Why does the 2015 Malibu have two batteries?

The 2015 Malibu uses two batteries only on models equipped with GM’s eAssist mild-hybrid system, combining a traditional 12-volt battery with a separate energy-storage battery for hybrid functions. This setup supports electric assist, stop-start operation, and energy recovery without overloading the main battery.

Two-battery setup explained

The Malibu’s two-battery arrangement is designed to power different parts of the car’s electrical system without cross-loading them. The standard 12-volt battery handles starting the engine and running conventional accessories, while a separate energy-storage battery powers the hybrid components and keeps the electric motor/generator supplied with energy for assist and regenerative braking.

Before listing the key functions, note that only Malibu models with the eAssist mild-hybrid system use this dual-battery configuration. The gasoline-only versions rely on a single 12-volt battery.



• Electric motor assist: The secondary battery supplies energy to the hybrid motor to provide additional power during acceleration.


• Stop-start functionality: With a dedicated energy-storage battery, electronics can run while the engine is off, allowing smoother start-stop behavior without draining the main battery.


• Regenerative energy: The hybrid battery stores energy recovered during braking for later use by the electric motor.


• Electrical isolation: Keeping the hybrid system on a separate battery protects the 12-volt system from high current loads, improving reliability.

In short, the dual-battery setup in the Malibu’s eAssist variant is a targeted design choice to enable mild-hybrid capabilities while preserving the traditional 12-volt system for standard vehicle functions.

Models and systems involved

Within the 2015 Malibu lineup, the two-battery arrangement is tied to the eAssist-equipped model. Those seeking a purely conventional gasoline car without hybrid features typically receive only one 12-volt battery.

To clarify how the system differs across configurations, consider these points:



• eAssist-equipped Malibu: Includes a dedicated energy-storage battery for the hybrid system alongside the standard 12-volt battery.


• Non-eAssist Malibu: Uses only a conventional 12-volt battery for starting and basic electrical loads.

Ultimately, the presence of two batteries hinges on whether the car uses an eAssist mild-hybrid system, which relies on additional energy storage to bolster efficiency and performance.

How the two-battery system works in practice

Practically, the vehicle’s control systems manage power flow between the two batteries via a DC-DC converter and hybrid electronics. The 12-volt battery continues to power starting, lighting, and accessories when the engine isn’t running, while the separate battery stores and supplies energy for the electric motor, regenerative braking, and related hybrid operations.

Before the list, here are the core components and their roles:



• 12-volt battery: Standard automotive battery for engine start and vehicle electronics.


• Hybrid energy-storage battery: Powers the electric assist motor and stores recovered energy.


• Electric motor/generator: Provides mild-hybrid assistance and regenerative braking input.


• DC-DC converter: Manages power transfer between the high-energy battery and the 12-volt system.

These elements work together to deliver improved efficiency and smoother operation without compromising the traditional electrical responsibilities of the Malibu.

Summary

In the 2015 Malibu, two batteries appear only on models with GM’s eAssist mild-hybrid system. A conventional 12-volt battery powers starting and typical car electronics, while a separate energy-storage battery supports the hybrid functions—electric assist, stop-start, and regenerative braking. This dual-battery approach helps enhance efficiency and performance for hybrids while keeping the rest of the vehicle’s electrical system simple and reliable.

What is the small battery in the trunk of a 2015 Malibu?

The small battery in the trunk of a 2015 Malibu is an auxiliary battery that powers accessories and the Start-Stop system, while the main battery under the hood is for the starter motor and main electrical demands. The auxiliary battery ensures electronics like the radio, lights, and climate controls continue to operate during a stop when the engine shuts off. 
You can watch this video to learn how to replace the auxiliary battery: 58sSteeveGMTECHYouTube · Jan 18, 2024

• Purpose: This secondary battery provides dedicated power for the start-stop system and other electronics, such as the audio and climate control systems, during engine restarts. 
• Identification: A common code associated with a failing auxiliary battery is P305F. 
• Symptoms of failure: Symptoms of a failing auxiliary battery can include dimming lights or the radio cutting out when the engine restarts automatically after a stop. 
• Main battery vs. auxiliary battery: The battery under the hood is the primary battery that starts the car, whereas the one in the trunk is specifically for the accessories and start-stop feature. 

Why does a 2015 Chevy Malibu have two batteries?

Your 2015 Chevy Malibu has two batteries because it is equipped with an engine start-stop system. The main battery under the hood is for starting the engine, while a smaller, auxiliary battery in the trunk powers the vehicle's accessories and electronics when the engine is temporarily shut off by the start-stop feature. This system is designed to improve fuel efficiency and reduce emissions.
 
Main battery

• Location: Under the hood.
• Function: Powers the engine starter and the car's electrical systems when the engine is running. 

Auxiliary battery

• Location: In the trunk. 
• Function: Powers all electrical components when the engine is off, such as the radio, lights, climate control, and power windows, to keep them from being interrupted by the start-stop system. 

How it works together

• When you come to a stop, the engine turns off, and the auxiliary battery powers the accessories. 
• When you release the brake or press the accelerator, the main battery handles the power needed to restart the engine quickly. 

What does the auxiliary battery do in a 2015 Chevy Malibu?

In a 2015 Chevy Malibu with the Auto Stop system, the auxiliary battery in the trunk powers electrical accessories and systems when the engine is off, while the main battery under the hood is for starting the engine. The auxiliary battery is responsible for keeping systems like the radio, lights, and computer controls active during Auto Stop events to prevent them from shutting down. If the auxiliary battery fails, you may notice dimming lights or a radio cutting out when the engine automatically restarts.
 
This video explains what an auxiliary battery is and the symptoms of a malfunction: 39sStudent LessonYouTube · Aug 9, 2024

• Auto Stop Power: The auxiliary battery provides the necessary power to run the car's electrical components, such as the radio, lights, and dashboard displays, when the engine automatically shuts off during an Auto Stop event. 
• Main Battery Function: The primary battery, located under the hood, is still responsible for starting the engine. 
• System Draw: A failing auxiliary battery can put a parasitic draw on the main battery while the car is parked, potentially leading to the main battery draining and the car failing to start. 
• Location: The auxiliary battery is typically located in the trunk, distinct from the main battery under the hood. 

This video explains the function of an auxiliary battery in electric vehicles: 55sBosch MobilityYouTube · Apr 10, 2025