What Is The Voltage Of A Fully Charged AGM Battery?

What Is The Voltage Of A Fully Charged AGM Battery?

Your battery isn’t performing as it should do. So you want to know what is the voltage of a fully charged AGM battery? Because you want to check if it is on its way out. The battery won’t charge properly, discharges very quickly, or keeps going flat.

Sounds like you might have a problem with it. You feel checking the voltage when it is fully charged might give you the answer. Or you have checked it and it doesn’t seem right.

So you are looking for some answers well here we go!

What Is The Voltage Of A Fully Charged AGM Battery?

“The voltage of a fully charged 12V AGM battery should be around 12.8 volts without a load. Without a load is also known as an open-circuit. The voltage for a 12V Lithium Iron Phosphate (LiFePO4) battery should be around 14 volts or more. This is important to know when gauging how healthy your battery is. If the battery can’t be charged to around those voltages it might be on its way out.”

Below is a chart showing the approximate voltages of AGM and LiFePO4 12V batteries. At various states of discharge. Fully charged being 0% depth of discharge (DOD) and 100% DOD being fully discharged.

Depth of Discharge (DOD) Percentage 12V AGM Battery 12V LiFePO4 Battery
0%12.8V14V
25%12.6V13.2V
50%12.3V13.1V
75%12V13V
100%0 – 11.8V0 – 10.4V

Remember the above figures are an approximation only. The actual voltage readings from your battery can be affected by other factors like your multimeter.

Depth of Discharge or DOD

In case you don’t know what depth of discharge DOD means IPS (Independent Power Systems) says;

To define the depth of discharge, we must first establish the term battery capacity. Battery capacity is the total electrical energy supply available from the battery, expressed as a unit of power over time, such as kilowatt-hours (kWh).

The depth of discharge is the percentage of the battery that has been discharged relative to the total battery capacity. For example, if you discharge 6 kWh from a solar battery with a capacity of 8 kWh, the battery’s depth of discharge would be 75% (6 kWh ÷ 8 kWh).
(Source; solarips.com)

Notice that in the table above the battery voltage drops as it is discharged. It is recommended that you do not discharge an AGM battery below 80% DOD. If you want the battery to last many years it is best not to discharge it below 50%. 

Never discharge it to less than 11.8 Volts or 100% you will almost certainly damage it beyond redemption. It is best to treat a LiFePO4 battery in a similar way by never discharging it below 90% DOD.

At 90% DOD, the voltage will be hovering around 12V and at 80% it will be around 12.9V. So it is probably best never to discharge a LiFePO4 battery below 80% of its capacity. It is always better to be safe than sorry.

Seasoned RV and boat owners try hard not to let their battery banks drop below 80% to 50% capacity. Because they understand the cost of having to replace them prematurely.

If you want to know a bit more about the depth of discharge or DOD take a look here:

Related == >>>>>>>>>> What Is A Deep Cycle Batteries Allowable Depth Of Discharge (DOD)?

This is a video from Aussie Batteries & Solar. Q&A: How to check your battery charge level and troubleshoot issues?

How Do You Test The Voltage Of An AGM And LiFePO4 Battery When Fully Charged?

If you want to measure the voltage of your battery you are going to have to find the volt or multimeter. The best way to test the voltage of both AGM and LiFePO4 batteries is to take 5 readings. This means you will need a pen and paper to average the readings out.

The first thing you are going to need to do is fully charge the battery with either a charger or solar panels. Make sure your charger is set to the correct battery type either AGM or LiFePO4. And the same with a solar charge controller if you are using solar panels.

In case you don’t know an AGM battery is charged at between 14.6V -14.8V. A LiFePO4 battery is charged at between 14V – 14.6V,

Taking Voltage Readings From Your Volt Or Multimeter

It’s a good idea to take a reading of your battery voltage before charging and make a note of it. So it’s time to connect the multimeter to your battery. 

Connect the positive wire of the meter it will be colored red to the positive terminal on the battery. Then connect the negative meter wire to the negative terminal on the battery normally colored black. Remember to disconnect the multimeter while the battery is charging. 

Initially, after the battery is fully charged if you take a meter reading. You will notice that the voltage will show a much higher reading than you expect. This is normal and you need to go have some lunch for an hour while the battery stabilizes.

What Is The Voltage Of A Fully Charged AGM Battery? - Multimeter 1
The Test Results

Now hook the meter up again and take the first reading then disconnect it. Do this another four times taking a reading each time you will now have five-meter readings. To find the average add them all up and divide by 5.

This will give you an accurate measurement of your battery’s voltage. With an AGM battery, you will be looking for the reading to be as close to 12.8V as possible. A LiFePO4 battery will need to be as close to 14V as possible.

If they are not you may have a problem so to make sure do the tests again. If it’s the same it could be time to think about a replacement battery.

Whether you have an erroneous reading or not as a further check retest several hours later. If in the intervening hours the voltage has dropped by a volt or two. You may be looking at having to replace the battery. 

If you would like to know more about charging batteries with solar panels check out this post:

Related == >>>>>>>>>> How many batteries can a 200 watt solar panel charge?

How Long Will It Take To Fully Charge An AGM Battery?

That will depend on how far it has been discharged but as an example let’s use a 100Ah battery. We will assume it has been discharged by 25% so that means 25Ah needs recharging. 

Battery with meter

Time to use the multimeter to check the volts prior to charging. Yes, it seems obvious but if you are using a multimeter double-check it is set to volts. Then attach your meter to the battery with the terminal wires. Red to the positive battery terminal and black to the negative terminal.

At 25% discharge, the AGM battery should read around 12.6V. And the LiFePO4 should read around 13,2V.

