159 2.4 20007 plate
19,500 DPF/EGR by
AO Silver Member
Join Date: May 2009
Location: United Kingdom
Do some reading on battery voltage with ambient temperature change - if air temp is low it makes a measurable difference to the voltage of a battery. Also note that it seems a calcium battery has higher open circuit voltage than pure lead acid.
Also don't just leave the bonnet up as you may have a current drain problem with the car, if you want to use battery voltage to check battery state then you must disconnect the battery for at least 4 hours though most manufacturers will quote 24hrs.
You alternator output voltage does seem a bit low but not by much. I would put it under load (main beam, heated seats, heated rear screen, heater blower motor all on etc.) and see what the voltage across the terminals was at idle and again at 2,500 rpm.
You haven't explained why you started investigating; I assume the battery isn't as good as it used to be.