Rick's Free Auto Repair Advice

Charge a car battery

Best way to charge a car battery

Here’s a clue—not with your car’s alternator

The best way to recharge your dead battery is with an intelligent battery charger, not the alternator in your car. Everyone will tell you to drive the car for 20-mins to recharge a dead car battery. Ignore that advice because it’s not good for the alternator or the battery.

Your alternator is NOT a charger

Yes, you read that right. An alternator is designed to MAINTAIN your car battery, not recharge it from a deeply discharged state. It’s designed to run at only 35-50% of it’s fully rated output. When you use your car’s alternator to recharge a dead battery, you force it to run at nearly 100% of its rated capacity and that overheats the alternator and dramatically shortens its life.

Do the math on battery charger versus alternator replacement

Let’s do the math on an alternator replacement versus the cost of a battery charger. We’ll use a 2010 Chevrolet Impala as an example. The alternator for this vehicle is fairly easy to replace with a flat labor rate of just 0.9 hrs. The parts price for a genuine GM alternator is $354.35 for a 125-amp unit and $680.01 for the 150-amp unit. Add in a standard diagnostic charge and your total comes to $566 for the 125-amp unit and $702 for the larger alternator.

charge a dead car battery with a charger not your alternator

Clore Pro Logix 20 amp charger

A Solar Pro Logix 20-amp car battery charger costs around $140. It doesn’t take a college degree to figure out that charging your battery with a battery charger is a heckofa lot cheaper than burning up your car’s alternator.

What happens inside a dead battery?

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 sulphated crystals and return them to solution. If you skip that step and just try to recharge the battery, the high resistance will generate high heat and that can warp the plates and damage the battery. Keep that in mind for later.

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

By the time your battery is discharged, the sulfuric acid (battery acid) has lost most of its sulfate and hydrogen. All that’s left is water (H2O), which is why a discharged battery freezes in cold weather. The expanding ice warps the plates and bulges the battery case. Once frozen, the battery is destroyed. You cannot and should not even attempt to recharge it. And it doesn’t even have to be fully discharged to freeze up, because as it discharges, the acid/water stratifies, separating into water and acid layers. So the water layer freezes even at partial discharge.

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 linerar 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.

Don’t rely on your car’s alternator to recharge your car battery

Why? Well, let’s take a look at how alternators/generators work. A generator spins a coil of wire past magnets mounted on the generator case. Car makers stopped using generators back in the 60’s, opting instead for alternators. An alternator works differently than a generator. It spins an ELECTROMAGNET (rotor) past a stationary wire coil (stator) to create power. Get it? It uses an ELECTRO-magnet, which means you need power in order to generate power. Tell me, how are you going get power to energize the electro-magnet if your battery is dead? Oh yeah, that IS a problem isn’t it? Ever hear your friends say they jumped the car and drove it for a while but the battery was dead the next time they tried to start it? Yeah, now you know why. I rest my case.

Second, even if your battery has some power, it’s STILL NOT A GOOD IDEA to rely on the alternator to recharge your battery. Why? Because alternators were never designed to be battery chargers. They were designed to replenish just the battery power that was used to start the car and then to provide enough power to run the vehicle’s accessories. If you use your alternator to recharge a dead battery, you risk overheating the alternator and damaging it. How much is a new alternator? Around $500 including the diagnostic fee, part and labor. How much is a real battery charger? Around $100. Even people who are bad at match can figure that one out.

“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..

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.

How car battery reconditioning works

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 sulphated 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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