Yunsailing 2 Pcs 200A High Precision Watt Meter Power Analyzer Battery Consumption Performance Monitor Amp Meter Monitor with Backlight Digital LCD Screen for RC Solar Voltage Current Power (100V)

I have this one. Just needed to add MC4 connectors on each side. But it is only 'instant' - no app. https://www.amazon.com/dp/B0C5M8YMH6

Sounds like a battery monitor is what you need. I have one of these and I like it. Draws only a couple milli-Amps and has negligible voltage drop, but tells you a whole lot about what's going on with your battery.