@flightquay What you're seeing is actually the intended behavior. I've simulated several different battery dynamics in my aircraft, including internal resistance and temperature effects.
A 24V battery has a nominal voltage of 25.8V when fully charged. When current is drawn from the battery, the voltage will droop in proportion to the amount of load. Here's we see ~0.9V droop when the battery master switch is placed in the on position. Try adding load, or engaging the starter, and you will see the voltage drop further. With the starter engaged, we go all the way down to 17V, and back up slowly, as the propeller begins to spin. When the generator comes online, the voltage will jump to 28V, which charges the battery.
This also affects the lighting in the aircraft, and other system too. You can read more in the "Voltmeter & Ammeters", and "Voltage-Based Light Dimming" sections of the manual.
Just for fun, you can see the LO VOLTS annunciator illuminated and the lights dim when the starter is engaged in this video. These are the details that I enjoy most about aviation
In the meantime, let me know if there is anything else I can do for you!