Rick's Free Auto Repair Advice

Charge a car battery

Best way to charge a car battery

You can charge a car battery two ways. You can use a battery charger or charge a battery by driving it at highway speed for at least 20-mins. Unless you’re driving a new car (2013 or newer), your best bet is with a computerized battery charger that has a battery reconditioning feature. Late model cars have a battery reconditioning routine built into the power management system. Older vehicles don’t have that battery reconditioning feature and will charge a car battery at a high rate, possibly damaging the lead plates. A smart battery charger can actually recondition a car battery to remove sulphation. Here’s why a battery charger is better than driving.

An alternator needs power to produce power. And it needs high RPMs to generate maximum output. Even if you can get your dead battery up to the minimum voltage to operate the alternator, why risk burning up a $400 alternator when you can just buy a high-end battery charger for about $75?

Why is it better to charge a car battery with a charger?

A much better way to charge a car battery is to invest in a newer style charger that has specific settings for the type of battery you’re charging. Flooded lead acid batteries found in cars and trucks require a different charging protocol than absorbed glass mat batteries found in vehicles with start/stop technology and many newer motorcycles. And valve regulated sealed lead acid batteries (GEL) used in electric start outdoor power equipment and uninterruptible power supply (UPS) units require a different charging protocol. limits voltage to a safe level and runs a charging routine based specifically on your battery’s condition.

battery charger, charge a battery

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

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

How car battery reconditioning works

As you can see on the chart below, 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

WARNING: If you have old battery charger and want to use it to charge the battery in your late model vehicle equipped with computers, you must heed this warning. Older chargers are capable of putting out up to 20 volts. That’s enough voltage to fry every computer on your vehicle. So, before you attach the battery charger, you must first remove the vehicle’s battery cables. That’ll isolate the battery and protect the computer. Even then, old battery chargers don’t have computerized battery reconditioning cycles, so they can actually damage a severely discharged battery.

© 2012 Rick Muscoplat

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Posted on by Rick Muscoplat


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