Rick's Free Auto Repair Advice

How to Charge a car battery using a battery charger

Learn how to charge a car battery without making these common mistakes

When your car battery is dead, you want to slap on a charger and go. But rushing the process isn’t good for your battery. When you recharge a battery, you’re trying to push the electrons off of one plate and make them go back and sink into the plate they came from . A quick charge is like dampening the surface of a sponge and then wondering why it doesn’t hold much water. I’ll show you the correct way to charge a car battery and avoid the most common mistakes.

Car batteries recharge in three phases

absorption phase. 

The Reconditioning Phase

When a battery is discharged, the plates accumulate a coating of non-conductive sulfate crystals. These crystals reduce the plate area that’s exposed to the electrolyte, so the battery can’t produce enough power. So the first phase of recharging is referred to as the reconditioning phase, where the charger starts to break down the sulfate crystals and return them in liquid form to the electrolyte solution.

Mistake #1 Boosting charging voltage and current to speed up the process winds up damaging the battery

If you set the charger to maximum output and jam in high amperage at a high voltage, the high resistance from the non-conductive sulfate crystals will generate high heat that can warp the plates and damage the battery. Low and slow is the best way to recharge a battery and not cause more damge.

The bulk mode

In the bulk mode phase, the charger outputs high amps at low voltage. Recharging a dead battery in bulk mode takes between five to eight hours. The bulk mode accounts for about 80% of the battery charge. The rest is accomplished through the absorption mode.

Mistake #2: High amps and High Volts in Manual Mode

Many DIYers put the charger into manual mode and crank up the amps to speed up charging. But in manual mode, the charger also puts out maximum volts. As you can see, bulk mode needs high amps but low volts. If you want to get maximum power into the battery, leave the charger in automatic mode. Does rush this.

Absorption mode

During the absorption mode, the charger lowers the current (amps), while raising the voltage. A complete absorption mode for a dead battery can take as long as seven to 10 hours to fully saturate the lead plates. If you cut short the absorption mode, the plates will sulfate, shortening the life of the battery.

Mistake 3: Driving to charge a car battery

The problem with driving the vehicle to recharge a dead battery is that you never drive long enough to complete all three phases.

“It’s a myth that the vehicle alternator will bring the battery up to 100% charge – it won’t. This means that over time battery capacity and battery life will be reduced”—Bobbie Dumelle, CTEK INC.

Even if you manage to complete the recondition phase, you’ll never complete the bulk charging mode, which means you’ll never even enter the absorption mode. So your battery will be in a constant state of discharge.

What happens if you use your alternator to charge your car battery

An alternator is designed to MAINTAIN your car battery, not recharge it from a deeply discharged state. Modern alternators are designed to output very high amperage, but only for short periods. They are not designed to operate at continuously high current outputs for long periods. Using your car’s alternator to recharge a dead battery is a great way to overheat the diodes in the alternator and damage it. Let’s see; battery chargers cost less than $75, while an alternator costs around $$800 –  $1,000 at a shop, including installation.

Idling your engine is the absolute worst way to charge a car battery

At idle, your alternator only outputs around 35% of its maximum rated output. When you use your car’s alternator to recharge a dead battery at idle speed, you’re forcing it to run hard at low RPMs. That dramatically shortens its life and you never finish the bulk mode or absorption mode. That would take 4-10 hours at idle speed.

Mistake #4: Not cleaning your battery terminals before starting the charging process

Battery terminal corrosion puts high resistance into the circuit and that’s not good for when your car uses power or tries to charge the battery. It takes just about 10-minutes to clean battery terminals and battery posts and that dramatically reduces the high resistance.

Dirty battery terminals can kill your battery and your charging system. Here’s how to clean battery terminals.

For more information on what causes battery sulphation, see this post.

Start with a visual inspection and battery test

Never attempt to recharge a battery until you’ve checked for

battery tester

Old fashioned battery tester

freezing. Look at the case and check for bulges. If you see bulged sides, you’re buying a new battery—PERIOD. If there’s no sign of freezing, use a modern battery tester to check it’s internal condition. Don’t rely on one of these ancient “toaster” testers that just heats up wire elements. You need a modern battery tester that tests internal resistance and conductance.

