The Essential Steps on How to Charge a Car Battery

Charging a car battery can take 4-8 hours with a 12-volt battery charger. You can recharge your car battery at home, parked in a well-ventilated garage. Charging a battery can take most of the day or all night. Even so, it’s a good idea to recharge your car battery every so often if it sits parked for a few days between drives. Parasitic draw, as in the amps your car’s electronics pulls from the battery when the engine is off, can drain a battery faster than you think. Charging the battery will keep your car’s electronics responsive and reliable. On the other hand, discharged car batteries can become permanently damaged in just a few hours.

Let's go through the essentials, including the steps to charge a battery, how many volts your battery should have and how to pick the right charger. Let’s get started.

How to charge a car battery

Disconnect the negative terminal to protect your car’s electronics, and then connect the charger’s clamps before you plug it into a power outlet. Check the settings and then turn your charger on. Depending on how weak your battery is, it can take longer than 4-8 hours to charge.

Let’s go through the steps to connect your battery charger, what settings to use and more.

Make sure you’re outdoors or in a well-ventilated area. Take off any jewelry, put on gloves and put on safety glasses. Then, as a precaution, connect a backup battery to the On-board Diagnostics II (OBD-II) port. Your car’s onboard computers need power all the time. Losing power wipes their memory, which can cause new car problems, including erratic idling.

1. Disconnect the negative terminal. It’s usually the black one. Put a rag or a glove around the terminal to prevent it from touching any metal. This step protects your car’s electronics from the charging voltage.
2. Connect the charger to your car battery terminals, positive to positive and negative to negative. Be careful not to touch your charger’s terminals to the negative terminal you just disconnected.
3. Plug in your battery charger. Consult any specific instructions, such as voltage settings or battery type settings.
4. Set the voltage to 12 volts and choose “flooded” or “wet” for the battery type. Flooded and wet are industry terms for a regular car battery, as opposed to an AGM battery, enhanced flooded battery or a lithium battery. If you’re using a smart charger, it may detect the voltage and battery type for you. Charging an AGM battery is different.
5. Start the charger and wait. Depending on your battery charger, it may take 4-8 hours to charge your battery enough to start the car a few times. It may take 10-24 hours to charge your battery up to 100%. The longer you charge it, the more strength the charger can put in the car battery.
6. When it’s done, disconnect the charger. A fully charged car battery will have 12.88 volts when the engine’s off. Your charger’s indicator light will signal when it’s done. Again, avoid letting the charger touch the car’s loose negative clamp.
7. Reconnect the car’s negative terminal to the battery. Disconnect the backup battery.

Now you’re ready to hit the road with a fresh start.

How to pick the right battery charger for your dead battery.

Car battery chargers vary significantly. Trickle chargers, smart battery chargers and battery maintainers — it might sound like a marketing ploy, but these are all different devices designed for different tasks.

The biggest difference between battery chargers comes down to how long it takes them to charge a car battery.

A smart or automatic battery charger will charge a battery in 4-8 hours.

These devices are designed to charge a car battery. A smart battery charger like the Interstate Guardian 4 will carefully monitor voltage and amps as it charges your car battery. Depending on the model, some smart battery chargers can tell if they’re connected to an AGM battery or regular battery, as well as the right amperage it needs to charge the battery as quickly as possible. Now, how long it takes to charge your battery with a smart charger depends on how weak the battery is and which amp settings your specific charger uses. Your car battery will be 100% charged when it’s done. Smart battery chargers are an excellent tool to keep in the garage for that one time you suspect you’ve got a weak battery.

A trickle charger will charge a car battery in a few days.

Another common type of battery charger is the trickle charger. Trickle chargers such as the Interstate Guardian 1 use just an amp or two to push power into a car battery, and they can take several days to fully charge a car battery. Most trickle chargers plug into a wall, and some are solar powered. Most trickle chargers will stop charging when the battery is full. Trickle chargers are a smart choice for vehicles you don’t drive often.

A battery maintainer will charge a car battery in weeks.

Also sometimes called battery tenders, battery maintainers don’t exactly charge car batteries. Instead, they detect a battery’s voltage and send more power into it to keep the battery at the same voltage. If you hook a battery maintainer to a battery that’s 75% charged in the fall, it’ll still be 75% next spring. They turn on after a battery drains down and then turn off when the battery gets back up to the level when they were connected. Most automatic battery chargers will do the job of a battery tender after they’ve fully charged a battery. Battery maintainers are excellent choices to protect deep-cycle batteries like the ones used in boats.

How many volts is a car battery supposed to have when fully charged?

A fully charged car battery will have 12.88 volts. Cars use a 12-volt electrical system, and the difference between a fully charged battery and a completely dead one is just 1.04 volts. In fact, the difference between a fully charged one and a battery that might fail in a couple of weeks is just 0.2 volts. If your battery shows just 12.68 volts, then it’s weak and needs to recharge.

