How to Use a Car Battery Charger – Step by Step, Safely and Effectively

Have you recently bought a car battery charger, but have never used one before?

If you own a car, there’s a good chance you’ve experienced the frustration of a flat battery. And although it happens all year round, winters can be quite tough on batteries.

Chargers can either top up your battery or jumpstart your vehicle when needed, so keeping a charging device in your trunk can avoid being stranded.

But do you know how to use a car battery charger?

Car battery charger clamps connected to a battery inside a car engine compartment

Misuse of a charger can result in minor burns and other incidents while operating the device. Following the correct procedure and taking safety measures while using it is necessary.

In this article, we’ll explain what a charger is and cover the tools needed to operate it. By the end of this guide, you’ll know all the steps required to safely prepare, use, and to finish disconnect and store your charging device.

Skill level: Beginner.

Time to Complete: 2–24 hours depending on the charge speed.

Tools needed: Before using your charger, ensure you have the required tools and gear to operate it safely.

  • The proper charger with its instructions.
  • Insulated cable set with its storage case—if they don’t already come with the charger.
  • Thick gloves and safety glasses.

Choosing the Right Charger

Not all battery chargers are the same, and there are a few things to keep in mind when choosing the right one for your vehicle(s).

What is a battery Charger?

A charger is a convenient device, often small enough to fit most cars’ trunks. Versatile, it can be used to either increase your battery charge or to jumpstart your car.

There are many reasons why your battery may fail, such as cold weather, old age, or accidentally keeping the lights on overnight. Planning ahead for unforeseen circumstances can save you a lot of time and frustration.

How do you know if your battery needs a little help? Dim lights when running, or even the lights dimming when starting your car, are a sign that the charge is low.

A hydrometer might also be a useful tool to check how much charge is left but is a bit of a specialist tool that not many will own or even need.

How Does a Charger Work?

A battery doesn’t collect and store electricity. Energy is created through chemical reactions, converting it into electricity. Inside a car battery, you’ll find lead plates and sulfuric acid.

A chemical reaction between the lead and acid pushes electrons to travel from the negative to the positive plates, creating an electrical current. But this reaction is finite, and once enough electrons have travelled from the positive plate to the negative, a balance is reached, and no more charge will flow.

So what does a charger do? Simply put, it reverses the direction of the electrons. During charging, they travel from the positive to the negative plates, effectively recharging the battery.

A charger comes with both a rectifier and a transformer. This enables the device to change the mains voltage and AC current of 110 or 220 volts, into a DC current of 12 volts, matching and able to charge a vehicle battery.

What Makes a Good Charger?

Before using or purchasing one, here are a few things worth considering:

The Right Charger for the Right Battery

Before using a charger for the first time, or purchasing a new one, ensure that you’ve checked the following two points.

First, you’ve read the instructions thoroughly.

Secondly, the charging device is the right fit for your car.

Not all chargers fit every battery. Batteries come in a variety of different types, from valve-regulated lead-acid to gel, and absorbed glass mat to lithium batteries. Each requires a particular kind of charger.

Larger cars—such as SUVs—need larger batteries. To charge the battery of an SUV, you might need a charging device providing at least 75 amps at a time.

And carefully read the product features, as they don’t all come with a jumpstart functionality, which is something we recommend to look for.

Charging Time

Most chargers should allow a charge between two to 10 amps per hour. Some even provide a charge of 40 amps per hour. Unless too old—in which case they won’t hold the current—most batteries can be recharged.

They often come with switches to allow both a slow or a fast charge. This can mean the charging time varies between two and 10 hours or more. We’ll expand on charging time further down.

The higher the output, the faster the recharge will be. These devices also tend to come at a higher price point.


You might want to consider where you’ll use your charging device. If you’re looking to charge at home, portability might not be an issue.

If you need it on vacation or at work, you’ll want to select one that will easily fit in your car. Smaller compact devices are usually more expensive, but they’re easier to carry around.

Gauge or Screen Display

Most charging devices come with a gauge. As you’re charging, the arrow should move from the red zone back to the green charging zone. While charging, this feature is handy to keep an eye on the charge level.

Other charging devices feature a screen display showing the percentage of charge left. The best ones will even inform you if the issue is coming from the device itself.

