Your battery has a B.T.S. battery temp sensor. It monitors your batt temp and determines the battery charging rate.
The PCM sends 5volts to the sensor and is grounded on a return line. As temperture increases, resistance in the sensor decreases and the detection voltage at the PCM increases.
System voltage will be higher at colder temps and is gradually reduced as things heat up.
The sensor is located under the battery in the tray.
disconect the pig tail harness then pry the sensor up through the tray.