I had 2 Link10 units which were the predecessor to Xantrex Link Pro and Xantrex Link Lite. I believe that Link 10 was either the first or one of the first battery monitors capable of calculating SOC using Pukert’s law. https://en.wikipedia.org/wiki/Peukert%27s_law
The quality suffered after Xantrex purchased the Link 10 company years back. Xantrex is owned by the Schneider Electric Co now. I believe the Xantrex Pro has the capability to trigger a small relay based on the SOC level which could be used to start a generator.
Some popular units of today are by Victron; BMV 700 and BMV 702 with different features. There is also the venerable battery monitor by Bogard Engineering, still looks like from the previous century.
All of these battery monitors use a shunt mounted between the negative battery post and the ground/loads. These shunts are calibrated resistors and the battery monitor measures the drop of voltage across which is proportional to current. Voltage polarity changes with current flow direction by which the monitor knows if battery is being charged or depleted. SOC (State of charge) is often used as a primary gauge to see the battery state.