Reasons Why the Start-Stop System Is Not Working

Start-stop technology makes driving more economical and environmentally friendly. As a result, by 2020, almost every third car on our roads was equipped with this technology. For example, when the car stops at a traffic light, the start-stop system shuts off the engine. As soon as you press the clutch pedal or release the brake in the case of an automatic transmission, the engine immediately restarts.

In addition to the frequent engine starts, which the battery must support for this process, it also powers all the electronic consumers even when the engine is off. You could say that the car battery is the heart of the system in modern vehicles. It is often aided by the intelligent battery management system (BMS), which acts as the brain of the car’s electronics. The reason the start-stop system may not function can be due to an issue in either of these two essential systems.

Battery Management System (BMS) Disables the Start-Stop Function

The main priority of any battery is to start the engine. However, if the battery is only partially charged, the battery management system (BMS) disables the start-stop function to ensure engine starting. A similar protective mechanism is activated in the following cases:

• The wrong battery technology is installed, which can only support a small number of charge cycles.
• The external temperature is too high or too low. If the battery requires too much energy to power the fan, the start-stop function is disabled. When and whether this happens depends on the comfort settings of the specific car manufacturer. It may also happen that the start-stop function still works, but the air conditioning system is automatically turned off.
• The engine temperature is too high or too low. If consumers or the battery do not heat the engine, it must generate its own heat through combustion, which requires a higher starting current. The result: The start-stop function is not activated. If the engine is at risk of getting too hot, the radiator fan must cool it, as airflow is insufficient or nonexistent when the vehicle is stopped at a traffic light. The fan requires high power, so the start-stop function is not activated.

Incorrect Battery Technology Causes Problems

Another cause is improper battery replacement in the vehicle. For example, if the technology or capacity of the battery is not correctly entered, the BMS cannot use the proper algorithm. If the battery is not registered as new in the vehicle after replacement, the vehicle may remember the data from the old battery and not use the full potential of the newly installed battery.

You can read more on this topic in our article about the risks of replacing a start-stop battery with a regular battery.

There Are Also Factors That Interrupt the Start-Stop Function, Which Are Not Related to the Battery:

• The seat belts are unbuckled
• The car doors or hood are not properly closed
• The parking assist system is active
• The car is driving up a steep hill (too steep of a slope)

If the start-stop function doesn't work very often or doesn't activate at all, it is recommended to visit a service center as soon as possible. In this case, experts can often perform a free battery test to determine if the battery needs to be replaced.