I may be full of crap, it's been over 15 years since I worked on this stuff. You have been warned. The systems I worked on used multiple deep discharge, cold

I may be full of crap, it's been over 15 years since I worked on this stuff. You have been warned. The systems I worked on used multiple deep discharge, cold weather hardened lead acid batteries. Deep discharge batteries have an important distinction from automotive batteries. They tolerate significantly more deep discharge cycles but can't produce as many amps. If you're just trickling a more or less steady amount of power into some electronics, you don't need massive amounts of cranking amps that an auto battery provide. Solar PV and battery tech have definitely improved quite a bit since then, so were I now building these same installations, I might do something entirely different. But "back in the day" your advice of "buy another battery" was more or less exactly how we handled charge rate limitations.