Know the Essentials of How to Charge a Car Battery

Let’s walk through how long it takes to charge a car battery, how to do it step by step and how to avoid overcharging.

We are so spoiled by our phones and household electronics. We just plug in our phones, Nintendo Switch or electric toothbrushes whenever their batteries get low.

Charging your car battery takes a few more steps because it’s a manual process.

Your car battery isn’t designed like other electronics. Most devices in your life (and their batteries) have electronics programmed with charging instructions. They’ll follow exactly the right steps automatically. For instance, your phone battery has a thin strip of electronics. That’s how it can talk to your phone’s processor. They chat about voltage, amperage rates and float charge times. It's a lot of computation to lengthen that battery’s lifespan.

Your car battery doesn’t have any of that.

Once your battery starts the engine, the alternator charges it while you drive. Meanwhile, the charging system and the onboard computer regulate the alternator’s output. The alternator does enough to maintain a car battery through a normal battery lifetime of 3-5 years. That’s usually just fine.

However, you may need to charge your car battery

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

If that’s you, your car needs help. Regular driving won’t fix it. Your car’s alternator cannot fully recharge your car battery.
That’s why you might need to hook it up to a charger.

How to recharge your car battery

Make sure you’re outdoors or in a well-ventilated area. Take off any jewelry, put on gloves and put on safety glasses.

1. Plug in your charger. Consult any specific instructions for your charger.
   
2. 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, and that can cause real car problems, including erratic idling behavior.
3. Disconnect the car’s negative (usually black) terminal. Then put a rag or a glove around the terminal to prevent it from touching anything else. Disconnecting the negative will protect your car’s electronics from the charging voltage.
   
4. Connect the charger to your car battery terminals. Your charger’s clamps should match the posts. Be careful not to touch your charger’s terminals to the negative terminal you just disconnected.
5. Set the voltage to 12 volts and choose “flooded” or “wet” for the battery type. Flooded and wet are car 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.
6. 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 percent. The longer you charge it, the more strength the charger can put in the car battery.
   
7. Disconnect the charger from the battery when it’s done. Your charger’s indicator light will signal when it’s done charging the battery. Again, avoid letting the charger’s clamps touch the battery’s loose negative clamp.
8. 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.
   

Charging your car battery will warm it up. 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. The solution is to charge your battery slowly. You want to raise its charge without raising its temperature.

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 feel better with a fresh, fully charged battery.

What types of battery charger should you use?

Use an automatic battery charger that adjusts its charging voltage for the fastest charge.

Chargers vary significantly, even within their types. From trickle chargers to smart chargers and maintainers, the biggest difference between battery chargers comes down to how long do you plan to charge the battery:

• Use a smart or automatic charger for 10-24 hours. It depends on how weak the battery is and which amp settings your specific charger uses. Your car battery will be 100 percent charged when it’s done.
• Use a trickle charger for several days to a week. The most common charger type, trickle chargers, use far fewer amps but can push enough power into the battery to charge it slowly. Some trickle chargers are solar powered. Others plug into a wall. They all provide a steady trickle of power.
• Use a battery maintainer for months. These don’t charge batteries. If you charge a battery to 75 percent and then hook it up to a maintainer in the fall, it’ll still be 75 percent next spring.

Another device you might find sold beside a battery charger is a jump-starter.

However, 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.

How a car battery charger works (and why it takes so long)

Charging a 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 it’s almost full, you’ve got to blow as hard as ever. At that point, you’re working just as hard to keep air in the balloon instead of blowing back into your face.

Car battery chargers go through three phases as they charge a car battery:

1. The bulk phase. The charger raises the battery up to 75 percent in a few hours because it doesn’t have to raise its voltage much to fill it with amps. The charger takes this process slow to keep the battery from getting too warm. By the way, a car battery at 75 percent 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 up, the battery’s voltage returns to a normal 12 volts. The charger needs more voltage to finish pushing power into the battery. Higher voltage can heat up the battery, so the charger goes slow. 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 percent, so the charger turns into a trickle charger. After all, an idle battery slowly loses its charge.

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 to charge it 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 that it’s working properly first.

How many volts is a car battery supposed to have?

A fully charged car battery will have 12.88 volts.

Cars run on a 12-volt electrical system. A car battery’s voltage may vary but only in a narrow range, from a fully charged 12.88 volts to a totally dead 11.80 volts.

The difference between a fully charged car battery and a dead one is only 1.04 volts. That slim window means, you should use at least a digital multimeter and not a simple voltmeter with an analog dial.

