Everything You Should Know About 12-Volt Batteries on Electric Vehicles

Everything You Should Know About 12-Volt Batteries on Electric Vehicles

Did you know that all EVs require a regular 12-volt battery to start? Every EV relies on a low-voltage battery system, which is recognizable from traditional internal combustion engine (ICE) vehicles. Many are surprised to learn that their cutting-edge battery electric vehicle still relies on this seemingly old technology.

Just like any regular vehicle, EVs need a 12-volt battery to start. This article aims to demystify the 12-volt battery on electric vehicles, providing a comprehensive guide to its function and maintenance. Understanding this crucial component will help for a smoother, more informed EV ownership experience, ensuring you are never caught by surprise by the unnoticed hero of electric mobility.

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What is a 12-volt battery in an EV, and why is it necessary?

In the exciting world of today’s electric vehicles, the focus is almost always on the massive, high-voltage (HV) lithium-ion battery pack –  the heart that drives the vehicle silently and efficiently. However, this advanced powerhouse cannot function in isolation. The 12-volt battery is tucked away, often out of sight, but it is essential to the operation of every battery electric car. This seemingly common low-voltage battery is the hidden hero, performing a host of critical tasks before the main traction battery even comes online, and continuing to support essential functions throughout your drive. 

So, what exactly is this 12-volt battery in an EV? In most electric cars, particularly earlier models and many current ones, it’s very similar to the battery you’d find in a traditional internal combustion engine (ICE) vehicle. Commonly, it’s a lead-acid battery, often an AGM (Absorbent Glass Mat) type, which is sealed and maintenance-free, making it suitable for various mounting locations within the vehicle. 

Some newer or premium EVs are now transitioning to using 12-volt lithium-ion phosphate (LFP) batteries, which offer benefits like lighter weight and longer lifespan, but the core principle of providing 12-volt DC power remains the same. Its capacity is much smaller than the HV traction battery, typically ranging from 30 to 60 Amp-hours (Ah), compared to the hundreds of Ah found in the main pack.

The 12-volt battery plays an important role due to its wide range of essential operations. Firstly, it’s responsible for “booting up” the EV. When you press the power button or unlock the car, the 12-volt battery springs into action, powering up the vehicle’s various electronic control units (ECUs), the dashboard displays, the infotainment system, and performing initial system checks. Think of it as the battery that wakes up the car’s brain.

Most importantly, one of the most important tasks of the 12-volt battery on an electric vehicle is to energize the contactors. Contactors are heavy-duty electromechanical switches that connect the high-voltage traction battery to the electric motor and other HV systems. Without the 12-volt supply to close these contactors, the HV battery remains isolated, and the EV cannot enter a “ready to drive” state. This is why a dead 12-volt battery will leave you stranded, even if your main traction battery is 100% charged.

Another vital role of the low-voltage battery is to provide a stable power source for critical safety systems in the event of an HV battery disconnect, such as during an accident. If the main HV system is compromised or automatically shut down for safety, the 12-volt battery ensures that hazard lights, emergency call systems (eCall), and door locks can still operate. It also maintains power for memory functions in various ECUs when the car is off.

In simple terms, the 12-volt battery on an electric vehicle acts as a bridge, enabling the sophisticated high-voltage systems to come to life and then supporting all the ancillary systems that make a car functional, safe, and comfortable. Its health is critical to the smooth running of the entire EV ecosystem.

Everything You Should Know About 12-Volt Batteries on Electric Vehicles

How to charge and maintain a 12-volt battery in an electric vehicle?

The 12-volt battery is a common reason for curiosity and uncertainty among new EV (Electric Vehicle) owners. Specifically, how does this critical low voltage battery get charged in a vehicle without a standard alternator powered by an engine? What efforts may be taken to keep it healthy and extend its life inside the unique ecology of a battery-electric vehicle?  

In an EV, the 12-volt battery is primarily charged by a component called a DC-DC converter. This complex piece of power electronics takes high-voltage DC power from the main traction battery (which can range from 300 to over 800 volts, depending on the EV model) and “steps it down” to a lower voltage, typically around 13.5 to 14.5 volts, suitable for charging the 12-volt battery and powering the vehicle’s low-voltage systems. Think of the DC-DC converter as the EV’s equivalent of an alternator, but instead of being mechanically driven by an engine, it’s electronically powered by the main HV battery.