Solar BA9 battery tester

Modern intelligent battery tester. This one is made by Solar. It’s the BA9 battery tester

Set up the battery tester

Program the tester to match the type of battery in your car. Choose either standard lead-acid (SLA) or absorbed glass mat (AGM). This is important because the tests are different for each type. Then enter the battery’s cold cranking amp rating (CCA)—find that on the battery label. Start the test and read the result.

For more information on how to test a car battery, see this post.

Recharge the battery, but not with your grandfather’s battery charger to recharge your battery

“Many traditional old-style linear chargers don’t have the capability to listen to the needs of the battery; so they either undercharge as a precaution or overcharge the battery”—Bobbie Dumelle, CTEK INC.

Modern maintenance-free (MF) SLA and AGM batteries should never be charged with an old linear non-intelligent battery charger. The plate chemistry used in MF batteries is different than older batteries with removable vent caps. Old battery chargers were designed to charge at high voltage and current, some as high as 20-volts. High voltage and high current can overheat an MF battery. That warps the plates and boils off the water. Worse yet, that high voltage can fry all the computers in your car. Trust me on this, don’t try to charge your battery with an old relic—send it off to be recycled.

More on the battery reconditioning phase

As you can see on the chart below from Solar, a car battery charger with a reconditioning feature starts the recharging process at around 4-volts and 7-amps. That’s called the energizing phase. Then it boosts charging current to around 13.5 amps while slowly ramping up the voltage to a peak of around 14.5 in phase 3. In the third or absorption phase, it maintains constant voltage while ramping down the amps. The 4th phase drops the voltage slightly and the amperage a bit further. In the 5th or resting phase, it drop current flow to 0 while maintaining around 12.5 volts. The 6th phase exercises the battery by pulling a load on the battery until it reaches the final restoration phase where it raises amperage and voltage one last time.

how to charge a car battery

Notice the charging pattern for reconditioning a battery

Low and slow beats high and fast

I know you’re in a hurry to recharge your battery. But cranking a battery charger up to its highest current isn’t always best for your battery. I mean, if you want to get going quickly and you’re willing to destroy your battery in the process, why not just bite the bullet and buy a new battery? Fast charging may get you going today, but what good does that do you a week from now when the battery dies?

Where to get an intelligent battery charger?

Several reputable companies make intelligent battery chargers. Here are a few.

battery charger, charge a battery

Buy a battery charger with settings for standard flooded lead-acid, GEL, and AGM

Solar Pro Logix

The Solar Pro Logix battery charger shown below has a “recondition” cycle for severely discharged and sulfated batteries. That reconditioning cycle can bring them back to life. And, you can use it to charge traditional “flooded” lead-acid batteries, as well as gel-cell and absorbed glass mat (AGM) style batteries. Many electric start lawn mowers use gel-cell batteries and motorcycles are now using AGM batteries.

battery charger, car battery charger, restore dead battery, revive dead battery

One example of a logic-based battery charger that’s safe for modern vehicles


TheCTEK Model CT5 TIME TO GO charger works with SLA, absorbed glass mat (AGM), GEL, Maintenance Free and Calcium (Ca-Ca).

The unit includes

CTEK CT5 Time To Go battery charger

CTEK CT5 Time To Go battery charger

LEDs that indicate how much longer it will take to charge your battery. Plus, it tells you when the battery is charged enough to try starting.

Schumacher SC1359

The SC1359 intelligent charger has a 15A rapid charge mode and a 3A maintainer features. It’s microprocessor controlled and automatically adjusts the amperage rate to charge and maintain your vehicle battery. The charger works with SLA, AGM and GEL batteries.

Schumacher SC1359 Battery Charger

Schumacher SC1359 Battery Charger

Can a jump start damage your car? See this post

What’s the average life of a modern car battery? Clue, it’s shorten than you think. See this post.

Replacing your car battery? Times have changed. Follow this new procedure.

© 2012 Rick Muscoplat





Posted on by Rick Muscoplat

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