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Can You Revive a Completely Dead Car Battery?

Power - by Bryan Veldboom - updated on 6/22/2022

Man in green shirt looking at car battery

Car batteries need to stay charged in order to remain healthy. Normally, that's not a problem. If you drive your vehicle on a regular basis, the alternator will do a perfectly adequate job of keeping your battery charged. But what about vehicles you don't drive all that often? When vehicles sit unused, their batteries become discharged over time. Today we'll be taking a look at what happens when your battery becomes discharged and what you can do to recover it.

At What Voltage is a 12V Battery Discharged?

Most modern cars use 12 volt batteries to turn over the engine. In a 12 volt battery, there are six separate cells. When fully charged, each of these cells will contain about 2.15 volts of power. That means that a car battery is considered fully charged at around 12.6 volts.

When your battery's voltage drops even a small amount it can make a big difference in its overall performance. For instance, a total voltage of 12.1 volts means that your battery is operating at only 50% of its total charge. Once it drops down to 11.8 volts, the battery is almost completely discharged.

Can a Battery Be Too Dead to Jump?

If your vehicle refuses to start, the first thing you'll want to do is try and give it a jump start. There are a couple of different ways to do this. You can jump start your car using jumper cables and a second vehicle or with a portable jump pack. Keep in mind that if your battery has discharged too low you won't be able to jump start it. If jumping your vehicle doesn't work, your battery is most likely suffering from a condition called sulfation.

What is Battery Sulfation?

If allowed to discharge too low, your battery will reach a point where it can no longer be recovered and needs to be replaced. This occurs because of a process called sulfation. When a lead acid battery discharges, small sulfate crystals made of lead and sulfur form on the battery's plates. This is a natural part of the discharge process, which becomes reversed when the battery is recharged. If you leave a battery discharged for too long though, these soft deposits transform into hard, stable crystals that impede the battery's flow of electrical current, eventually killing the battery.

Can Battery Sulfation Be Reversed?

Sulfation can be reversed in a flooded lead acid battery if it is detected early enough. You can do this by applying an overcharge to a fully charged battery using a regulated current of around 200mA (milliAmps) for a period of roughly 24 hours. This allows the battery's terminal voltage to rise between 2.50 and 2.66 volts per cell, which helps to dissolve sulfate crystals.

If your battery is discharged too low to attempt this, there are battery chargers available that are designed to recover severely discharged batteries. These devices work by sending pulses to the battery terminals in order to reverse sulfation. If you're looking for a charger that can restore a sulfated battery, we suggest the X2Power 7.5 amp charger, which comes with a Recovery mode for deeply discharged or sulfated batteries. Keep in mind that sulfation is particularly difficult to reverse in AGM and thin plate pure lead AGM batteries, since they don't react well to low amp charging.

How Long Can a Car Battery Be Dead and Still Be Recharged?

A car battery that sits unused will start to deteriorate after about a month. It can drain even faster if the battery is stored in an excessively hot or cold temperature, or if there is an excessive parasitic drain on your battery. If you have a vehicle that's going to be sitting for a month or longer, you'll want to pick up a battery maintainer. Battery maintainers are automatic chargers that keep your battery at its ideal state of charge. They shut off once the battery reaches a full charge and kick back in once the charge drops too low. This allows you to keep a battery plugged into a maintainer indefinitely without fear of overcharging it.

Should You Remove the Battery from a Car When Not in Use?

If you have a vehicle that you plan on storing for a whole season, it's a good idea to remove the battery entirely. Store the battery in a battery box and place it in a dry, well-ventilated area with a steady temperature of between 40℉ and 60℉. This will help extend the life of the battery.

Even though disconnecting the battery will slow its rate of self-discharge, you'll still want to check the battery's charge every month and charge it up any time it's at a 70% charge or less. You can also attach it to a maintainer the same way you would if it was connected to your car.

Have Your Battery Tested at Batteries Plus

Are you experiencing issues with the battery in your car or truck? Why not bring it to the experts at your nearest Batteries Plus and have it tested for free. If you do need a replacement, our associates can help you find the best car battery for your vehicle, plus most of our locations offer free battery installation too (valid on most makes and models). We also carry a number of additional auto essentials like battery chargers, jump packs, jumper cables and more in our Auto Center.

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