You should be getting 12.7V with the engine off and the battery has been left to sit for over an hour. That is a fully charged battery. Is the 12.5V measured with the CarPC powered on? If not, maybe see what your voltage is when you turn on some accessories, lights, CarPC, etc... Voltage might not be a good indication of battery health. Sometimes the open terminal voltage on a battery might be 12.5V, but under the slightest load, it drops dramatically. That would be a sign of a bad battery, or bad alternator which isnt keeping the battery charged.