The charging of the 12-volt battery via the DC-DC converter generally occurs under several conditions:

• When the EV is “On” or in “Ready to Drive” Mode: As soon as you power up your EV to drive, the DC-DC converter becomes active, supplying power to all the 12-volt systems and simultaneously topping up the 12-volt battery as needed.
• During HV Battery Charging: When your EV is plugged in and charging its main traction battery, the DC-DC converter is also typically active. This ensures that the 12-volt battery is maintained or recharged while the HV battery is also receiving its charge. This is an important distinction from many ICE vehicles, where the 12V battery only charges when the engine is running.
• Periodically When the EV is Off (Maintenance Charging): Many modern EVs have intelligent battery management systems that monitor the state of charge of the 12-volt battery even when the vehicle is parked and turned off. If the system detects that the 12-volt battery’s charge is dropping too low (due to parasitic drain from systems like telematics, alarms, or keyless entry), it can automatically activate the DC-DC converter for short periods to bring the 12-volt battery back up to a healthy level. This process draws a small amount of energy from the HV traction battery. You might occasionally hear faint humming or clicking sounds from a parked EV; this can sometimes be the DC-DC converter at work maintaining the low voltage battery.

One common concern among EV owners is whether the charging of the 12-volt battery significantly depletes the main HV traction battery, thereby reducing driving range. In general, the energy required to maintain the 12-volt battery is relatively small compared to the total capacity of the HV battery and the energy used for propulsion. The DC-DC converters are also quite efficient. 

While there is some energy use, its impact on the overall range is typically negligible for a healthy 12-volt battery system. However, if the 12-volt battery is old, failing, or there’s a significant parasitic drain, the DC-DC converter will have to work more often and for longer periods, which could lead to a slightly more noticeable draw from the HV battery over time, particularly if the vehicle is left parked for extended periods.

What happens if the 12-volt battery dies completely? As discussed previously, the EV will not “start” or enter ready mode. The DC-DC converter itself usually requires a minimum 12-volt supply to activate. So, if the 12-volt battery is too flat, the DC-DC converter cannot start to recharge it from the HV battery. This is why jump-starting or externally charging the 12-volt battery is necessary in such a scenario.

Maintenance tips for the 12-volt battery on an electric vehicle share some similarities with ICE vehicles, but with a few EV-specific nuances:

• Regular Checks (If Accessible): If your EV’s 12-volt battery is accessible, periodically check its terminals for corrosion. Clean, tight connections are vital. Ensure the battery is securely mounted. Most 12-volt batteries in EVs are sealed AGM or lithium-ion types, so they don’t require checking electrolyte levels.
• Understand Warning Signs: Be attentive to any dashboard warnings related to the battery or electrical system. Dimming lights, slow power windows, or infotainment glitches can be early indicators of a weakening 12-volt battery.
• Typical Lifespan Awareness: The lifespan of an EV’s 12-volt battery can sometimes be shorter than in an ICE car, often cited as 2-5 years, though this varies greatly by EV model, battery type (lead-acid vs. lithium-ion), climate, and usage patterns. Lead-acid batteries generally have a shorter lifespan than the 12V lithium-ion batteries now appearing in some newer EVs. Heat is a major enemy of lead-acid batteries, so vehicles in hotter climates might see shorter 12V battery lives.
• Driving Habits and Parking Duration: If an EV is driven very infrequently or only for extremely short trips where the DC-DC converter doesn’t have much time to operate, it could potentially put more strain on the 12-volt battery, especially if there’s a moderate parasitic drain. Conversely, if an EV is left parked for many weeks or months without being driven or plugged in, the periodic maintenance charging of the 12-volt battery will gradually deplete the HV traction battery. Most EVs have a “deep sleep” or “transport” mode to minimize this for long-term storage, but the 12-volt battery can still self-discharge over time.
• Software Updates: Vehicle manufacturers sometimes release software updates that can improve the management of the 12-volt battery charging system, addressing issues like excessive parasitic drain or optimizing the DC-DC converter’s operation. Keeping your EV’s software up to date can be beneficial.
• Avoid Draining Accessories When “Off”: While the car might allow you to use accessories with the car not in “ready” mode (sometimes called “accessory mode”), be mindful that this directly drains the 12-volt battery. If done for extended periods without the DC-DC converter active, it can deplete the battery. It’s generally better to have the car fully “on” if you plan to use high-power accessories for a long time, so the DC-DC converter can support the load.
• Consider a Battery Tender for Long-Term Storage: If you plan to store your EV for a very long period (months) and are concerned about the 12-volt battery, you could consider using a smart 12-volt battery tender/maintainer, provided your vehicle manufacturer doesn’t advise against it. Always connect it according to the manufacturer’s instructions for both the tender and the vehicle.

Some advanced EVs, particularly those with 12-volt lithium-ion batteries, incorporate more advanced Battery Management Systems (BMS) for the low voltage battery, similar to the BMS for the main HV pack. These systems can offer better monitoring, balancing, and protection, potentially extending the battery’s life.

