An electric vehicle’s largest, most important and most expensive component is its high-voltage battery pack. Many newer bespoke EVs are built around their battery pack, and some have even incorporated it as an integral part of their chassis structure. The battery pack must store the vast amounts of electricity required by an EV, not only to provide enough power for daily driving but also to ensure a sufficient range to minimize the owners' charging anxiety.

This high-voltage battery is also known as the traction battery in an EV, and it has dozens of kilowatt-hours (or even over 100 kWh, in some EVs) of stored electricity and 400 or 800 volts of electricity flow. That's enough to provide hundreds of miles of range and excellent acceleration in an EV, keeping the lights on and the refrigerator running in your house for about a week or giving power to any power tool or appliance for many hours through bidirectional charging.

However, even though you’re literally sitting on a huge battery in your EV, it still requires a small 12-volt battery of the kind you find in your conventional combustion car. To understand why a big battery on wheels needs an additional small battery, we need to look back at the history of the car and how the 12-volt battery became ubiquitous throughout the industry, and it’s remained a staple even as we move into the electric age.

A Look Back At Car Batteries

BMW Group Plant Leipzig - battery module production

BMW Group Plant Leipzig - battery module production

Cars have been around for well over 100 years, but they initially didn’t have a battery because they didn’t need one. The magneto, a precursor to the later alternator, provided the electricity needed to power their ignition coils and generate spark for their engines. The magneto generated power as the engine ran, and they could do without an electric starter because all early cars were started with a hand crank.

By the 1930s, the hand crank was seen as old-fashioned and dangerous, and many cars already had electric starters powered by an onboard battery, marking an irreversible shift in the industry. These batteries initially had 6 volts, but by the 1960s, most manufacturers had moved to 12 volts so that they had enough power to crank larger displacement engines with higher compression ratios. 12 volts became the standard for car electrical systems that have remained in place until the present day.

The battery’s main role is to power a car’s ignition system, and once its combustion engine starts, it uses the alternator to keep charging the battery, which in turn powers everything in the vehicle, from the headlights to the electric windows. It also serves to regulate the voltage provided by the alternator, provides additional power when the alternator can’t keep up, and acts as a buffer to protect the various electrical components from electricity spikes.

The vast majority of modern cars still use a 12-volt battery because, over the decades, an entire industry has been built around this voltage despite limitations that became apparent over time. However, a shift is happening, and some vehicles are moving to a 48-volt system, which has many advantages. These include a huge reduction in the cable mass required and having more power to feed systems like active suspension that can’t run on 12 volts.

EVs Need 12-Volt Batteries

BMW battery module

BMW battery module

Even though electric vehicles don’t need a jolt of power to spin a starter motor and crank over a combustion engine, which is a 12-volt battery’s most important role in an internal combustion car, it is needed for everything else. The lights, door locks, electric power steering, infotainment, safety systems, and everything in between were essentially borrowed from traditional combustion engine cars and they usually run at 12 volts with current supplied from the 12-volt battery.

It’s worth noting that the 12-volt batteries used for EVs have a lower output than those used to start combustion engines. When cranking a combustion engine, the battery needs to provide a burst of power quickly—between 200 and 600 amps—while in an EV, the requirement is many times lower.

EVs use their DC-to-DC converter to step down the voltage to 12 volts to power all auxiliary systems without passing the current through the 12-volt battery. The most important function served by the 12-volt battery in these vehicles is to wake them up and keep some systems online while the vehicle is off. What the 12-volt battery is used for and when it’s used varies from EV to EV.

When an electric vehicle is turned off, its traction battery is disconnected from its electrical system using special contactors, which are electronically controlled switches. The 12-volt system also powers these in an EV, and the battery needs to be disconnected to prevent power leakage through the system as well as to make EVs safer when parked by having its high-voltage system de-energized.

It therefore needs an additional power source for the electrical contactors that connect the traction battery and turn on the EV. That’s where the 12-volt battery comes in, and in this role, it performs a task similar to providing power for an ICE car’s electrical starter.

Taking on the role of the alternator in an ICE car is a DC-to-DC converter, which steps down the voltage and essentially trickle-charges the 12-volt battery from the traction battery. It only runs when needed, when the 12-volt battery reaches a lower state of charge because it would otherwise drain power from the high-voltage battery all the time.

In most EVs, if you leave them parked for an extended stretch, they will periodically wake up, click on the electrical connectors for the traction battery, and charge the 12-volt battery so that it doesn’t run dry. If the 12-volt battery in an EV fails, you probably won’t be able to unlock and start it, even if the high-voltage battery still has juice.

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