What Should A 12 Volt Battery Read When Fully Charged?

Batteries are interesting pieces of equipment operating on miraculous-like operations that give them their qualities. For an average joe, it’s simple and plain, and they are rechargeable and non-rechargeable units used to run various electronic devices.

But for scientists or experts, along with the curious ones, there’s a lot more to understand when it comes to batteries. There are confusions, theories, and thoughts to be explored as well as to answer.

One of these is the relation of the voltage and the state of charge of batteries along with several related complex aspects. Let’s take a twelve-volt battery; a lot of people are confused about why it has more than twelve volts when it is fully charged.

What should a 12-volt battery read when fully charged? Generally, a twelve-volt battery should have about 12.7 volts or higher when it is fully charged. This will depend on several factors that make this aspect about batteries very confusing for some.

Cells have voltage setpoints apart from the actual voltage that it has within it. These setpoints indicate the amount of voltage that sets the condition of the battery.

Typically, a fully charged twelve-volt should have a rest voltage of 12.6 volts or more, depending on the manufacturer’s specified amount. When it comes to deep cycle batteries, they absorb voltage the cells should receive and maintained to reach a fully charged state ranges from 14.2 volts to 14.8 volts.

Finally, there’s the so-called float voltage, which is a set point, which is the amount of voltage for a specific system to operate with an efficiency that usually around 13.8 volts.

With all of these details, the chance of a confused person is expected to be high. However, the most conclusive answer to the question is that a twelve-volt battery should have 12.6 volts or more when it is in a fully charged state.

Learning About Voltage Reading and State of Charge of Cells

When it comes to testing a battery’s voltage regardless of the device to be used, the readings will be affected by its current state of operation.

When the battery is on the charging or discharging state, the voltage reading will be referred to as “load voltage” When it is idle, the reading is called “rest or open-cell” voltage.

Whether the battery is at the state of receiving a charge, powering a circuit or at idle will affect the voltage reading.

On that note, you have to keep in mind that voltage reading is referred to in two terms. Learn and understand the below.

1. Charge Voltage (Battery is Charging)

When a battery is in the charging state, its plates are polarized and start to develop an internal resistance leading to the so-called surface charge. As this happens, the surface charge is added to the cell’s voltage, increasing its voltage reading.

Due to this, the device is not showing the battery’s actual state of charge. This explains why some people get confused when their meters read the high voltage on the tests they do.

On the other hand, when the battery starts to deliver current to its load, it will be on the discharging state. All the surface charge that its plates generated will diminish, including its actual state of charge.

With that said, the cell’s voltage will be able to recover the moment the load is taken out from the circuit.

2. Open Cell or Rest Voltage (Battery is Idle and Rested)

When the battery is fully charged and in the idle state, that is the best time to get a voltage reading if you want to see its actual state of charge.

However, you have to keep in mind that forgetting the rest voltage. The battery should also be rested for at least four hours. This is to give it enough time to cool down, allowing the accumulated surface charge to dissipate within its electrolyte gradually.

Suppose you are going to take a closer look based on open-cell and load voltage. The confusions with regards to the state of charge of a battery and its voltage are due to these fluctuations in voltage.

It also suggests that the state of charge of a cell can be reflected on its rest or open-cell voltage because it is the state of the battery with no loads, and the surface charge has dissipated.

State of Charge and Battery Voltage: Taking a Closer Look

A battery’s state of charge is defined as the amount of power available within it represented as a percentage. It also describes the cell’s ability to deliver enough current to run or sustain the load for a specific time.

The state of charge also reflects how much capacity the battery has within it, giving users an idea when the battery needs to be recharged.

There are several ways to determine the state of charge of a battery, and each of these methods uses distinct devices or tools. For a flooded lead-acid power cell, the most accurate way of determining its state of charge is by carrying out a specific gravity test.

However, for other types like AGM, gel, and sealed lead-acid batteries, the test is not applicable. That is why voltage is used to determine the state of charge for these kinds of batteries.

How Voltage Is Related With the State of Charge

It is true that the voltage of a battery is used as a basis to determine its state of charge, especially for battery types that can’t be tested with a hydrometer. However, you have to keep in mind that due to the fluctuations, voltage is not considered to be a fundamental basis for the state of charge.

With that said, it is best to use the battery voltage change as a reference for the changing condition of the battery, which can be considered its relative charge state.

The only way to have an accurate voltage reading is with a reliable and credible BMS or battery monitoring system. Without one, we can only have a rough estimate on a battery’s state of charge since we will only have the voltage as our only basis.

Using Voltage to Determine the State of Charge of 12-V Sealed Batteries

As I mentioned earlier, it is an established truth that the most accurate way of determining the state of charge of sealed batteries is to use voltage as a basis. This is because a hydrometer won’t be able to read the specific gravity.

When using voltage as a basis, you have to keep in mind that for accurate voltage readings. It is recommended that the battery is fully charged and resting for around four to six hours, but preferably twenty-four hours.

Voltage testing a battery immediately after charging it or when it is connected to the circuit will ruin the readings due to the fluctuations. To determine the state of charge, the result or interpretation will then be compared to a chart that shows the state of charge.

Here’s a table that you can use for determining the state of charge with the voltage reading of twelve-volt sealed batteries.

Final Thoughts

After going through all this information, I understand that it can be a lot to digest, but just to be clear with the subject. A fully charged twelve-volt battery should have a voltage reading from 12.6 volts to 12.95 volts, depending on the type.

Remember that this is the reading of the open circuit or rest voltage of the battery. In case the reading is higher, then it is possible that the voltage reading is done while the battery is charging or it has been recently charged and not rested.