What Happens When Your Hybrid Car Battery Dies?

While electric vehicles (EVs) have come a long way since their inception and are now some of the most satisfying vehicles to own, plenty of people would rather use a hybrid that combines an internal combustion engine with an electric motor. It's the best of both worlds. However, while most people know that you can't drive a regular gas-powered car without any fuel or an EV without a charged battery, what happens when a car that uses both runs out of electricity? Turns out hybrids use batteries for more than just momentum, and if the battery dies, your car may die along with it.

Hybrid vehicles maximize fuel efficiency by switching between a gas-powered engine and an electric motor in different situations. At lower speeds, the car utilizes the battery-powered motor, but when you drive faster or climb hills, the gas engine takes over. Hybrids know which method is the most efficient at any given time and swap between them accordingly. 

Like standard gas-powered cars, hybrids have a small 12-volt battery that runs basic electronics and helps wake the car up. If that 12-volt battery dies, the car generally will not start, no matter how much gas is in the tank. But they also have a separate high-voltage traction battery, which is more like the large battery pack people think of in an EV. The trickier question is what happens when this bigger battery starts failing or dies, because it's what powers the electric motor and supports the hybrid system while you drive.

Even with a dead or failing high-voltage traction battery, some hybrids can still run, at least in a limited mode. These include plug-in hybrid vehicles (PHEVs), cars that have parallel systems like the Prius, and older models such as the Honda Civic Hybrid. However, to paraphrase Dr. Ian Malcolm from "Jurassic Park" (played by Jeff Goldblum), just because you can do something doesn't mean you should.

While hybrid vehicles rely on their gas engines when traveling at high speeds, these components receive an extra nudge from the electric motors because they aren't as powerful as the engines in straight-up gas-powered cars — a necessary evil in the hybrid vehicle design. Without a battery to run the electric motor, hybrid performance takes a nosedive because the engine doesn't have the strength to work on its own and consumes extra gas in a futile attempt to do so. To add insult to injury, driving hybrid vehicles without a battery puts additional strain on many of the car's systems, especially the engine.

Even if your hybrid car can technically drive without a battery, don't take the risk. Plug in your hybrid car whenever possible (assuming you can) to maintain its fuel efficiency and component health.

Given all the ways a dead power cell can hamstring a hybrid car, you should definitely keep your battery as healthy as possible and change it out before it flatlines. While some techniques that help extend EV driving distances, such as utilizing regenerative braking, can help, you should always be wary of indicators that your hybrid battery is on its way out.

Many of the warning signs are kind of obvious. Poor fuel economy, a significantly reduced range, and charging problems are all symptoms of a dying battery. Strange noises are also a good indicator — listen for annoying electrical hums, pops and clicks, or an unusually loud battery-cooling fan. However, the most reliable indicator is also the most obvious: the warning light. If the "Check Hybrid System" light comes on (often a triangle surrounding an exclamation mark), that's your sign to take your car into the shop. 

Of course, the light won't tell you if the problem lies with the battery or the hybrid system itself, but you need to fix the issue either way. While swapping out a new battery is expensive — usually thousands of dollars — it's better to catch a failing battery early and bite the bullet on your own time rather than finding out your hybrid car's battery is dead when you need it the most.