Volts

[vic008 17]

Have 20v coming out of panel(overcast day) then thru the regulator but only 12.3v on the start battery. Have never seen a reading over 12.3. Sound like the regulator faulty, or the battery is in great condition ?

[Island Time 1,235]

Sounds like neither of those to me. Sounds like the battery is pretty flat, and the solar power has not yet charged it. How big is the panel, what's the battery, is it full of water?

A fully charged 12v flooded lead acid battery should be about 12.8v at rest after a few hours ( no load, no charge). 12.3 with some charge happening is bad. However, if the batt is ok, the voltage should slowly rise (to 13.3-14.8v depending on controller) while the panel is in the sun. Small panels can take a long time - days or even weeks - to charge a battery, so it would be useful to know the charge rate being accepted (amps).

Voltage at the panel is NOT indicative of available power - volts is more like pressure, amp hours more like volume.

[wheels 543]

Yep I agree with IT.
Things we need to know.
What is the battery type?
How old is it.
What size is the Panel/s
What happens with the battery voltage when the Engine is started?
What does the Solar Reg output V read when you disconnect the battery?

Something that can happen with FLA type batteries is a thing called Stratification. The Acid is Heavier than Water and as such, settles out over time and sinks to he bottom of the battery and the water to the top. Thus the top half of the plate is in water and not acid and the battery does not produce full charge. The only way to over come this problem is to charge the battery with an "equalization" charge. You need a special charger that can do this. It charges the battery at a higher voltage of upto 17V and very low current. This results in excessive gassing of the cells and the bubbles stir up the Electrolyte. This should actually be carried out Monthly believe it or not. Failing that ability of form of charging, the only other way is a real good fast charge to try and bubble the cells, but that requires a good sized alternator and a good charge regulator.
      Normal plain Jane Alternators with no regulator, only ever take a Battery to 13.8V and that's it. A Start Battery never becomes fully discharged and thus never gets a good hardy full charge and stir up. So a Start bank often never lasts as long as House Banks properly looked after, do.