If you’re checking your battery with a multimeter, here’s a handy breakdown of a battery’s voltage and its charge.

Car Battery Voltage Chart: A multimeter will show you a battery’s resting voltage, which tells you how much charge the battery has.

Car batteries need to stay between 90% to 100% charged all the time or else they can degrade and fail sooner than expected.

When should I recharge a car battery with a battery charger?

You may need to charge your car battery if

• you jumped your car recently.
• you notice odd behavior in your accessories.
• you left an interior light on overnight (but the car still starts.)
• the ignition sounds different to you.
• your car had a slow or sluggish start recently.

Most devices have electronics pre-programmed with charging instructions to help their lithium battery last longer. They follow the right steps every time automatically.

Your car battery doesn’t have any electronics, so it relies on your car’s computer and the battery chargers you connect. The charging system and the onboard computer regulate your alternator’s output as it charges your car battery. The alternator does enough to maintain a car battery through a normal battery lifetime of 3-5 years. That’s just fine, usually.

However, if you’ve faced any of those situations listed above, your car needs help. Perhaps an aftermarket modification is adding too much parasitic draw. Perhaps another failing part is affecting the charging system. In the end, regular driving won’t fix it because your car’s alternator cannot fully recharge your car battery. That’s why you might need to hook it up to a charger.

Now, there might be other issues why your car won’t start.

How long does it take to charge a dead car battery?

It takes about 4-8 hours if you’re using a typical battery charger and the battery isn’t completely dead. You could charge it faster with an industrial-grade battery charger. If you’re using a trickle charger, battery tender or battery maintainer, it could take days to charge a dead car battery with power from a wall outlet.

The better question might be “Is it worth the time to recharge a car battery?” The short answer is yes.

Recharging your battery can add months to its lifespan. It also strengthens the battery so it can better serve your whole engine. The car battery does more than start your engine. It protects your onboard electronics from the engine’s stray power spikes. It also backfills your alternator if your engine or electronics need more power than the alternator can give at any moment.

If you’ve ever noticed your car is more responsive right after getting a fresh battery, that’s why. Your car really does work better with a fresh, fully charged battery. Recharging your car battery may be just as important as cleaning battery corrosion.

Why does it take 4-8 hours to charge a car battery?

Charging causes heat, and heat can damage car batteries. If it gets too hot, the water inside the battery evaporates. In turn, the liquid inside gets more acidic. That means the battery’s insides corrode much faster.

Car battery chargers are designed to provide low, slow charging with relatively high voltage. Battery chargers can reach voltages up to 13-14 volts. If you test a car battery’s voltage with a multimeter while the car is running, a healthy battery will show 14 volts because the alternator is trying to charge it.

Think of voltage as electrical pressure. A battery charger needs to use a higher voltage to push more power into a mostly charged battery.

Charging a car battery is like blowing air into a balloon. At first, you can push a lot of air into the balloon without much effort. Then you have to blow harder as it fills up. When the balloon is almost full, you’ve got to blow as hard as ever just to keep air in the balloon instead of blowing back into your face. In the same way, battery chargers raise their voltage to recharge a battery.

But, 16 volts is the limit for charging a car battery still hooked up to a car. Never let a charger reach 16 volts. That much voltage can damage your car’s onboard electronics. Automatic chargers will monitor the voltage and cut power to protect a car, but it will take longer to finish charging the battery.

To charge a car battery, most chargers use three phases:

1. The bulk phase. The charger raises the battery up to 75% in a few hours because it doesn’t have to raise its voltage much to fill it with amps. The charger takes this process slowly to keep the battery from getting too warm. But it’s far from done. A car battery at 75% is not going to reliably start your car for long.
2. The absorption phase. Now the charger must raise its voltage to push the last 25 percent into the battery. As it charges, the battery’s voltage returns to a normal 12 volts. The charger needs more voltage to finish pushing power into the battery. More voltage could heat up the battery, so the charger slows down. It may take hours to absorb its new power.
3. The float phase. This phase keeps the battery’s voltage up until you’re ready to take it off the charger. Now your battery is up to 100%, so the charger turns into a trickle charger. After all, an idle battery will lose charge over time.

Protecting your car battery from heat is why it takes so long to charge it. So, charge your battery in a cold place to protect its lifespan. Expect it to take longer than one afternoon. It may take all day to get your battery up to 100%. Don’t take your it off the charger early. Or else, you may risk needing a jump-start later. Don’t expect a discharged battery to do the job of a fully charged one.

Can you charge a car battery from a jump-starter?

