Can you Jump a Car Battery with an Extension Cord?

Have you ever found yourself running late with a dead car battery and wondered if you could use an extension cord to give yourself a jump?

Car batteries cannot be charged with an extension cord from a wall outlet without a charger as an intermediary since they operate with direct current (DC) power and are rated at 12-volts. Wall outlets operate with alternating current (AC) power and are 120 volts. The power types and outputs are not compatible.

Attempting to strip the end of an extension cord of the female end to expose the wires and attach them to the terminals of a car battery will certainly result in massive property damage, likely bodily harm, and possibly death.  

In order to use AC power to jump your car battery, it must first be converted to DC power and then reduced to the proper charging range to avoid massive overcharge which will destroy your battery and almost certainly result in fire or explosion.  

What is the difference between DC and AC power?

The primary difference between AC and DC power is the flow behavior of the electrons in the circuit.  Alternating current, when plotted on a graph, would show peaks and valleys as a visual representation. 

The peaks represent when the flow of the electrons are going in one direction, and the valleys depict when they reverse and flow in another direction.  

The electrons in an AC current rapidly alternate back and forth in the circuit to give you the power to utilize your devices and appliances, hence the name alternating current.

Direct current, on the other hand, would be plotted on a graph to depict a straight line.  The electrons move only in one direction (from the negative terminal of a battery to the positive side and through any device that you hook up to it).  

These are two very different types of power and cannot be used interchangeably without a device specifically designed to convert the forms of energy, such as a DC-to-AC power inverter, an AC-to-DC converter, or the AC adapter on your laptop or TV cords.

What voltage does my car battery have to be to start my car?

Despite the fact that your car battery is labeled as being 12 volts, that does not mean that it is charged when a reading of it indicated 12 volts.  If that were the case, a 12-volt reading would mean that your battery was only about 45% charged and would likely not be able to start your car.

Your car battery should ideally have a voltage reading of 12.6-12.7+ when at rest.  A reading in this range would indicate a charge of 95-100%. When you start to get around 12.1 (50% charged) it starts to become a coin toss of whether or not it will turn over when you crank the key.

Can I substitute another battery to get it started and then replace the old battery when it’s running?

Perhaps you have a marine battery in your garage and you want to know if you can get the car running with it, and then disconnect it once running to hook up the original battery so that the alternator will charge it.  

Unfortunately, no.  You risk causing thousands of dollars of damage to your car’s computer system and electronics if you attempt to swap batteries while the car is running.  It’s a great idea in theory but will damage your sensitive electronics that are everywhere within your car if you attempt it.

How much would a wall outlet overcharge a battery?

Overcharging a car battery by only a couple volts is enough to get your attention.  Even in controlled overcharges, such as when a charger initiates a desulfation or equalization process, the battery will be brought up to 15.5-16.4 volts.  At that rate, you will hear the battery vigorously bubbling from across the room and it will be rapidly venting out gases such as hydrogen, oxygen, and hydrogen sulfide.

Adding more of a charge on the battery would rapidly increase the odds of structural breakdown as the battery would not be able to absorb the increased electricity (a controlled overcharge already means that it can’t fully absorb the charge as is). The battery would likely become extremely hot, the side walls would buckle, and the battery would melt and/or explode resulting in a fire or with sulphuric acid being thrown everywhere.

Even if the wall outlet was in direct current like the car battery, the 110-120 volts supplied is simply out of the question to be hooked up to a 12-volt battery that can barely tolerate 16.5 volts.  Put plainly, it would be a messy way of attempting suicide.

How can we safely convert wall outlet AC power to DC power to charge a car battery?

If you cannot get a jump from another car’s good battery using a set of jumper cables, you can certainly use the wall outlet to charge your car.  The caveat is that you must have the proper tool to convert the high voltage AC current to a low voltage DC current. 

The most common tools for this include a standard battery charger, a battery charger with a jump start feature, or a portable jump start battery pack.  

Battery chargers are rated in amps, and the math is fairly simple to figure out how quickly it will charge your battery.  

The average car battery is about 45 AH (ampere-hours). Say we fully discharged that battery so that it is at (zero) 0AH, and probably has a voltage of 10.5.  

If you have a battery charger that is rated at 30 amps, it will roughly replace 30AH in your battery per hour.  So, it will take roughly 1.5 hours to come up to a full charge, but you will likely only need 50-55 minutes to turn the engine.  After that, the car’s alternator will charge the battery as you drive around.

If you’re in a hurry, a standard smart charger is probably not the most ideal choice.  However, it is always handy to have one around. There are also smart chargers that have a jump start feature (such as this one seen here on Amazon). They are affordable and would be a great option for you to invest in.  You’ll never need to interrupt a neighbor or friend’s day to have them drive over to give you a jump again!

Finally, there are portable jump start battery packs.  These are essentially a 12-volt battery with jumper cables and some bells and whistles attached to it (lights, USB ports, etc.).  They can also be a lifesaver, but remember to keep them charged at all times and to charge them immediately upon purchase! 

This is one on Amazon with extremely high ratings and it comes in different sizes.  Best of all, you can fill out your car’s details on Amazon to see if this would be compliant or if a different would be best!  

It also can charge your cell phone and other small devices as well.  It’s definitely worth a look and could come in very handy when boating or camping as well!

When you purchase portable jump starters you do not know how long that battery has been stored in the box (this is critical if you get a model that has a lead-acid battery).  It has been slowly self-discharging (losing its charge) since it left the manufacturer. 

If you let it get low enough (around 12.4 volts) it will begin to sulfate.  The battery will then begin to lose its conductivity and its lifespan will be reduced.  The longer you keep it in a sulfating state, the more irreversible the damage will be.

Final Though:

In the end, you can certainly use that extension cord to help get your car started, but you just need to use a battery charger or a portable jump start battery pack to plug into first!

Robert Van Nuck

Robert lives in central Michigan and enjoys running, woodworking, and fixing up small engines.

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