The 12 V systems in internal combustion engine (ICE) vehicles and electric vehicles (EVs) have a lot in common, but the differences make them interesting from an engineering perspective. This FAQ reviews the similarities and differences between the two applications — including the lead-acid versus Li-ion batteries. It will close by examining the use of 48 V Li-ions in EVs.
EVs get their efficiency from using a high voltage to drive the traction motor. High voltages like 800 Vdc reduce the cable size needed to deliver the tens of kilowatts this motor requires. The smaller cables reduce cost and weight and simplify installation.
However, the high voltage is a potential source of danger and is not compatible with electronic control units (ECUs), relays, small motors, lighting, and many other functions in an EV. The 12 V battery also acts as a backup power source to ensure the uninterrupted operation of critical systems like brakes, steering, and windshield wipers.
So, 12-V lead-acid batteries are common in EVs and ICE vehicles (Figure 1).
Figure 1. The 12-V power system in EVs provides uninterrupted power to safety-critical functions and general systems like lighting and infotainment (Image: Varta).
An important difference in the use of 12 V lead-acid batteries in ICE vehicles versus EVs is a need for about 500 A peak output current to start an ICE, which is not required for an EV. However, due to the low cost of existing heavy-duty batteries, they’re commonly used on both types of vehicles.
The high current required for IEC starting breaks down deposits that can build up on battery plates, extending battery life. EVs don’t draw high peak currents, and 12 V lead-acid battery failures are more common in EVs than in ICE vehicles.
Replacing existing 12 V lead-acid batteries with alternatives like a DC/DC converter, delivering 12 V from the high-voltage battery pack, or using a Li-ion battery presents several challenges.
For example:
- Lead-acid batteries are cheap, and the alternatives are significantly more costly.
- The use of lead-acid batteries simplifies the EV design process, enables the use of a large ecosystem of 12 V-rated automotive components, reduces complexity, and maintains a high level of reliability.
- Lead-acid batteries are proven safe and replacing them with an alternative like Li-ion can raise safety concerns.
EVs require contactors
The contactor is an essential component in EVs not found in ICE vehicles. It requires low-voltage power. The contactor is used for two purposes:
- To control the inrush current when the vehicle is first turned on and the capacitors in the motor drive are charged
- To isolate the main battery pack when the vehicle is off or has been in an accident.
Isolating the main battery pack when needed is important for EV safety. The main coil in the contactor is a low-voltage circuit and uses the 12-V auxiliary power source. The reliability of lead-acid batteries makes them well-suited for powering EV contactors.
Li-ions and 48-V systems
There have been considerations about replacing lead-acid batteries in 12 V systems with Li-ions. In the case of IEC vehicles, there are several challenges to deal with. The Li-ions are more costly and sensitive to cold in the winter and hot in the summer. Another problem is that Li-ions cannot be directly charged using the alternator, requiring additional charge management circuitry, further increasing the cost.
Even though Li-ions weigh less and can marginally contribute to a greater range in EVs, the problems with their use have generally eliminated them from consideration. That’s changing. EV makers are replacing the 12 V auxiliary power with 48 V.
For example, Tesla’s CyberTruck will have 48-V auxiliary power and use Li-ion batteries. The use of 48 V results in smaller, lightweight wiring and electrical components, further reducing the weight of an EV. It’s also possible that future EVs will use a mix of 48 V for the main auxiliary power distribution, with 12 Volts still used to power certain functions (Figure 2).
Figure 2. The use of 48-V auxiliary power distribution in EVs will not necessarily result in the complete elimination of 12-V power (Image: Vicor).
Summary
The 12 V systems on IEC vehicles and EVs are similar. Both use the same types of lead-acid batteries, powering a range of auxiliary systems.
In EVs, the 12 V power is also used by the contactor, which provides important safety and performance functions. EVs are slowly moving away from 12 V systems toward 48 Volts and hybrid 12 and 48 V systems where the lead-acid battery is replaced by a Li-ion one.
References
Eliminate the 12V battery and increase EV performance, Vicor
Tesla Will Switch to 48V Low-Voltage System in Cars to Significantly Reduce Use of Copper, Tesmanian
The role of lead acid batteries in electric vehicles, Varta
Why Electric Cars Still Use Ordinary 12-Volt Batteries, Jalopnik
Filed Under: FAQs, Featured, Power Management