How Do Battery Chargers Work?

If you’ve ever dealt with a dead car battery, you know firsthand what a pain jumping the battery can be. Digging out the jumper cables and flagging down a passerby is frustrating, annoying, and inconvenient – and that’s even if anyone stops to help in the first place.

That’s where battery chargers come in. These lifesavers endeavor to leave jumper cables in the past by offering easy ways to recharge dead batteries or maintain the charge of an idle battery.

There are numerous battery chargers on the market these days, and in order to shed some light on how these chargers work, we’ve decided to dig into the details of these helpful tools. But before we dive in, one thing is for sure: these chargers sure beat pulling out the jumper cables when you find yourself with a dead battery.

What is a battery charger?

Simply put, the typical automotive battery charger is a small device that works by trickling a small amount of power into a dead battery over a long period of time. One end of the charger plugs into a household outlet, while the other end features a pair of cables with alligator clips that connect to the engine and battery.

Amperage, which measures how fast electrical currents flow, is the key determinant of how battery chargers work. A battery charger claiming it can charge a battery at three amps is saying that electricity is flowing back into the battery at the rate of 3 amps an hour. To go a bit deeper in the what and why you may consider the amperage and the capacity of your car’s battery.

For reference, a single amp represents the quantity of electricity traveling past a given point in the span of one second. If it helps to think of this in more technical terms, bear in mind the differences between amps, volts, and wattage.1

What is a car battery?

Car Battery

Image courtesy of Pixabay

The car battery is nothing but a storage facility for energy. When the car is started, for instance, the starter taps into the electrons held in the battery. Once the car is started, the starter kicks off and the engine begins to run on its own. Any electrical draws from then on – whether that be the headlights or the radio – is then handled by the alternator, which runs off the engine. The alternator also pushes current back into the car battery, helping replenish whatever energy the battery dispelled in starting the car.2

Over time, the battery loses its ability to store electrons. Eventually – usually after five or six years – it won’t be able to maintain the level of charge necessary to start the car.

Alternatively, a healthy battery may drain down completely if an accessory is unwittingly left on overnight. Once the car is turned off, all electrical components are powered directly by the battery rather than the alternator, which inevitably drains the battery. It’s easy to accidentally leave the headlights on overnight and come out in the morning to a dead battery.

Even when your car isn’t running, the battery is still working to remember radio presets, keep the clock going, alarm your car, and maintain other similar accessories. None of these power draws are large, but given enough time they will drain the battery. Most cars can’t sit idle for more than two or three weeks before the sum power draw of these background accessories drains the battery completely.3

How does a battery charger work?

The typical home battery charger taps into a 120-volt household outlet for power then steps down that voltage in order to trickle that power to a 12-volt car battery.

The rate at which the battery is actually charged can be usually be selected by adjustments found on the battery charger itself. Selecting higher amperage results in a faster charge but may be more detrimental to long-term battery life. A slower rate of charge – between two and four amps – is recommended.

The average car battery is rated for 48 amps – meaning it can provide one amp for 48 hours or, conversely, 48 amps for just one hour – so at a rate of four amps per hour you can expect a car battery charger to fully charge a battery in about 12 hours.

Most battery chargers now feature technology that recognizes when a battery has reached its optimal state of charge. This is important, as overcharging a battery is much more dangerous than undercharging one. As more and more electrons are shoved into a battery, they begin to heat up in an attempt to dissipate and escape. Ruthless overcharging can even end in the battery exploding, so always practice caution in charging batteries.

How to operate a car battery charger

First, you’ll want to disconnect the battery cables leading off your car battery. First, disconnect the black wire (the ground), then follow suit with the red wire.

With your battery charger unplugged, attach the black wire coming off the charger onto a stable metal component in the engine bay, such as a portion of the frame. Do not attach to the negative terminal on the car battery. After putting the black charger cable in place, fix the charger’s red cable to the positive terminal.

With everything properly connected, confirm the charger is off before plugging it in. Now return to the charger and switch it on, being careful to ensure the correct voltage and amperage settings are in place. Assuming you don’t need a quick charge, choose the lowest amperage setting you can.

For voltage, choose the setting that corresponds to the electrical system in your car. Antique cars built before the 1960s often had six-volt electricals; some new luxury cars feature 48 volts. When buying a car charger, ensure it is compatible with the electrical system in your car.

Battery chargers vs. jump starters

Jump Starters

Image courtesy of Pixabay

When shopping for battery chargers, you’ll find there are both battery chargers such as those we’ve discussed as well as portable jump starters. Don’t consider the two terms synonymous: these are two different devices built for two different functions.4

The chargers we’ve gone into detail within this article are for maintaining a charge or replenishing a charge over hours. Portable jump starters, on the other hand, are designed to provide a quick jolt of power when you’re in a jiff. If you come out of work and find your battery dead, a battery charger won’t help. You’ll need a jump starter to get back on the road.

Portable jump starters are great tools that beat jumper cables by a mile for their convenience and ease of use if you need to give your battery a jump. But they can’t maintain a charge, if used too often they may damage a battery. If you want to keep a battery charged during long-term storage or otherwise have the hours needed to slowly juice up a dead battery, use a battery charger.

Parting thoughts

Battery chargers are excellent tools, especially for the car collector or anyone else who may want to keep a car battery charged for months. Many casual shoppers may be better off with a portable jump starter, however, which is best for those roadside emergencies when you need to get back on the road and fast.

Once you’ve decided what is right for you, you’ll be all set to make the purchase of a battery charger. With these indispensable tools, you can finally ditch those old, pesky jumper cables.

Article Sources

Motor Sports Village uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.

  1. Batteries and Chargers: Understanding Volts, Amps & Watts. Accessed 27 Sept 2021.
  2. Alex Muir. Using a car battery charger. Accessed 27 Sept 2021.
  3. Bartlett J. How hot weather affects your car battery and what to do about it. Published 4 May 2019. Accessed 27 Sept 2021.
  4. Car Battery Charger vs Portable Jump Starter – What’s the difference? Accessed 27 Sept 2021.