I wonder if I should stick a tiny volt meter on top of that battery, or buy one of those fancy Battery Monitor Meters from Amazon?
Maybe instead of looking at voltage and guessing, read the actual Ah used?
Like this:
https://www.amazon.com/dp/B013PKYILS/
Or this:
https://www.amazon.com/dp/B07FN3VXH4/
Anyone used one of these before?
I've inserted a 70A breaker in the + side, so the battery can be cut completely off from the load of the meter and the heater's controller standby current draw.