Hi. I take the following table from a 60's european car manual:

Battery 12 volts, 60 Amp/hour:

100% charge = 12.66 volts
75% charge = 12.45 volts
50% charge = 12.24 volts
25% charge = 12.06 volts

measured with voltimeter between (+) and (-) battery posts, engine off.

After start the engine the voltage would be 13.8 to 14.5 volts or more (all gadgets off - lights, radio, vent, etc., the voltage is function of the power generated by the alternator - more powerfull alternator, more amperes, but also, more load to engine).

The 0,2 volts of difference (13.8 teorical vs 13.6 your car) could be associate to bad earth on some part of the car due oxide, after all, our cars are some old.

On my rojo car (a 121 amazon), after switch to on the auxiliary fogs lights, the AMP idiot light starts glowing lightly, ditto with the internal fan and high lights on. I understand it is a usual behavior on Amazons (please, more experienced people on the board, confirm or denied it). On my car, and using a Daewoo Tico alternator (see pic), I have 14.2 volts with engine on.

Truly yours (and excuses for my espanglish),

Joaquin Novara
Rojo 121 Amazon
Peru