Why Your Car Won’t Start With a Jump?

Jumper cables on car.

As an Amazon Associate, Modded gets commissions for purchases made through links in this post.

Getting stuck with a dead battery can range from a mild annoyance to a dangerous or even life-threatening situation. We hate getting stranded for most of us, but that is why we keep a set of jumper cables in the trunk. Usually, we can find a good samaritan willing to pop their hood and hook their car to ours, but it doesn’t always work. What can it mean if your car won’t start with a jump?

What Causes Batteries to Die?

The average car battery will last between three and five years under the hood. Other than age, what causes batteries to stop functioning and leave you stranded?

Deep or Frequent Discharges

Automotive batteries can discharge now and then, but running them empty and recharging them will shorten their lifespan. 

Extreme Heat

Lead-acid automotive batteries use a diluted acid as their electrolyte. Exposure to excessive heat can cause the water to evaporate, which will weaken its charge and can cause the lead plates to rust as the acid gets replaced with oxygen. 

Extreme Cold

Automotive batteries lose much of their starting power when they get cold. Once it gets to 0 F, it can lose up to 50% of its power. Exposure to cold weather can also decrease the lifespan of the battery.

Excessive Vibration

Automotive batteries can withstand some vibration, but they can handle only so much. Too much vibration can cause the cell connectors to break apart, interrupting the circuit and causing it to stop charging. 

Fast Charging

Waiting for a discharged battery might take a while, but running a fast charger too frequently can damage the internal components and shorten the battery’s lifespan.


The very best outcome from overcharging is just a dead battery. The other potential side effects can be downright dangerous. Overcharging your battery can boil the acid electrolyte mixture. The heat causes the water to evaporate and, in extreme cases, can even melt or warp the battery case. 

Testing Your Battery

There are two different ways to test your battery. The first requires a multimeter, which you may already have in your garage. The second requires a load tester. If you don’t have one of these, you may be able to get your local auto parts store to test your battery for you. 

For the first test, set your multimeter to volts and touch the sensors to the positive and negative terminals of the battery. You should see a reading of 12.6 volts in a properly charged battery. Anything lower than that could indicate a problem. If you drop below 12 volts, the chances are pretty high that the battery won’t start the car. Once the vehicle starts, the multimeter reading should be between 14.4-14.6 volts if the alternator is functioning properly. 

The second test puts the battery under a load to simulate when the car starts. From the 12.6 volt baseline, your battery shouldn’t drop below 12 volts when placed under a load. Even if the battery starts the vehicle, a low reading could indicate a dead or dying cell and might tell you that the battery needs replacement. 

Troubleshooting Jump Failure

If your battery dies and it doesn’t start when you hook up the jumper cables, what could be causing the problem?

The battery is completely dead. 

If your battery is empty, no amount of time hooked up to the jumper cables will make the car start. 

The battery terminals are damaged or corroded.

Corrosion or damaged terminals prevent electricity from flowing freely through the cables and will prevent them from being able to start your car. 

The starter isn’t working.

It isn’t always the battery that causes the problem. If the starter fails, the car will not start. 

The neutral safety switch is failing.

The neutral safety switch tells the engine computer when the transmission is in park or neutral. The engine won’t start if this switch fails, even if the transmission meets all other parameters. 

The fuel pump isn’t working.

Without fuel, the engine won’t start. If the fuel pump fails and the gasoline can’t make it from the tank to the engine, the car won’t start, and jumper cables won’t make any difference. 

The jumper cables are failing.

When was the last time you used your jumper cables? They can deteriorate over time. If the insulation breaks down, it can be dangerous and cause electrocution. 

Keeping Your Car Running

Having a set of jumper cables in your trunk can be handy if you have a dead battery, but it won’t always solve the problem. If the battery is entirely dead or there are other points of failure in the vehicle, you won’t be able to rely on your jumper cables.