No, jump-starters do not charge your car battery. They only send power (through your car battery) to the starter so you can get going again. These devices, from the large box profile to jump packs small enough to fit in a purse, all can save you time and money if you need to get going. Jump-starters are not a great source of power for your car battery because they have a much smaller power profile. They’ll send a dozen amps through 14-15 volts to turn the starter. That’s a far cry from the hundreds of cold cranking amps in a typical car battery. These amps also pass right by the battery. If you notice your car battery working better after using a jump-starter, it’s because the alternator did some recharging while you drove. That said, you need to drive fast for a long time to recharge your car battery.

How long does it take to charge a car battery from driving?

Too long. To charge a car battery by driving, you have to travel at highway speeds for about 4-8 hours. If you’re trying to pick a good battery charger, then the alternator is a terrible choice. Whatever myth you’ve heard, you cannot charge your car battery significantly with just 30 minutes of driving. Take it from the battery experts.

Now, your car’s alternator does charge the car battery, but its real job is running the onboard electronics. Every engine sensor, light switch, digital screen, clock, USB port and more gets its power from the alternator (if the engine is on) or the battery (if the engine’s off.) The alternator gets its power from the engine turning, the rpm on your dashboard’s tachometer. Most cars need to be running at more than 1,000 rpm before the alternator starts charging the car battery.

Bottom line: Your alternator turns gas money and a road trip into an almost recharged car battery. Even then, it will still take 4-8 hours to fully recharge a car battery.

Can you recharge a dead battery?

No, you cannot recharge a totally dead battery to 100%. If any car battery has dropped to zero percent charge, then permanent damage already started. The only question is if you can recharge it soon enough to prevent more damage. If you’re fast, you may recharge it significantly. The right battery charger might even reactivate some deadened internal components.

It’s because of a process called sulfation. Literally, the parts that held electricity harden. Hardened patches and spots on the lead can’t hold an electrical charge anymore. If enough of those patches and spots appear on the battery’s lead, then it can’t hold as much power.

As a car battery gives power, the sulfuric acid and water inside bonds to the lead plates. The lead turns into lead sulfate while the liquid mixture becomes mostly water. Recharging the battery separates the lead and sulfur molecules.

If you don’t recharge it within hours, that bond hardens in place.

A damaged battery won’t ever recharge to 100% again. If the battery was at 50% when it sulfated, then it might only charge up to 50% and then stop. Even after several days charging it.

Instead of trying to rehab a dead car battery, recycle it.

A permanently damaged car battery is a real drain on your car. It will only hold back your engine. At worst, it could wear down your alternator or leave your electronics short on power if the alternator needs help. Running on too little power can introduce pesky problems that are hard to diagnose. Shop techs know them as gremlins. They can be as mild as windows that hesitate to go up or down or as severe as irregular revving when the transmission shifts gears.

In general, one of the best ways to avoid these gremlins is to give your car a fully charged car battery.

How long does it take to recharge a car battery with jumper cables?

It will take forever to charge a dead battery using jumper cables. Do not try this. It doesn’t work because the current from the running car is passing through the dead battery, going straight to the dead car’s starter and not doing a thing to the dead battery itself.

If you start the dead car and leave the jumper cables connected, you might think you’re helping the dead car charge its battery. Instead, you’re risking damage to both cars.

If you rev the good car’s engine, it will raise the voltage and flow of amperage to the alternator. The alternator’s not turning the extra power to charge the dead car battery. Instead, it’s spreading power evenly to the rest of the electronics. If you rev the dead car’s engine, it’s going to try raising voltage to the battery for a few seconds, but that voltage will also spread to the good car.

At best, you’re spending gasoline to brighten another car’s dashboard lights. At worst, using jumper cables to charge a dead car battery could damage both cars.

Remember how dangerous 16 volts can be to a car? Revving cars connected by jumper cables is generally a bad idea.

The signs of overcharging a car battery and how to avoid overcharging

Use your senses of smell, hearing and touch to check your battery for overcharging. Turn off the charger if you sense any of these while charging your car battery

1. The smell of rotten eggs
2. A hissing sound from the battery
3. Heat from the plastic case

These signs mean you are hurting the battery. If you hear a hiss or smell rotten eggs near the battery, that’s a sign that water vapor is escaping. The battery is getting so hot that water is leaving the battery.

To avoid overcharging, check your charger’s settings. Look at your car battery’s label to make sure you’re using the right settings. Most car batteries in the market today are a wet, or flooded, car battery. Using the wrong setting could hurt your car battery.

Only use the AGM setting if your battery is an absorbed glass-mat battery.

Only use the lithium setting if your car battery is a lithium battery.

Only use the 6-volt setting if you’re charging a 6-volt battery. Car batteries are 12 volts.

Check your charger’s instruction manual. That should answer most questions about which charging program works best for your car battery.

Plus, always check on the charging battery. Despite whatever the charger’s box advertises, you should never connect a charger to a battery and walk away. Monitor it. Make sure it’s working properly.