How to resuscitate a deeply discharged AGM battery?

If you discharge a battery too deeply, you risk losing full use of it. If you discharge it deeply and don’t recharge it quickly and completely, its chemical memory may reset. Then you cannot every recharge it 100 percent again.

However, you can take steps to recover a deeply discharged battery. And this is true even if it has sat on your shelf for months. Let’s learn how to resuscitate a deeply discharged AGM battery.

Here are 3 Methods to Resuscitate AGM Battery

Option 1: Recharge with a Modern Charger

One way to try to salvage a nearly completely discharged or dead AGM battery is to connect it to a modern charger. Get one that has a setting for desulfation and maintenance modes.

Let the charger slowly charge the battery. Run sulfation mode to break down the lead sulfate crystals on the battery’s plates. 

Then switch the battery charger to maintenance mode or trickle mode. That will continue to add power to the battery and full charge it, but this setting eliminates the risk of over-charging it.

What is the Next to test the AGM battery?

The next step is to test the battery. Connect it to a power draw and check the voltage. Make certain it is putting out a healthy voltage. Over-discharging or discharging it at 1.5 to 2 times the rated capacity of the battery will ruin it. This will typically ruin it.

If the battery is over-discharging in this way, disconnect everything and discard the battery because you cannot recover it.

What if the battery is putting out the expected voltage level? 

Connect it to something and monitor it. If it runs out of power too soon, the chemical memory has reset to less than 100 percent. Batteries in this condition should be disposed of.

If the battery lasts the two to six hours you expect it to power various devices, and then fully recharge it when you’re done. And use a trickle charge to top off the battery and maximize its performance.

An AGM battery that’s been recovered is prone to accelerated sulfation. Desfulate it with the proper mode on the battery charger every couple of recharge cycles.

Option 2: Recharging the DIY Way

This do-it-yourself tactic comes with more risk, but it is an approach you can try with AGM batteries if you don’t have an advanced battery charger. It still requires a battery charger and cables, but you’re going to try to trick it into fully charging the battery. 

You’ll also need a good battery that has at least 12.2 volts capacity, a volt-meter and a timer or watch.

Set everything up in an area with good ventilation and no ignition sources. During the first part of this process, the battery charger is going to be off to the side. Connect the positive terminal of the good battery to the bad one. Connect the negative terminals.

Now connect the battery charger. Turn it on, and let the battery charger try to charge both batteries in series.

Check the temperature of discharged battery

Check the discharged battery’s temperature. It should warm up a little as it recharges, but it should be disconnected if it is getting hot. If you see any bubbling at the vents or smell gas, disconnect everything. In either scenario, the discharged battery should be discarded because it isn’t safe to charge.

Suppose the battery is gently warming up and there are no signs the battery is overheating. It is going to take anywhere from one to three hours to recharge. Monitor it so you don’t accidentally blow up the battery.

Use the voltmeter to check the charging battery’s level. When it hits 10.5 volts, you can disconnect the good battery and the newly recharged battery. In a couple of days, use a conventional battery charger to top off the charge.

Verify the battery with the voltmeter

Now you can test the battery voltage with the voltmeter, and verify that it is in the normal range. You can connect the battery to a power source, if the output voltage is normal. If it isn’t, you can try to use it as long as you aren’t connecting it to a battery array.

If the voltage of your recovered battery is significantly lower than the others, it can actually discharge the other batteries faster than they otherwise would. Monitor its performance. If the voltage sags far faster than expected, you can’t really rely on the battery and should get rid of it.

Suppose the voltage drops 2 volts after a day on the shelf instead of an ominous 3 or 4. Fully recharge the battery after every use so that it doesn’t experience a chemical memory reset.

As soon as possible, borrow an advanced battery charger so that you can run a desulfation cycle on the AGM battery. AGM batteries will develop sulfation as soon as it approaches total discharge.

Option 3: Recharging the nearly discharged battery.

The first two methods we described are good for completely discharged and possibly dead batteries. What if the battery is 90 to 95 percent discharged? You’re still at risk of the battery chemistry resetting so that it can’t be fully recharged. The process to recover it is similar but has some minor differences.

Check the voltage. If the voltage is much lower than the safe voltage recommended by the manufacturer, your battery chemistry has been altered for the worse. (Less than 11.5 volts is bad.) It may develop a higher self-discharge rate than normal because of this damage. If the voltage is near the recommended range, you can continue on to the next step.

What is the good steps to run a desulfation cycle?

A good first step is running a desulfation cycle. Make certain the battery is cool and isn’t developing high internal pressure. (For example, don’t do anything more with it if it is outgassing or gets hot.) Then attach a smart battery charger to it to try to charge it to the 80 percent mark before switching to a trickle charge.

Avoid a fast charging cycle, since you don’t know if the battery can handle that influx of power. Instead, fully charge it with a slow normal charge from an AGM battery charger.

What to do when your AGM battery nearly fully charged?

When the battery is nearly fully charged, your battery charger should switch to maintenance mode or trickle charge mode. Get the battery to 100 percent charge. If it can’t, the battery may be bad. In this case, discard it.

Once the battery is fully charged, test the voltage. If the voltage is within acceptable levels, try to use the battery. And fully recharge the battery after every usage.

You’re also going to want to run a desulfation cycle after two or three charging cycles, since an AGM battery experiences sulfation soon after it discharges.

Option 4: Call the pros.

If you want to know how to resuscitate a deeply discharged AGM battery, this is the simplest solution.

You can take your discharged AGM batteries to a battery professional. They will often check the batteries for free, though they will tend to recommend buying new batteries.

You may or may not be charged if they recharge the batteries. However, they know how to do so safely.


If your battery has almost completely or totally discharged, there are several ways you can try to save it. We’ve also shared tips on how to know when the battery is beyond recovery and how to prevent it from becoming a useless brick in the future.

John S.

Hello guys! I'm a 37-years-old author, traveler, writer, blogger, and a camper. I enjoy life as much as I can and love to visit beautiful places in my RV. That's why while traveling I have decided to dedicate some time to share my experiences with everyone that might be interested in traveling, camping, and RVs.