A fully charged battery supplies around 12.6 volts. If this falls to 12.0 volts, the state of charge drops from around 100% to around 25%. Conductivity is reduced by low temperatures, so if your battery is not holding its charge your car may not start in cold weather.
Sludge builds up in the bottom of the case and can short the cell out. Deep discharges, heat, vibration, over-charging, under-charging or non-use all speed up the ageing process. In the end, if not replaced the battery will fail, and one day your car won't start.
We check the battery's ability to hold its charge. We may need to replace the battery if it's not holding a charge, or look at why it's not charging when you drive.
At every service.