Charging An AGM Battery With A Battery Charger

How fast the battery will charge will be down to the amperage of your battery charger. Remember we are using a 100Ah battery discharged by 25% or 25Ah

Battery Charger Hours To Full Recharge
10A2 Hours 30 Mins
20A1 Hour 15 Mins
30A50 Mins
Charging An AGM Battery With A Solar Panel

For the solar charging, we will assume we have a 12V 200-watt monocrystalline panel. The amp production is 10.5 amps per hour. It is a lovely sunny day with no clouds around and a light wind. The temperature is 77°F/25°C and the panel is perfectly orientated towards the sun. 

Sounds like standard test conditions (STC) doesn’t it?

Charging An AGM Battery With A Solar Panel

So our 200w solar panel will produce 10.5A per hour under these conditions. This means it will recharge the battery in approximately 2 hours 25 minutes. 

A LiFePO4 battery will charge even faster because it has less internal resistance. Some manufacturers claim up to 5 times faster.

If you are looking for more information about 200w solar panels take a look at this review

Related == >>>>>>>>>> Newpowa Solar Panel Review – 12V 200w Rigid Monocrystalline

What Are The Differences Between An AGM And LiFePO4 Batteries

The difference between AGM and LiFePO4 batteries is in their chemistry. They both differ from flooded lead-acid batteries because neither contains sulphuric acid. But they work from the same basic principle. 

When charging they convert incoming electrical energy to chemical energy. When discharging they convert outgoing chemical energy to electrical energy. This is achieved by means of what is called the voltaic cell and several of them together make a battery.

I am not going to go into a big chemistry lesson here. But simply put to make the cell you need two electrodes made from different metal components. These act as positive and negative voltage sources. 

They are suspended in a liquid electrolyte that transfers electrons between the positive and negative electrodes. To get the battery completely charged the direct current (DC) source for example solar panels. Must be connected to the battery terminals with the correct polarity.

The DC source must be of a higher voltage rating than the battery. Because electrical current always flows from higher to lower voltage.

AGM Battery

You probably already know but in case you don’t  AGM stands for Absorbed Glass Mat. They are a type of sealed valve-regulated lead-acid (VRLA) battery. This means that if there is a build-up of pressure inside the battery the valve automatically opens.

Under normal operation, the valve will stay closed. But if there is something wrong with the battery like a short circuit the valve will open. This is a safety feature but even though AGM batteries are classed as sealed lead acid. If there is a problem there will be a gas and contamination escape.

How Long Will It Take To Fully Charge An AGM Battery?

So I guess you could say they are partially sealed. The whole idea of a sealed lead-acid battery is to make it maintenance-free. This is true of an AGM battery. Because it recombines oxygen and hydrogen through an electromechanical process.

This allows the battery to release water back into the cell instead of evaporating it. It means that you do not have to top it up with water like other lead-acid battery types. But does that mean because of the regulation valve the battery has to be kept in a ventilated area?

Well if there is a problem with the battery and it vents hydrogen and oxygen in an unventilated area. There is a possibility of an explosion. So yes the area you keep the battery in has to be ventilated and not a sealed space. So take note RV, trailer, and boat owners.

Like LiFePO4, AGM batteries can be installed on their side or at an angle. Because as I mentioned earlier they do not contain sulphuric acid. This negates the necessity to keep them flat.

If You are looking for more information about AGM batteries follow this link;

Related ==>>>>>>>>>> How Much Does An AGM Battery Weigh? – 100Ah Deep Cycle

LiFePO4 Battery

LiFePO4 stands for Lithium Iron Phosphate and is a type of sealed lithium-based battery. The battery is truly maintenance-free because it is completely sealed. Unlike an AGM battery, it doesn’t have a safety valve.

Your thinking yikes what happens if something goes wrong inside the battery will it just go boom?. No, it won’t it has a battery management system or BMS which takes care of everything.

The management system will protect the battery from;

  • Over-Voltage
  • Under Voltage
  • Over-Current
  • Short-Circuit
  • Over Temperature
  • Cell Imbalance

The system will stop the charging process to protect you and the battery. There is no need to worry about ventilation as it doesn’t give off any combustible or noxious gases. Recreational vehicle. trailer and boat owners can choose where to install the battery.

Renogy Smart 100Ah Lithium Iron Phosphate Battery
Renogy Smart 100Ah Lithium Iron Phosphate Battery

If you would like to take a look at a LiFePO4 battery you can do so here;

Related == >>>>>>>>>>> Renogy Smart Lithium Iron Phosphate Battery Review 12V 100Ah

My Closing Thoughts

When your battery is on the blink it can prove to be a bit of pain in the neck. A good way of testing it is with a multimeter to find out its voltage when fully charged. At least it will give you a ballpark idea of how your AGM or LiFePO4 battery is performing.

If you need to change it and it is only a couple of years old you will need to think about the DOD you allow. This is because the lower you discharge a battery the shorter the lifespan will be. A good way of making sure you get longer out of your next battery would be to;

  • Only allow the battery to be discharged below 50% capacity if absolutely necessary. As a reminder that is 12.3V for an AGM and 13.1 volts for a LiFePO4 battery
  • Never ever let the battery drop below 11.8 volts or 100% discharge.

There are a few practical ways that can help you with this. 

  1. Carry a multimeter 
  2. Keep the battery in a battery box with a built-in voltmeter.
  3. Attach a separate voltmeter
  4. A low voltage cut-off device. This will disconnect all loads if the battery voltage becomes dangerously low
  5. If you have been out using the battery but don’t know if it is fully charged. When you get home put it on charge straight away.

If you would like to know more about AGM batteries check out this post;

I hope you enjoyed my post and have found it helpful. If you have any questions about AGM batteries and voltage. Or want to leave your own personal review, please feel free to leave a comment below.

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