12.5 volts sounds a bit light for a fully-charged battery. To begin, have you disconnected the battery for a spell to discharge the system then re-connected it? You might need your radio code beforehand to reset that. It helps others if you give details of your vehicle as well - they might have experienced your problem with a similar vehicle..
2015 2.2 CDTi SENav 163ps manual 4x4 in Carbon Flash with 19\" alloy spare towing a Bailey Unicorn III Madrid @ 1500kg.