I wouldn't hold too much store (pun?) by measured off load voltages after charging or discharging Peter. Both situations take place at the battery plate to electrolyte interface, but the electrical storage of a lead/acid battery is chemically within the depth of the plates so it takes a while for the measured voltage to stabilise after either condition. See table below for a guide to battery condition, only relevant after resting for several hours from use.
![[Linked Image]](https://i.imgur.com/ymKsuRN.jpg)
Please note that specific gravity of the batteries electrolyte is a better guide to battery condition rather than off load voltage. This is measured with a hydrometer but only if you have access, i.e. no help with a sealed battery. Even then it gives no indication of the batteries capacity which can only be assumed as advertised when new or nearly so. Part of the ageing process is sulphation where the lead sulphate on the plates of a discharged battery can crystallise, effectively reducing their surface area in contact with electrolyte. This situation is greatly accelerated if battery is left unused in discharged state, hence importance of battery charger on float/maintenance mode when vehicle out of use for several weeks.
A sulphated battery will have a much diminished capacity, hence will charge and discharge quickly. It may show the full 12.7 volts indicating it's fully charged but only within the constraints of its limited capacity which even then may not be enough to turn engine over long enough for it to fire, especially mid winter, and hence my initial comments. The "repair" mode on your charger will be an attempt to reverse the sulphation by deliberately raising the charge voltage and often called equalisation. You usually get a warning in the chargers instructions not to have anything connected during operation in this mode since a 12 volt battery can hit up to 16 volts during such charging.
A lead/acid battery even left without load will still loose approx 5% of its capacity/month due to internal discharge. Having said all this I would be very dissapointed if a normally regularly used battery, in a new or old car, wouldn't start the vehicle after at least three weeks, in an airport car park for example.
