Think back to the days when you mowed your lawn with a gasoline-powered mower. Do you remember how you started it? You probably pulled that long cord a handful of times, attempting to get the engine to crank. Early vehicles had an equivalent starting mechanism, in which the user had to use a mechanical hand crank system to start the car. Compared to modern day vehicles, these early vehicles had no electric demands due to the absence of headlights and interior electronics. 

In the 1920s, the electric starter was invented to replace the mechanical system. This system required an electricity source, and thus began the inclusion of batteries in vehicles. Initially, these batteries were small, typically a 6-volt lead acid battery. As vehicle size and electricity demands increased, the industry switched over to a 12-volt battery. To this day, lead acid remains the battery choice for engine cranking. However, lead is toxic and environmentalists have been pushing for an alternative for years. In Europe, lawmakers succeeded in keeping nickel-cadmium batteries out of consumer products, and authorities want to do the same with the starter battery. The standard alternative is lithium-ion, but poor cold-start performance and high prices have prevented the widespread adoption. 

Just like a gasoline vehicle, an electric vehicle has a 12V battery to power accessories and security functions while the car is parked. 12V systems have remained popular due to their minimal wire gauge requirements and the fact that the same parts can be used for both gasoline and electric vehicles. While it may seem logical to simply use the main high-voltage battery pack to power the vehicle’s electrical systems instead of a separate 12V battery, there are several reasons why this isn’t practical:

1. The main high-voltage battery pack is designed to power the electric motor and propel the vehicle, not to provide a steady supply of electricity for the vehicle’s electrical systems. Using the high-voltage battery for this purpose would put a lot of strain on it and could potentially reduce its overall lifespan.
2. The main high-voltage battery pack is much larger and heavier than the 12V battery, which would add unnecessary weight to the vehicle. Using a separate, smaller 12V battery helps to keep the weight of the vehicle down and improve its overall efficiency.

Roles of the 12V battery

The 12V battery serves several important functions, some of which are shared roles between gasoline and electric vehicles.

1. It powers the vehicle’s electrical systems when the car is not in use, including the central locking system, alarm, and other security features [shared with gasoline vehicles].
2. It powers the vehicle’s onboard computer and diagnostic systems, which is essential for starting and monitoring the main high-voltage pack.
3. It acts as a buffer for the main high-voltage battery pack, helping to smooth out any fluctuations in the high-voltage battery’s output, ensuring a steady supply of electricity to the vehicle’s electrical systems.

Charging the 12V battery

In gasoline vehicles, the 12V battery provides a large jolt of electricity needed to start the vehicle's engine. This energy needs to be replaced, otherwise the battery will not have sufficient energy to start the vehicle next time. In order to charge the battery, gasoline vehicles utilize an alternator, which acts like a small electric generator, converting mechanical energy into electrical energy. Electric vehicles, on the other hand, do not have engines with pulley systems, and therefore do not contain an alternator. So how do we charge the 12V in an EV?

The 12V battery in an EV is typically charged through a process called “trickle charging.” This involves using a small amount of electricity from the main high-voltage battery pack to keep the 12-volt battery charged while the vehicle is in use. Trickle charging helps to ensure that the 12V battery always has enough power to perform its various functions, even when the main high-voltage battery is being used to propel the vehicle. In addition to keeping the 12V battery charged, trickle charging also helps to maintain the health of the battery and extend its lifespan. This process of charging also occurs when charging the larger high-voltage battery.

It’s important to note that the 12V battery in an EV is not always the same as the battery found in a traditional gasoline vehicle. The 12V battery in an EV is often a special type of battery called a “deep cycle” battery, which is designed to be regularly discharged and recharged. This is in contrast to the “starter” battery found in a traditional gasoline vehicle, which is designed to provide a burst of power for starting the engine and is not meant to be regularly discharged.

Demands on the 12V battery

A 12V battery offers the high current needed to start an internal combustion engine, and the entire electric architecture of an ICE vehicle has been designed to be powered by the 12V battery when the engine is turned off. In an ICE vehicle, this initial large demand of energy  is the biggest draw over a short period of time that the battery will experience, and only occurs at the beginning of each new drive cycle. Similarly, vehicles equipped with automatic start-stop function often have a specialized battery with higher cycle life to account for the additional demand. 

Replacing the 12V battery

The idea of eliminating the 12V battery in an EV has been proposed, particularly because they add cost, weight, and take up valuable space in a vehicle that already has a large battery. If we hope to replace the 12V battery system, manufactures need to address three things:

1. Cost - lead-acid batteries are cheap, and in most cases, it costs more to employ a DC-DC conversion system to turn 400V (from the battery pack) to 12V for all the electronics.
2. Complexity - redesigning anything in a car is complicated work, and lead-acid batteries offer proven durability and a relatively long lifespan.
3. Safety - most of the low-voltage systems are in very close proximity to the passengers. Lead-acid batteries allow the higher voltage to be isolated by disconnecting the main battery back from critical systems. High voltage inside the passenger compartment would require many layers of safety protection. 

Many automotive systems, especially safety systems, must respond quickly to sudden changes in power. Historically, small batteries have much better response times than DC-DC power converters. Additionally, until recently, few options existed for safely and reliably converting 800V or 400V down to 12V with a fast response times, and without adding unwanted volume or weight. Some companies, such as Vicor, have proposed high-performance power converters as a way to “eliminate the 12V battery and achieve enhanced transient response, decreased weight and additional package space—all of which contribute to extended range and better overall performance.”

Battery wear in EV vs ICE

Browsing EV forums across the internet will suggest a common theme - increased rate of 12V battery replacement. Many owners report replacing the battery in just 2-3 years, which is far sooner than typical replacement in an ICE vehicle. Additionally, a poor-performing battery has clear symptoms in an ICE vehicle, often being slow to start. In an EV, however, it’s quite difficult to assess 12V battery issues, as it becomes a software issue. Owners often report strange vehicle behavior such as flashing dashboard lights or infotainment issues. Engineers in the space have suggested that a likely culprit is the software itself. EV manufacturers took the easy road when designing the split battery system, adopting the existing ICE architecture because it worked. The issue is likely in how and when the high-voltage battery charges the 12V battery, and how the additional electronics play a role (e.g. BMS, users checking SOC).