How Do I Charge An AGM Battery From An Alternator?

AGM batteries are easy to charge using a regular 1-10 amp charger. But, some facts may lead your AGM battery to charge with an alternator. Most probably, you’ve heard people’s interrogation over the internet- is an alternator safe for AGM? They also ask- how do I charge an AGM battery from an alternator?

Well, charging your AGM battery is all about connecting the terminals of the AGM and the alternator. The positive and negative terminal of the battery goes for the positive and negative terminal of the alternator. This isn’t an effortless process because the steps might require repetitions. 

However, an alternator isn’t that much risky for an AGM battery. It may not properly charge the battery and hence, the discharge rate will be tremendous. That’s how an AGM battery might be endangered. 

Can you charge AGM with an alternator?

An alternator is the best option for your AGM battery when the battery is preserved. In most winter countries, their temperature falls to minus and the battery requires storing. In that circumstance, charging your AGM battery with an alternator is recommended. 

AGM batteries have a self-discharge character. So, it’ll be fully discharged after a certain time. An alternator is used only for emergencies like this. It’s not better than the regular method because it’s unable to charge the battery properly.

How do I charge an AGM battery from an alternator- Just 5 Steps

Charging an AGM battery with an alternator isn’t different from charging any deep cycle battery. Here you know about some steps to charge an AGM battery using an alternator. 

Step:1- Checking alternator compatibility

First, ensure that the alternator is compatible with the battery. Some alternators are old-versioned and compatible except for AGM batteries. Using an inappropriate alternator may damage the AGM battery permanently. It can endanger the battery by overcharging or fast discharging. 

Step:2- Go through the manual

Checking the manufacturer’s instructions is mandatory because there might be something you should care about. By disobeying any alert, you may damage the battery as well as the alternator. 

Step:3- Connecting AGM battery with alternator

There are two terminals of an AGM battery: the positive terminal and the negative terminal. The same goes for an alternator. Attach the positive portion of the battery to the positive terminal of the alternator. Then, repeat the similar process for the negative terminal. Don’t forget to equip yourself with safety equipment before diving into this process. 

Step:4- Dealing with the engine

Start the engine to make it warm first. You should run it for not more than a few minutes. After warming up the engine, turn it off and check the voltage of your AGM battery. The battery is okay to deal with the engine if its voltage is between around 13 volts. 

Step:5- The last task: Repetition

One trial might not be sufficient to charge your battery properly. Better, try to repeat the process until you get your desired battery voltage. A partially charged battery isn’t worth the engine during critical situations. 

Now, have a look at the alternator voltage for AGM batteries from the chart below. 

Battery levelVoltage
100%13.00 V
90%12.75 V
80%12.50 V
70%12.30 V
60%12.15 V
50%12.05 V
40%11.95 V
30%11.81 V
20%11.66 V
10%11.51 V
0%10.50 V

How long does it take to charge an AGM battery to get fully charged?

It takes around 5-6 hours to charge an AGM battery properly. Charging an AGM battery fully means its voltage should be 13V. To gain this voltage level, you can either use a regular charger or an alternator. The regular charger will ensure to charge your battery just in one attempt. It passes through three stages of charging. That’s why it takes some extra time  


This section reveals two common questions about charging an AGM battery. 

What is the best way to charge an AGM battery? 

AGM batteries or lead-acid batteries require charging with a low amp charger. It’s better to use a 1-10 amp charger to charge your AGM battery. This kind of charger can cope with a three-step charging system and fully charge the battery.

Does the AGM battery charge with an alternator?

AGM batteries get perfectly charged only with the regular charger. Using an alternator is encouraged only in emergencies. An alternator can’t help the battery to get its ultimate voltage level. This charging system isn’t well-equipped as it doesn’t have a three-stage charging facility.  


AGM batteries require a full charge to ensure their longevity. A partially charged AGM battery isn’t good enough to supply sufficient energy. There are facts like winter or rough weather to use your AGM battery safely. The most mentionable part is charging your AGM battery. 

You may have a regular charger and it’s fine to ensure 13V for your battery. Then, why are you thinking about an alternator? You should try to ensure not to use the alternator as long as possible.

An AGM battery is easy to charge, but it takes a little bit extra time. It’s because AGM batteries have an engineered function to be charged in three steps. If you are storing your AGM battery for a long time, an alternator is the right pick.

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Marcelo Araujo-Xavier

Hi, Marcelo Araujo-Xavier is here! I am the author of Batteryhint. I have been working in the battery industry for over 20 years and have a passion for helping people find the best battery for their needs. I have personally tested and reviewed hundreds of different batteries. I'm certified by the International Association of Automotive Battery Manufacturers. I have always been interested in batteries and how they work, so I decided to start this website to share my knowledge with others. I hope you find the information on this website helpful and informative. I will be regularly updating this website with new resources, If you have any questions or comments, please feel free to contact me. I would love to hear from you and help you find the best battery for your needs.

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