Seems like most people are incorrectly connecting their battery tender to their C8s, must be done like this !

C8 owners find that the battery tender they use does not report the battery is 100% State of Health (SoH) but the State Of Charge (SoC) is 90% and battery voltage around 12.6 volts
Each of the 6 cells of battery puts out 2.2 volts = 12.6 volts so as the remote battery monitor shows 12.6, thus SoH is 72 %

The reason SoH is not 100% is the C8 and other new GM vehicles have a battery monitor module (also needed for the new E-Ray), and it controls the charge to the battery when the charger is hooked up as GM states (shown below)

In short, the monitor is controlling the total charge to the battery to prevent overcharging battery

The electronic battery sensor is an important part of the vehicle energy management.
The sensor informs the car of the exact battery status, measures the temperature and controls the charging voltage and charging current accordingly.

The massively increased on-board currents in current vehicle models, and in particular the micro-hybrid start-stop applications and recovery of braking energy, place new demands on car batteries. Increasingly, kinetic energy is also converted into electrical energy during coasting (= free rolling of the car) and this charges the battery.

Use and function of the sensor

Energy management continuously records parameters such as voltage, current and temperature via a battery sensor.
These measured values are used to calculate important parameters such as state of charge (SOC), state of function starting ability (SOF) and overall condition age of battery (SOH State Of Health) and a variety of other values.

The data are sent to the vehicle computer.
The driver then decides on the energy balance in the vehicle and, if necessary, restricts the use of comfort consumers such as heated seats or air conditioning, switches off the start-stop function and signals this to the driver on the display with the corresponding information.
If the alternator then recharges sufficiently, the control unit automatically reactivates all consumers and functions.

The ultimate goal is to ensure that the engine is started at all times and, in doing so, avoid vehicle failure.
The sensor also supports intelligent alternator control, which reduces CO2 emissions, saves fuel and protects the environment.