How to Charge a Car Battery

An automotive mechanic using a jump cable to jump start a dead car battery. — A car battery charger is an essential tool to have in your car; it may save you when you least expect it. warodom changyencham / Getty Images

Learning how to charge a car battery is a critical skill for any driver, ensuring that your vehicle remains reliable and ready to hit the road at a moment's notice. Whether you're dealing with a dead battery or just performing routine maintenance, understanding the correct and safe way to charge a battery is essential.

Fortunately, we've got you covered. In this article, we'll walk you through the process of charging a car battery step-by-step. Even if you're a beginner, you'll be confidently charging car batteries like a pro in no time!

Contents

Reasons Why Car Batteries Die
The Deal With Positive and Negative Terminals
Use Caution When Dealing With a Car's Battery
How to Charge a Car Battery

Reasons Why Car Batteries Die

One of the most common reasons for a dead battery is improper charging practices. Both overcharging and undercharging can significantly reduce a battery's lifespan. Overcharging can lead to excessive heat and pressure build-up within the battery, causing it to deteriorate faster. On the other hand, undercharging results in a partial state of charge, leading to sulfation, where lead sulfate crystals form on the battery’s plates, diminishing its capacity and performance.

It's also important to be mindful of accessories that could drain your car's battery even when the vehicle is off. To prevent these issues, it's crucial to follow the manufacturer’s guidelines for charging and use a reliable charger that can regulate the charge appropriately. Regular checks on the charging system and battery health can also play a pivotal role in ensuring your car battery receives the right amount of power, prolonging its life and ensuring consistent performance.

The Deal With Positive and Negative Terminals

In a car battery, the positive and negative terminals play critical roles in the flow of electrical current. The positive terminal, typically marked with a plus sign (+), is connected to the battery's cathode. This is where the electrical current flows out from the battery to the vehicle's electrical systems.

On the other hand, the negative terminal, marked with a minus sign (-), is connected to the battery's anode and serves as the point where the electrical current returns to the battery after circulating through the vehicle's systems. This flow of current is essential for starting the engine and powering electrical components.

The chemical reactions inside the battery between the lead plates and the sulfuric acid electrolyte facilitate this flow of electrons from the negative to the positive terminal, creating a circuit that powers the car's electrical needs.

Use Caution When Dealing With a Car's Battery

A car battery contains chemicals that produce hydrogen gas during use. Hydrogen gas is volatile and has been known to explode under certain conditions, causing serious injuries. For example, a car battery may explode while starting the car, while jump starting or by carelessly shorting the terminals with a screwdriver. In fact, under certain conditions a car battery may explode while just sitting in a parked car or on a table [source: battery-chargers].

How to Charge a Car Battery

Keeping all of that in mind, here's how to safely use a car battery charger:

Use the correct charger for your particular car battery. Check the car owner's manual for information (or ask your car dealer or trusted auto mechanic).
Read the charger's instruction manual and make sure you understand how the charger works and what precautions are necessary. Pay close attention to all the warnings.
Remove both battery cables from the battery terminals. First remove the negative cable (the black one) and then the positive cable (the red one).
Check that the charger is not plugged in to an electric outlet and that the power switch is off.
Connect the positive (usually red) charger cable to the positive (+) battery terminal and the negative (usually black) cable to the negative (-) battery terminal.
Decide whether you want to charge the battery slowly (trickle charge) or quickly. Select a lower charging voltage and current for a trickle charge and a higher setting for a quicker charge. Check the charger manual for the proper settings. Note that trickle charging gives the best results.
Begin the charging process. Select whether the charger will run for a specific amount of time or automatically shut off when the battery is charged. Not all chargers have this feature.
Leave the charger or cables in one place while the battery is charging.
Turn off and unplug the charger when the battery is fully charged.
Disconnect the cables, starting with the negative cable, and then moving on to the positive cable [source: CarsDirect].

This article was updated in conjunction with AI technology, then fact-checked and edited by a HowStuffWorks editor.

Car Batter Charger FAQ

Can a battery charger jump-start a car?

A battery charger is designed to deliver a small charge over a long period, allowing the battery to absorb the charge and maintain a voltage and amperage. The charger does not create enough amperage immediately to satisfy the needs of the starter motor, and will be unable to jump-start the car.

Can I start my car while it's on the battery charger?

Yes, but getting enough charge into a flat battery might take some time.

What should I look for when buying a car battery charger?

When buying a car battery charger make sure that the charger is 10% minimum of your battery's Ah rating. For example, if your battery is a 100Ah battery, your charger should be a 10 Amp charger minimum. That said go for a greater amp charger if you want to charge the battery faster.

How do you charge a battery with a battery charger?

Usually, most batteries do not need a recharge for at least five years of their use, provided you are taking good care of your car. Before anything, first, check that the charger is unplugged. Now remove the battery cables by removing the negative cable first. Connect the positive charger cable to the positive battery terminal and the negative to the negative terminal. Select a charge setting (low or high). Leave the charger and cables while the battery is charging. Now turn off the charger and disconnect the cables.

Do you need to disconnect a car battery to charge it?

It's always a good idea to disconnect a car battery before charging.