Originally Posted by masterfly
Thanks; just for clarification: not space heater, AS furnace set at 50 deg overnight, and comes on occasionally when outside temps drop into 30’s. Monitor in AS not sufficient?
Any device that is just looking at battery voltage is only making a very rough guess. If it is wired directly to the battery posts the guess will be a little better, but still not very good at all. Most RV monitors do not go directly to the battery posts ( the typical AS monitor does not go to the posts).
A shunt based monitor is the only way to really know what's going on with your batteries. They are a common item on lithium installs. They are equally helpful with lead acids.
The voltage monitors only have limited information. The voltage of a full battery (or half full) changes by maybe as much as a half volt up or down as temperature changes over a normal "let's go camping" sort of range. The monitor does not read temperature. It does not understand that 12.0 (the normal stop using voltage) now is 12.6 (because it's *cold* out). It happily tells you the battery is at 100% when it's below 50%. Battery capacity goes down when it's cold, that also messes things up.
A good monitor runs a current meter ( a shunt ) on the battery. It looks at all the current in and all the current out. It also looks at battery voltage. The fancy ones look at temperature. You do some settings to let them know what the characteristics of your battery are. They do math to tell you what's left in the battery.
If you are going to be off grid / depending on batteries get one.
One of many:
A lower cost / simpler version:
There are a range of others out there. The Victron stuff is what a lot of us have used.