Common Issues and Troubleshooting for Your EV’s 12 Volt Battery

While the high-voltage traction battery in an EV receives the most attention, the 12-volt battery can often be the cause behind a surprising number of “no-start” situations or unusual electrical gremlins. Understanding the common problems associated with this low voltage battery and knowing basic troubleshooting steps can save battery electric vehicle owners significant time, frustration, and potentially a call for roadside assistance. 

One of the most common and definitive symptoms of a failing or dead 12-volt battery in an EV is the vehicle’s inability to “start” or enter the “ready to drive” mode. You might press the power button, and nothing happens, or you might get a cascade of error messages on the dashboard. Unlike an ICE car where you’d hear a struggling starter motor, an EV might simply remain inert. This is because the 12-volt battery is responsible for powering up the vehicle’s computers and, crucially, energizing the contactors (heavy-duty relays) that connect the high-voltage battery to the rest of the system, including the powertrain. Without sufficient 12-volt power, these contractors cannot close, and the car remains idle.

Other telltale signs include flickering dashboard lights or interior lights that are dimmer than usual. The infotainment system might fail to boot up, behave erratically, or reset itself frequently. Power windows and power locks might operate sluggishly or not at all. You might also see specific warning messages on the instrument cluster referring to the “electrical system,” “12V battery low,” or other similar alerts. In some cases, the key fob might not be able to unlock the car, or the charge port door might refuse to open, preventing you from even charging the main traction battery. These symptoms can sometimes be inconsistent at first, making them difficult to notice, but they usually worsen over time as the 12-volt battery degrades further.

When faced with a suspected 12-volt battery issue, there are several troubleshooting steps you can take. First, if accessible and you feel comfortable doing so, a visual inspection of the battery terminals for corrosion can be helpful. Clean, tight connections are essential for good electrical flow. However, many EVs make the 12-volt battery less accessible than in traditional cars, so this may not always be an option for every EV owner.

The next logical step, and often the most effective for a completely dead 12-volt battery, is jump-starting. Yes, you can jump-start the 12-volt battery on an electric vehicle, much like an ICE car, but there are critical differences and safety precautions. Most EVs have specific jump-start terminals under the hood or in a designated fuse box area, which might not be the battery itself. It is important to consult your owner’s manual to locate these correct jump points and understand the specific procedure for your model. 

Using the wrong points could damage sensitive electronics. A portable jump starter pack designed for 12-volt systems can be a lifesaver in these situations, offering more convenience than using another vehicle. When jump-starting, always connect the positive (red) cables first, then the negative (black) cable to the donor vehicle or jump pack, and finally, the negative cable to a designated ground point on the EV (again, always refer to the manual), away from the battery itself to avoid sparks.

With a multimeter, you can also check the voltage of the 12-volt battery.  A healthy, fully charged lead-acid 12-volt battery should read around 12.6-12.8 volts when the car is off. If it’s below 12.2 volts, it’s significantly discharged, and below 12 volts, it’s likely sulfated or failing. For EVs using a 12-volt lithium-ion battery, the voltage readings might differ slightly, so consult specific documentation if available.

A significant issue that can plague the 12-volt battery in an EV is a parasitic drain. This refers to a continuous, small electrical draw on the battery when the vehicle is turned off. While all cars have some level of parasitic drain to power systems like keyless entry receivers, alarm systems, and memory for various modules, an excessive drain can prematurely deplete the 12-volt battery. 

Common causes for the abnormal parasitic drain in an EV include additional accessories improperly installed (like dashcams or tracking devices wired directly to the battery), faulty electronic modules that don’t “go to sleep” as they should, or even software glitches. Diagnosing excessive parasitic drain can be complex and often requires specialized tools to measure current draw over time. If you frequently find your 12-volt battery dead after the car has been parked for a few days, a parasitic drain could likely be the reason.

Sometimes, issues mimicking a 12-volt battery problem can be software-related. A simple “reboot” of the car (turning it completely off, waiting a few minutes, and turning it back on) can sometimes resolve temporary glitches. For more persistent issues, a dealership visit might be necessary for software updates or more in-depth diagnostics.

If basic troubleshooting doesn’t resolve the issue, or if you’re uncomfortable performing these steps, it’s always best to seek professional help from a qualified EV technician or your dealership. They have the diagnostic tools and expertise to accurately identify the root cause, whether it’s a failing 12-volt battery, a charging system problem (like a faulty DC-DC converter), or an excessive parasitic drain. Attempting complex electrical repairs without proper knowledge can be dangerous and potentially void your vehicle’s warranty. Proactive monitoring of your 12-volt battery’s health and understanding these common problems can ensure your battery electric vehicle remains reliable and ready for the road.

More from Carnex Blog:

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As always, Carnex can provide you with detailed information about buying and selling used electric vehicles in Ontario.

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