Use this chart to compare a battery’s voltage to how much charge it has.

Voltaje	Nivel de carga
12.88	100%
12.64	75%
12.39	50%
12.09	25%
11.80	0%

You’ll notice that a battery charger can reach higher voltages, up to 13-14 volts. Think of voltage as electrical pressure. A battery charger needs to use a higher voltage to push more power into a mostly charged battery.

However, 16 volts is the limit.

Never let a charger reach 16 volts if you’re charging a battery still hooked up to the car. That much voltage can damage your car’s onboard electronics. Automatic chargers will monitor the voltage. They’ll cut voltage before going past 16 volts to protect the car’s electronics.

If you’re not 100 percent sure how to charge your car battery to 100 percent, we can help.

Visit any Interstate All Battery Center or a repair shop where Interstate is sold. The experts can charge it for you and answer questions about your specific charger and car battery.

How long will it take to charge a car battery?

About 10-24 hours, depending on how weak your battery is. If you’re using a trickle charger, expect it to take days.

Don’t expect an under-charged battery to do the job of a fully charged one. Take your battery off the charger early, and you may risk needing a jump-start later.

Protecting your car battery from heat is why it takes so long to charge it.

Moving electricity around generates heat. Heat hurts your battery. If your battery gets too hot, the water inside will evaporate, leaving too much acid for the internal parts. Of course, all batteries will wear down over time.

However, overheating your battery will shave years off its lifespan.

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 percent.

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

About 4-8 hours at highway speeds. But you can’t charge the battery to 100 percent that way.

Whatever myth you’ve heard, you cannot charge your car battery with 30 minutes of driving. The car’s alternator is not a car battery charger. Yes, your alternator can charge your car battery—if you’re driving on the highway for hours.

Driving for 30 minutes is enough time to warm up your car battery. Maybe the engine will feed it one or two amps.

However, your alternator cannot charge your car battery if you’re going 40 mph for a 10-minute drive and stopping at red lights. The alternator is too busy. It sends power to every electronic device, every light, every engine sensor. If it has amps to spare, yes, the alternator charges the battery a little. The alternator is not a battery charger. Its real job is running the onboard electronics.

So you just jump-started your car. You want to drive it around to charge it. You want to know how long it’ll take and how far you should go.

Better get ready for a road trip.

Let’s say your car battery is just 50 percent charged, which is possibly why you needed a jump-start. It’ll need 8 hours of charging to raise the charge up to 75 percent or 80 percent. Also, if you tap the brake or dip below 1,000 rpm, it stops charging the battery. Going 65 mph would generate enough power to send a worthwhile charge to the battery.

So, 65 mph for 8 hours.

That’s 520 miles. About the distance from San Diego to Sacramento.

To charge a car battery.

Even then, it won’t be fully charged. You could redirect more power to your battery. Turn off the AC, turn off the radio, turn off the headlights, stop charging your phone and use as few electronics as possible. However, you can’t make a big difference that way. Dozens of unseen electronics still need your alternator’s power.

Whoever told you that driving for 30 minutes will charge your car battery is wrong. That’s like saying you can charge your phone with 5 minutes on a charger. The science doesn’t support the claim.

Take it from the battery experts.

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

Forever. Don’t do this. Please, do not try to charge a car battery with jumper cables. The current from the running car is passing through the dead car battery, going straight to the dead car’s starter. The good car is pushing enough pressure in to turn the dead car’s starter.

And before you ask, no, revving doesn’t charge the battery substantially.

Revving the good car’s engine will increase the voltage and flow of amperage, but it’s not charging the dead car battery. All you’re doing is spending gas to brighten another car’s dashboard lights.

How bad is it if a car battery goes dead?

Car batteries get permanently damaged if they stay low on charge for a few hours.

If you had to jump-start your car, then your battery was at 40 percent or 50 percent all night long. Those eight hours were long enough to cause serious, irreparable damage to your car battery.

In fact, it may have cut your car battery’s lifespan in half.

It’s because of a process called sulfation.

You see, if you drain a car battery to 30 percent power and leave it there, the battery starts to die.

The science behind it goes like this.

Literally, the parts that held electricity harden. Hardened patches and spots on the lead can’t hold a charge anymore. When that happens, the battery can’t store as much power as it used to.

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 percent again. If the battery was at 50 percent when it sulfated, then it might only charge up to 50 percent and then stop.

Can a totally dead battery get recharged back to full health?

No, you cannot recharge a totally dead battery to 100 percent.

If its charge dropped to zero percent, 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.

That said, you might not get it back to 100 percent charge. 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 going 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.