Manual Versus Automatic

When selecting a manual charger, you’ll have to monitor the gauge or screen display to know when to turn it off.

Some models come with an automatic shutoff system, turning off the device when the charge is complete. Others go into a trickle mode—slow charge rate—when the device is fully charged.

Although they come at a higher price point, these extra features aren’t just convenient, they’re also safer for your battery and should expand its lifespan.

Ease of Use

If you’re a novice to the car world, selecting a charger with clear instructions should make your job much easier. Look for warnings and safety advice as well.

It’s preferable to choose one with adapted tools, such as insulated cables and clamps. Don’t forget to consider the cables’ length as well. Are they long enough? Can you store them safely in a storage bag?

Please check out our article on the best car battery chargers for more information if you’re still looking to buy and need some advice.

How to Use a Car Battery Charger

OK, enough small talk, let’s get into how to use a charger.

Following is a step by step guide that should work with almost all chargers, as the instructions for their use is pretty much always the same.

Getting Your Battery Ready

Depending on the model of your car, batteries can be found in different places in your engine.

You’ll almost always find them under the hood, but rarely they can also be located below the side of a vehicle or even under the back seat. However, they are very easy to spot and find.

Removing Your Battery

Car batteries can give life-threatening electric shocks when not handled with proper care.

If you need to remove the battery to charge it – if you cannot charge it in situ – please adhere to the following steps:

  1. Make sure to use proper insulated work gloves and glasses.
  2. Ensure that the engine is turned off.
  3. Remove the ground/negative battery terminal/black clamp first.
  4. Next, proceed with the positive cable.
  5. And when placing it back after charging, the positive red clamp should be connected first, and the ground clamp should be the connection made last.

Know Your Battery’s Specifications

Your owner’s manual should provide you with the battery voltage. Most of the time, it’s even displayed on the device itself. Most batteries are 12 volts, but it’s best to check as some can be 6v or even higher than 12.

If you have a sealed-type device, follow your manufacturer’s specifications closely. Pay specific attention to any warnings instructions.

Ensure a Good Ventilation

Although some batteries cannot be moved, it’s best to remove it from the car and select a safe location for the charging process. Some cells can give off a noxious gas while charging, and particularly so if something was to go wrong. Some batteries can even generate hydrogen gas while charging. A lack of ventilation could lead to an explosion.

So please do ensure good ventilation around the device while charging. A wide-open area with a draft would be ideal.

Flammable substances—such as lighters or gasoline—should be kept away. A simple static electricity spark could have disastrous consequences.

Cleaning the Terminals

Before starting charging, check the terminals and ensure that they’re clean. If you notice a white powdery substance on the terminals, they need cleaning.

At this point, you should be wearing gloves. Sulfuric acid can cause burns when handled with bare hands.

Take a scratchpad or sandpaper to clean the battery terminals, followed by a wet cloth with baking soda. Make sure to get none of the liquid on either your skin or your clothes as it will cause damage.

Remove the Cell Caps

Some batteries come with cell caps. When not removed, the caps prevent gases from escaping, so these must be removed before starting.

The caps are often located underneath a removable yellow strip, on top of the device.

Water Refill

A battery without water might not be able to hold the charge, and those that have suffered a prolonged period without enough water may have suffered permanent damage.

Batteries can, however, be refilled with distilled water and brought back to life with a charge. It doesn’t always work. The success of the procedure will vary depending on how long it has been dry and the quality of the battery. How old the device is will also come into play.

Unfortunately, you’ll only find out once you start charging it.

Hooking up the Battery Charger

Now that we have both the battery and the charger ready, let’s get them connected.

Disconnect all Power

Turn off the engine as well as every device that could use up energy. These include headlights, internal lights, or stereo systems.

Disconnect the Charger

Ensure that the charger power button is turned off. It also shouldn’t be plugged into an outlet.

Remove Cables from Battery Terminals

Both cables should be removed from the battery, completely disconnecting it from the car.

Start with the black cable—negative charge—before taking off the red cable—positive charge.

Attach the Charger Cables

Chargers come with two wires, one to attach to each of the battery terminals.

