The Ultimate Guide To Car Battery Chargers

Have you ever been stuck in the parking slot at work at the end of the day because your car just won’t start? Well 80% of the time, your dead car battery is the culprit. You see your 12 volt battery has a relatively short life span, and it will eventually give way if it isn’t properly maintained. The best way to maintain a battery is to charge it properly and know signs of dead car battery. This is why a car battery charger is very important device for your car. 

Most car batteries are usually rated 12-volt, 48 amps per hour, and require a 2 Amp charger to fully charge a flat one to fully for 24 hours. However, there are different battery chargers with varying charge rates available and the higher the charge output (which usually ranges from 2 to 10 amps), the faster your battery charges.

Why do you need a car battery charger? 

Having a battery charger for your car is very important to prevent inconveniences like the scenario painted earlier. If you’ve returned from a long vacation and find out your car battery is dead, you can actually fix this problem with a car battery charger. With the help of watts to amps calculator and a high-grade multimeter, you can measure the current capacity of your car battery and if it falls anything below 11.5 volts, you can get a fast battery charger to give it that jump and you are on your way.

portable car battery charger

Some chargers also double as car battery starters. These chargers will deliver a powerful current impulse to start your car in the case of an emergency. These fast chargers are super handy if you want to quickly top up your battery.

What kind of battery charger do I need?

Before you get a battery charger, you need to ask yourself the following questions:

  • What is my battery size?
  • How often do I use my battery and how quickly do I want the battery to charge? 
The battery size can affect charging speeds. By the way, we do not mean how big your battery is; we are talking about the amperage. For instance, a 50 amps battery will take about 27 hours to fully charge with a two amps battery charger. This will be too slow if you are in a rush.

If you have a battery with a large amperage (>70), it would be best to pair it with a moderately fast battery charger.

If you are the type who likes to take the bus to work and do not use your car often, then you can opt for slower battery chargers. These chargers will trickle charge your car’s battery over a long period. You can also go for a smart battery charger that will help to maintain your battery while not in use.

car battery charger on desk

Some chargers also double as car battery starters. These chargers will deliver a powerful current to start your car in the case of an emergency when your car battery is dead. These fast chargers are super handy if you want to quickly top up your battery.

There are also batteries with a microprocessor controller inside that stops the flow of current and prevent the battery from overcharging. There is also the option of a portable car battery charger. One example of such is a solar battery charger.

How to connect the battery charger properly?

Before you start using your battery charger, you must know how to connect it correctly to your car battery.

Not making the right connections can damage your battery and other electrical components in your car.

The following steps will help you connect your car battery charger to your car with ease. 

  • Remove the battery from the car. (You need all the right tools to help make your work easier. )
  • Connect the negative probe first to the battery.
  • Take the red or positive probe of the battery charger and hook it up with the battery’s positive terminal.
Many car chargers have alligator clips to help securely connect the probe.

How to charge with a car battery charger?

When you have connected the charger to the car battery, plug its power cord into the power outlet. Set the desired amps value if yours has the option to do so.

You should always charge your car battery in a well-ventilated area. Hydrogen is a by-product of the battery’s electrochemistry during the charge. Hydrogen is known to be explosive. Even static electricity can trigger an explosion.

Charging your battery in a ventilated area will help dissipate the hydrogen. You should also keep the battery away from any source of heat like cigarettes, flames, and lighters.

Safety should be your number one priority. We highly recommend you use safety equipment like goggles and rubber gloves while you work with your battery. 

How to read a car battery charger information?

After plugging your charger into the outlet, switch it on, and set the desired current value in amps on it. You will monitor the current values with the built-in amp meter on the auto battery charger. Look for the needle position or the LCD digits on the amp meter if it has such.

The amp meter’s needle will deflect on the metric scale to the set current value. While the battery is charging, you will notice that the needle will slowly return to its original position over time. This is perfectly normal.

Once the meter reads about the set value, it means the battery is fully charged. Unplug your battery and return it to the car.

how to read car battery charger

If your battery charger has two needles, you will read the amp meter differently from the method discussed above. Like before, you have to concentrate on the needles. 

One of the needles indicates the set value for charging. It shows how much current the charger is feeding your battery. The other needle indicates the amount of charge your battery has. When both needles are on top of each other, it means that the battery is fully charged.

Some smart battery chargers have an LCD screen to display information. This helpful display lets you see the amount of current and the voltage of your battery. You can also calculate the amp hours of your battery charger using the watts to amps calculator.

How to test if a car battery charger is working?

If your car is having issues starting or the electrical components are not working well, chances are the battery is faulty. This may not be due to the battery being bad., but may have something to do with your alternator or a battery charger.

You should test your car battery charger to make sure it is always working as expected. Not correctly charging your battery can permanently damage it. 

To test your car battery charger, you will need a digital multimeter. A multimeter is a versatile device that measures different electrical properties. The multimeter measures current, resistance, and voltages of over a range of values. If you do not have, you can get one online or from your local hardware store.

With your multimeter, you can go ahead to check your car battery charger. We advise that you carefully go through the manual of your battery charger to be familiar with it. Plug your charger into an AC outlet and flip on the switch.

A multimeter has positive and negative probes. These probes are usually labelled with +ve and -ve signs. It is also common to find them colour coded. The positive probe is usually coloured red, and the negative/ground is coloured black.

how to test a car battery charger

Insert the red probe in the port labelled as “V” and insert the black probe in the port labelled “COM” 

The battery charger outputs DC voltage to your battery. The multimeter has a dial to set the voltage type. Turn the dial and set it to DC. You have to set the voltage to the right value so that you won’t overload the multimeter. 

