My EV Is Dead, What Do I Do?

There's no need to panic if your EV won't start. Just be sure you know about both of your EV's batteries, and how to power them up safely.

With electric vehicles (EVs), most of the talk is about the high-voltage battery that drives the motor. How long do they last? How expensive are they to replace? How many miles you can travel between charges? And what happens to them once they reach end-of-life?

You may not know EVs come with a good old-fashioned 12-volt lead-acid battery. (Tesla uses a lithium-ion 12-volt battery.) Like its internal combustion engine (ICE) cousin, an EV will not start if its 12-volt battery is dead.

Luckily, jump-starting an EV 12-volt battery is virtually the same as jumping an ICE vehicle battery, with some really important safety considerations. Here’s what you need to know before throwing away those jumper cables.

Why Do EVs Have 12-Volt Batteries?

To power the accessories, lights, wipers, power windows and seats, heater blower fan, modem and airbags. It also powers the starting circuit and low-power computer systems that manage the high-voltage system.

Pushing the start button activates the motor controller relay (or contactor, a powerful electromagnet). That allows power to flow from the high-voltage battery to start the vehicle and electric drive motor.

Because EVs don’t have an alternator, the controller also recharges the 12-volt battery. A dead 12-volt battery cannot trigger the relay, which keeps an EV from starting, even if the high-voltage battery is fully charged.

How Do I Know the 12-Volt Battery Is Dead?

Just the same as on a traditional vehicle. If the power doors don’t open, the key fob doesn’t work or the brake lights don’t come on when depressing the brake pedal, it usually signals a dead battery.

A dead 12-volt battery can be caused by an EV sitting unused for a long time. The low voltage system in most newer vehicles is always “on,” powering various computers and control modules, even when the vehicle is “off.” A courtesy light accidentally left on, a malfunctioning low-voltage charging system, or a failing battery are also common causes.

Where Is the 12-Volt Battery?

It depends on the year, make and model of the EV.

Most 12-volt EV batteries can be found in or behind the frunk, usually hidden by a plastic trim panel. They can also be in the trunk. Check the owner’s manual if you’re unsure.

Some EVs come with remote battery terminals to connect the jumper cables. Remote battery terminals are deliberately placed away from the battery to reduce sparkling. If the battery is completely dead and you cannot open the doors, some manufacturers place a connector behind a removable bumper plug to power the battery. That allows you to open the doors.

Steps To Jump-Start an EV With a Dead 12-Volt Battery

Safety first: This information deals only with jump-starting an EV’s low-voltage, 12-volt battery.

NEVER attempt to jump-start an EV’s high-voltage lithium-ion battery. NEVER jump-start an EV’s 12-volt battery while charging the high-voltage battery. NEVER use another EV as a “helper” car to jump-start another EV or any other vehicle.

Always wear gloves and eye protection. Never attempt to jump-start a frozen battery, and never smoke when working around a battery. A frozen battery or sparks can cause the battery to explode. If you come into contact with battery acid, flush with plenty of water and get medical attention immediately.

Follow these steps using a quality set of jumper cables or a spark-proof portable jump starter with reverse polarity protection. NOTE: Only use a portable jump starter, never a lead-acid battery, to jump start a Tesla’s 12-volt lithium-ion battery.

Safely position the vehicles close enough to connect the jumper cables

To prevent a short circuit, never let the vehicles touch.

Prep the vehicles

Place the transmissions of both vehicles into park or neutral and fully set the parking brakes. Switch off all lights and all accessories in both vehicles. Check that all battery terminals are clean and tight.

Connect the positive (+) clamps

Attach one red positive (+) jumper or jump starter cable clamp on to the dead battery’s positive (+) terminal or remote positive (+) battery terminal. Attach the other end of the positive (+) jumper cable to the positive (+) terminal of the helper vehicle battery.

Connect the negative (-) clamps

Attach one black (-) jumper cable clamp to the helper battery’s negative battery (-) terminal. Attach the other end of the negative (-) jumper cable or jump starter to the remote negative (-) battery terminal or clean metal part only. NEVER connect the black (-) jumper or jump starter cable clamp directly to the dead battery’s negative (-) terminal.

Perform a visual check

Make sure the jumper cables or jump starter are away from moving parts, and the clamps are tightly latched onto the terminals.

Start the helper vehicle

Let the helper vehicle run for five minutes to charge the dead battery.

Start the EV

Once the EV starts, leave the cables connected to the helper car or jump starter for five to 10 minutes to ensure the EV stays running.

It is absolutely critical to remove the cables in the exact reverse sequence:

  • Negative (-) cable from the vehicle that was dead;
  • Negative (-) cable from the helper vehicle;
  • Positive (+) cable from the helper vehicle;
  • Positive (+) cable from the vehicle that was dead.

After disconnecting the cables, let the EV run for about 20 minutes. This will help charge the low-voltage battery, plus stabilize the entire electrical system.

If it doesn’t start, shut off helper vehicle’s engine, recheck all cable connections, wait five minutes and then try again.

Bob Lacivita
Bob Lacivita is an award-winning ASE and General Motors auto technician, educator and freelance writer who has written about DIY car repairs and vehicle maintenance topics. His work has been featured in The Family Handyman, a Reader's Digest book and Classic Bike Rider magazine. He has been a career and technical educator for 25 years teaching automotive technology, as well as writing state, federal and organizational foundation grants. He also helped design a unique curriculum delivery model that integrates rigorous, relevant academic standards seamlessly into career and technical education.