Why Some Mercedes Models Have Two Car Batteries Instead Of One

Typically, a car has one battery pack, stored under the hood alongside the engine and other vital components. It's the battery's job to provide for all of your vehicle's electrical needs, including starting the engine and powering various gadgets and accessories such as the car's lights, stereo, and more. That single battery pack has enough juice stored up to serve all of these vital needs. That, however, is precisely why it's understandable to be confused to discover two different batteries in your car.

A variety of cars produced by luxury brand Mercedes-Benz, including one of the best Mercedes of all time, the E500, are equipped with two batteries instead of just one. One of the batteries is located under the hood near the windshield, as you'd expect, but the other is in the trunk next to where your spare tire is stored. You might expect this to be some kind of emergency battery, but in fact, your Mercedes needs both of those batteries pumping power at the same time to start itself and run all the fancy gizmos in the cabin. To put it simply, these cars have quite a thirst for juice.

Certain Mercedes vehicles, like the E500, SL500, and S550, are equipped with two batteries in the hood and trunk: the main battery and the accessory battery. The main battery serves the typical purpose of a car battery, namely starting the car. The accessory battery, meanwhile, powers everything else you'd find in your Mercedes vehicle. This includes the lights, stereo systems, power locks, and any other gadgets present.

Why does a Mercedes need two batteries to do what most cars can do with one, though? It's because, as a luxury vehicle, a Mercedes has a generally much higher level of power consumption. Compared to simpler cars, a Mercedes is packed full of elaborate electronic features and mechanisms. The 2024 Mercedes E-Class has a range of luxurious features, like ambient lighting and an all-digital dashboard. If all of those tried to draw from the same battery as the starter, you'd never be able to start the car. 

Ergo, the starter has a dedicated battery specifically for turning the engine over, while other electrical features in a Mercedes draw power from the accessory battery. This is likely why the accessory battery is also known as the consumer battery, as these features consume quite a bit of energy. This can also be why the battery light on your dashboard may have come on, even if the car is starting fine. In that case, the main battery is probably doing okay, but something may have gone wrong with the consumer battery.