Determine lead acid battery safe float charge level for a range of state-of-charge
You will probably need an intelligent charger (micro controller based with a custom program to handle inputs from V&amp;A meters, solar charge controller, &amp; a power regulator).

Common methods of determining the SOC of a 12V(nominal) battery aren't very accurate.  Most often people use a generic table that shows V &amp; SOC %.  The problem with that data set is that it applies to a new/good battery and as a battery ages &amp; deteriorates the internal battery resistance increases, charge capacity decreases, &amp; charge rate decreases (i.e. the charge rate of an aged battery is slower at a specific voltage than a new battery)--so you can't rely on it as a reliable method to charge a battery with solar energy while the engine is off &amp; power is being drawn from the battery.

If you use a micro controller based smart charger with a good program, you should be able to do the following to determine the maximum safe voltage to apply to the battery while the engine is off:
1.  measure the battery V
2.  measure the A draw of the system
3.  To maintain the battery's current SOC, your power system must produce as much power as the system is consuming.  Your program would calculate the optimal charge A &amp; V to provide sufficient charge current to offset the power consumption rate.
4.  To add a net charge increase in the battery, your computer program will have to provide a higher charge rate than the charge consumption rate (you can decide that rate or make it a user configurable parameter).

Note: The regulated charging system of a running vehicle provides sufficient power to charge the battery &amp; maintain other electrical loads.  Your smart charger will have to emulate the same power availability while the engine is off (which might mean you'll need quite a bit of PV surface area (depending on loads).