Most car batteries have a voltage of 12V, and the charger should have a voltage that is higher than the battery. Set the multimeter to at least 20 DC Volts to compensate for the charger.

Connect the negative terminal of the charger to the negative probe of the multimeter.  

The next thing to do is touch with the red/positive probe of the multimeter to the corresponding terminal on the car battery charger. If you get a negative reading or no value on your multimeter, it may be that you reversed the polarity. 

Check the reading on the multimeter. It should correspond to the rated voltage of the car battery charger. If the charger is below the recommended value, you should get it fixed or changed.

How long can you leave a battery charger on a car? 

Most car battery chargers deliver 2-10 amperes of electricity and the average battery has a capacity of 48Ah. With a 2 amps charger, the charger will charge your battery for at least 24 hours. A 10 amps charger should charge your battery in about 5 hours.

If your battery is larger than the average, it will take a longer time to reach full capacity. Also if you have a fast charger with a large amperage, your battery will charge much faster.

Smart chargers and maintainers have made things much easier for us. These devices can be left almost indefinitely on your car, and they won’t overcharge or harm your battery. They are also friendly to your car’s electrical system. 

Some of these smart charges come with helpful features such as reverse polarity protection and surge protection. They monitor the battery and make sure it maintains a constant charge/voltage. 

How long does it take to charge a car battery with a battery maintainer 12v 3.8A? 

car battery maintainer

It can take up to 12 hours to charge a battery with a battery maintainer of 3.8A. You should know that a battery maintainer won’t charge a battery from a dead state. The battery maintainer needs to read a specific voltage from the battery before it begins to charge your battery. 

Once a battery maintainer has charged a battery to its full capacity, it will regulate it to ensure that the battery is always at its optimal charge without overcharging it.

How to fix a car battery charger?

You have to be confident in your electrical skills before you go ahead to repair your car battery charger. You can cause further damage to the car battery or even hurt yourself if not fixed correctly. 

It is also essential the car battery isn’t connected to any power source. Unplug your charger from the AC outlet and disconnect it from your battery.

Sometimes the battery may be faulty, before you go ahead, confirm if the battery charger is working. You can do this as explained above. Also, check that the plug and the plug fuse are well connected before you go ahead to disassemble the battery charger.

A few things can cause a car battery charger to develop a fault. Some of the common causes include:

  • damaged bridge rectifier (diodes),
  • fuses,
  • micro switches,
  • bad wiring.

You will have to open up the charger to fix it.

If it is the case of a busted fuse, all you have to do is to locate the fuse. It is usually covered with a clear plastic. You can go ahead to remove this with a flat head screwdriver or your fingers. Get the right replacement fuse, then gently put it back in its fuse holder.

Changing the microswitch requires a little more effort and patience. The electrical boards on battery chargers might be different but they all have common components.

Battery chargers that are water and dust resistant have rubber moulding and seals around ports and over screws. You can remove the rubber seals with a flat head screwdriver. Then go ahead to remove all the screws in the casing.

Remove the sandwich board in the case housing where you will find the panel which holds the microswitch and other components. Remove some of the screws holding the panel. You will find some wires connected from the panel to the case. You cannot remove the board, but carefully flip it over to reveal its underside where you will find the microswitch.

All you have to do is to find an exact replacement for the switch, preferably at a hardware store.

All that is left to do is to solder the new switch on the panel. You should align the pins correctly before you solder the board. When you are all done, put everything back into place and test your charger again.

What battery charger is the best?

The best battery charger depends on your preferences. The important things to consider are the size and voltage of your battery, the amperage of the chargers, and the total charging time.

There are a lot of car battery chargers available today, many of them bag different amp specifications, and as we mentioned earlier, the amp’s output of a car battery charger will determine how fast your car battery charges. So, without wasting any more time, let’s take a look at some best car chargers out there. You can read our review of the best battery chargers here.

Tender Plus 021-0128

Tender Plus 021-0128

First on our list is the old but not frail Tender plus 021-0128 charger. It has a portable design, and can relatively be used anywhere and anytime. However, this old boy has a charge output of 1.5amps which might not be much, but for general usage, it’s enough. 

In addition, the Tender plus auto battery charger instantly switches to float mode to maintain battery charge. It was one of the first car battery chargers to do this, and although many automatic car battery chargers now make this switch. The Tender plus works with lead batteries and flooded or sealed maintenance batteries, you also get 10 years warranty with this car battery charger.

Gooloo SuperSafe Car Jumpstarter

The Gooloo jumpstarter car battery charger also functions as a smartphone portable charger. It bags a hefty 18,000 milliamp-hour battery with a peak of 800A that can quickly jumpstart a 12-volt car battery. This car battery automatic charger can also charge your portable devices including your tablets and phones. It has an Alligator clip that is included to easily hook to your dead battery and help you bring it right back to life.

This car battery charger is a complete package, it can also be used to power your car refrigerator, it has a bright torch light that can be of great use if your battery goes dead in the dark and also to attract other people’s attention.

Schumacher SC1281 Fully Automatic Battery Charger

Schumacher SC1281

The name Schumacher should be meaningful in the minds of motorsport fans. This automatic car battery charger is not as portable the Gooloo. However, you can store it in your trunk or garage easily, it also has a smooth handle to move it easily. The real deal with this automatic car battery charger is the technical specifications: the device is actually split into two separate useful modes: there is the 30 amps charger for your car battery and also it has a 100 amps jump starter to quickly juice your car’s battery.


A battery charger is a device every car owner should get. However, they come in different capacities, and some of them serve some functions better than others. This can cause some confusion for car owners who have or are about to purchase one. We believe we have answered your most perplexing questions on car battery chargers.

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