Stump the Chumps Q&A: Battery Over Temp Fault

During the summer months some of my batteries are showing a Battery Over Temp fault on the charger.

During the summer months some of my batteries are showing a Battery Over Temp fault on the charger. Can I disable the temperature sensor to let the batteries charge?

Absolutely Not. Disabling the battery temperature sensor will cause the battery to overheat and is the worst possible way to address overheating issues. High internal temperatures can damage the battery and cause the cells to dry.

Battery heating occurs due to the resistance of the intercell connectors while the vehicle is in use. Additional heating due to this same effect occurs during charging. The higher amperage used during Opportunity and Fast charge exacerbates the problem. Because there is no cool down period following the charge as in a conventional charging application, the battery remains hot.

Cooling of the battery is very dependent on the design of the trucks’ battery compartment and the area of the battery case exposed to the room air for cooling purposes. The battery will usually be warmer than ambient based on the discharge rate, airflow around the battery, and other factors. As ambient temperatures rise in the summer months, the warmer air is less effective in cooling the batteries, causing the minimum temperatures of the batteries to rise. A warm battery placed on charge is closer to the temperature at which damage will occur. The Battery Over Temp fault occurs when the charger stops the charge to prevent battery damage.

For example, with normal ambient temperatures of 75°F, the heat added by discharging the battery brings the battery to a temperature of 90°F. Adding 10°F during a charge brings the battery to 100°F. In this case the battery has a margin of 30°F before issuing a Battery Over Temp fault.

Now consider an ambient temperature of 90°F during the summer months, the battery begins charging at 105°F. The increased heat from charging coupled with the reduced cooling effect of the higher ambient temperature can cause the internal temperature to become high enough to issue a Battery Over Temp fault.

Power Designers chargers can combat the overheating of a battery during charge by compensating for the battery temperature. Temperature Compensation adjusts the output of the charger based on the temperature of the battery. As temperature rises over 77°F (25°C) the charger automatically pulls back 2mV/°C/VPC (Volts per Cell). If the temperature reaches 125°F the charger pulls back an additional 2mV for a total of 4mV/°C/VPC before reaching the battery maximum temperature of 130°F. If the battery does reach 130°F the charge will pause allowing the battery to cool, before restarting the charge cycle. It will attempt this 5 times before terminating the charge cycle and issuing the Battery Over Temp fault. Power Designers can adjust this compensation to make it more effective; however, not every fault can be eliminated based on the application and battery temperature at the time the battery is connected to the charger.

When dealing with Battery Over Temp faults, some important considerations to prevent a battery from overheating include:

  • performing a weekly equalize charge followed by checking and filling the cells with water
  • increasing air flow around the battery while in the truck
  • increasing air flow during charge

To submit battery and charging questions or to seek help with tough battery and charging situations, e-mail us at