Originally Posted by Rubi4MyMrs
The reason for that sensor is to regulate the alternator charge rate (the voltage regulator monitors that sensor) depending on battery temp. If it gets too hot the charge rate needs to decrease or the battery can overheat & damage it. If you have ever noticed how hot a battery gets when it is being charged off a battery charger, especially a “fast charge” you will understand. Also battery voltage drops in cold temps so the alternator will charge more if the battery temp is low.
And if you tie the sensor back into the harness it will charge normally as if the battery isn't overheating, like when you use a AGM battery.
I've been running my duel Optima battery setup that way for over 8 years...
From the OEM FSM:
BATTERY TEMPERATURE SENSOR
The Battery Temperature Sensor (BTS) is attached
to the battery tray located under the battery.
The BTS is used to determine the battery temperature
and control battery charging rate. This temperature
data, along with data from monitored line
voltage, is used by the PCM to vary the battery
charging rate. System voltage will be higher at colder
temperatures and is gradually reduced at warmer
The PCM sends 5 volts to the sensor and is
grounded through the sensor return line. As temperature
increases, resistance in the sensor decreases
and the detection voltage at the PCM increases.
The BTS is also used for OBD II diagnostics. Certain
faults and OBD II monitors are either enabled
or disabled, depending upon BTS input (for example,
disable purge and enable Leak Detection Pump
(LDP) and O2 sensor heater tests). Most OBD II
monitors are disabled below 20°F.