal priority, and batteries accept all power at all times.

That's why you use batteries as a backup only or extra juice if you need a bit more.

Once you have access to uranium, there is zero reason to have more than single emergency battery imo.

They are always useful for stable power when you're using primarily solar or wind and don't want to build bazillion of these.

I thought there was a mod or script to hack in some sort of power priority to the grid but yeah, when you have fueled power (reactor or engine) batteries are a constant 20% drain. If you have solar or wind, it doesn't matter too much because those are infinite sources (until it gets dark or the wind dies off).

Event Controllers come in handy to automate your power grid and apply a priority system even in a vanilla no-script world. Switching batteries from Recharge to Auto or Discharge when needed and back again all on their own. Same for engines or reactors as the fuel might be scarce and too precious to waste.

Batteries are a useful additional source for intermittent heavy load situations, eg rapidly recharging a jump drive, or when peak power for thrust in multiple directions simultaneously is needed eg when trying to manoeuvre to evade a conflict.