The red wire should be connected to the battery’s red terminal—often labeled POS or A+. Then connect the black wire to the other terminal, the black one labeled NEG.

Make sure that the two wires never come into contact. This could create serious burns and even a potential explosion and is the reason we unplug the charger before connecting it, so this can never happen!

Setting up the Charge Rate

Set the voltage and amperage needed for the charge. You mainly have two options available.

Slow Charging Rate

If time allows, a slow charge is preferable. It’s also called the trickle charging method. It’s safer for your battery, and for you.

The charge will also be more efficient as it holds current for a more extended period of time.

Fast Charging Rate

If you’re in a rush, choose a higher amperage. A faster charge can take as little as two hours but represents a greater danger for your battery, and can shorten it’s lifespan.

Turn the Charger on

You’re ready. Plug in and turn the charger on. Don’t forget to set the timer as well.

The gauge should indicate that the device is being charged, and the arrow should slowly be moving back to the green zone. Don’t touch or move the cords during the process.

If the charge was very low and you’re using a slow charge rate, it will take many hours. Don’t expect to see instant results reflected in the charge meter, come back and check after some time.

How Long Should You Charge a Car Battery for?

Car batteries often hold 48 amp-hours of charge. Each hour can recharge about one amp.

A trickle charge—2 amps—might take 24 hours to charge back to a reasonable level. This may take up some of your time, but it’s preferable if you’d like to maintain the device in good shape.

40-amp chargers are efficient and will allow a faster charge. This fast charge speed shouldn’t be used frequently as it might damage the battery plate.

Some chargers will require you to manually monitor the charge, while others will shut off automatically when it’s fully charged.

Don’t think that overcharging is safer than undercharging. It’s the opposite, and an overcharge will damage your battery over time.

Once fully charged, Safely Disconnect

Once the charger indicates that the battery is fully charged, disconnect everything.

Doing so with the correct procedure will avoid sparks, burns, and other disagreements. Here are some guidelines.

  1. Turn the charging device off.
  2. Unplug the charging device from the electrical outlet.
  3. Remove the black—negative charge—cable first.
  4. Then proceed with the red—positive charge—cable.
  5. Place the charged battery back in the car.

Safety Tips

Beside wearing gloves and glasses, there are a few things to keep in mind to ensure your safety.

Avoid Sparks

A few helpful tips can avoid sparks and more potential harmful consequences.

  • At all times, avoid having wires or clips touching each other.
  • Only use insulated wires.
  • Have the charger disconnected from the mains at all times unless it’s fully connected to the battery and is in use.
  • Avoid touching any part of the car engine with any clamps or connections at any time.

Dealing With a Frozen Battery

A frozen battery can be a safety hazard. It prevents gas from escaping during charging and if you try to charge while frozen, the gas can expand until it explodes, spreading battery acid everywhere with it.

This is more likely to happen with AGM batteries, as they have a lower pH level and reach their freezing point faster.

Most batteries should withstand cold temperatures down to 50 degrees Fahrenheit. When distilled water and sulfuric acid don’t mix properly, the water can freeze inside the device.

Avoid jumpstarting if it’s frozen. If it shows cracks, it’s best to replace it. If you don’t notice cracks, warm it up before charging. In this case, a maximum charge of 10 amps should be used.

When to Seek Professional Advice

Towards the end of a charge, cells bubble and can emit gas. They can also emit a violent noise. In this case, the entire device might be failing, and you should have it checked by a professional.


Charging your battery might seem intimidating when taking on the project for the first time. Yet, it isn’t a complicated process. Even the most inexperienced car owners should be able to succeed.

Following the steps above will make the process faster, and it should also make it safer. For yourself and your battery.

Do you have any questions? Please leave us a comment in the section below. We read and answer them all.

From childhood go karting and motocross, to collecting and obsessing over scalextric, matchbox and radio controlled cars, I've always had an obsession with cars. Learning through manuals, books, trial and error, and more knowledgeable family members, I've also enjoyed tinkering with the mechanics and electronics of any vehicles I've owned. Now, over 3 decades later, I've started this site as a place for me to share my knowledge, to teach others how to care for and maintain their vehicles themselves, at home, so they can get the most of their vehicles and save a pretty penny compared to always seeking out professional help.

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