Rick's Free Auto Repair Advice

Charge a car battery

How to charge a car battery

How a battery charges

Car batteries recharge in three phases; reconditioning, bulk, and absorption phase. 

Reconditioning phase

The reconditioning phase is designed to break down the sulfate crystals that formed during the discharge and return them to the electrolyte solution. If you skip the reconditioning phase and just try to recharge the battery by jamming in high amperage, the high resistance from the sulfate crystals will generate high heat that can warp the plates and damage the battery. Once the reconditioning phase is complete, the charger enters the bulk mode phase

Battery 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

Battery 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.

What happens inside a dead battery as it discharges?

As a battery discharges, sulfate crystals form on the battery plates. The sulfate crystals reduce the plate’s active surface area, which, in turn, reduces the battery’s ability to produce power. Worse yet, the sulfate crystals act as an electrical insulator, increasing the battery’s internal resistance. That electrical resistance makes it harder to recharge the battery, so battery recharging should always start with a reconditioning phase to break down the sulfated crystals and return them to the electrolyte solution.

The problem with driving the vehicle to recharge a dead battery

“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 and damage your alternator. Let’s see; battery chargers cost less than $75, while an alternator costs around $400-$800 including installation.

Idling your engine is also not a good battery charging strategy

At idle, your alternator only outputs around 35% of the 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 never finishes the bulk mode or absorption mode.

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.

Here’s how an intelligent battery charger recharges your battery

An intelligent battery charger measures battery resistance conductance and chooses a charging protocol to match the battery’s condition. If the charger determines the battery is sulphated, it’ll start with a reconditioning or de-sulphation routine that breaks down the crystal formations and return them to solution. As the crystals break down, they open the pores of the plate material to allow charging. Then the charger can work it’s magic.

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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