Lead Acid Battery Voltage Charts

Battery voltage charts provide an easy way to estimate a battery’s state of charge. You can simply measure the voltage of the battery and use a voltage charge to estimate the current charging level of the battery.

But remember that each type of lead acid battery will have a different voltage range and that voltage charts only give a good general indication of the battery’s current charge.

In this comprehensive guide, we will be exploring lead acid battery voltage charts to understand how to read and use them. We’ll also cover how the battery voltage relates to the battery’s state of charge, how to measure open circuit voltage, and the impact current and temperature have on voltage.

Follow along as we break it down.

How Battery Voltage And State of Charge Are Related

Before we dive into the specific voltage charts, it’s important to understand the general relationship between voltage and capacity in lead acid batteries:

• As a lead acid battery discharges, the voltage decreases linearly. For example, a 12V battery may provide 12.6V when fully charged. After discharging halfway, the voltage will drop to around 12.3V.
• The rate of discharge impacts the voltage. Faster discharge rates result in lower voltages for a given state of charge.
• Higher lead acid battery voltages indicate higher states of charge. For instance, 12.6V means a 12V battery is fully charged, while 12.0V means it’s around 50% capacity.
• Temperature affects voltage, too. Cold temperatures increase the voltage while hot temps decrease it. The charts here assume room temperature.

Considering these principles, let’s look at some typical lead acid battery voltage charts.

SOC vs Battery Voltage Charts for 6V, 12V, 24V, and 48V Lead Acid Batteries

The battery voltage charts of lead-acid batteries vary slightly based on the battery type.

Below, we present the voltage charts of two types of lead acid batteries: flooded lead acid batteries and valve-regulated lead acid (VRLA) batteries.

6V Lead Acid Battery Voltage Charts

12V Lead Acid Battery Voltage Charts

24V Lead Acid Battery Voltage Charts

48V Lead Acid Battery Voltage Charts

These charts provide voltage guidelines for determining the state of charge of common flooded and sealed lead acid batteries at various voltages.

Remember to use the voltage values that match your particular battery type. And keep in mind these assume an open circuit reading at room temperature.

How to Read A Lead Acid Battery Voltage Chart?

Let’s walk through a sample voltage chart to understand exactly how to read it:

This shows a chart for a common 12V flooded lead acid battery.

The left column shows the battery’s state of charge or battery capacity remaining, in 10% increments.

The right column provides the open circuit voltage you can expect to measure at each state of charge level.

For example, when this 12V battery measures 12.29V, that indicates approximately 70% capacity remaining. If it drops to 11.63V, it is around 10% charged.

These voltage values assume the battery is at rest with no load or charging. The values may differ somewhat under load or charge. But this chart provides a handy guide for translating voltage to capacity.

Charts for different lead acid battery voltages follow the same format. Just multiply the voltages by 2 for 24V or 4 for 48V batteries.

Measuring Open Circuit Voltage

The only way to get an accurate reading of a lead acid battery’s state of charge from voltage is to measure its open circuit voltage. This means the battery must be disconnected from all loads and chargers and allowed to rest for several hours until its voltage stabilizes.

Charging or discharging a battery causes its voltage to fluctuate, sometimes quite drastically. So if you simply check voltage while the battery is wired up and in use, the reading won’t tell you much about the actual state of charge.

Once the battery is disconnected and the voltage has stabilized after a few hours, use a multimeter to measure across the positive and negative terminals. This resting open circuit voltage can then be compared to a battery voltage chart to estimate the state of charge.

When measuring open circuit voltage, the voltage alone gives a good ballpark estimate of the battery’s state of charge. However, current flow and temperature can also impact voltage. Let’s look at how these parameters impact the measured voltage.

The Impact of Current on Voltage

We know that to measure battery voltage, the battery must be at rest with no loads or charging current flowing. But what happens to the voltage when the current does flow?

When a lead acid battery discharges, the voltage decreases. The higher the discharge current, the greater the voltage drop.

On the other hand, when the battery is being recharged, the voltage increases. The higher the charge current, the greater the voltage rise.

This is due to the battery’s internal resistance. As current flows through the internal resistance, it causes a voltage drop or rise. This voltage-current relationship follows Ohm’s Law.

So if you measure battery voltage while charging or discharging, the reading will be higher or lower than the open circuit voltage, depending on the rate of current flow. This means resting open circuit voltage gives the most accurate correlation to the state of charge.

The Impact of Temperature on Voltage

Battery voltage is also affected by temperature. In cold conditions, lead acid batteries exhibit higher open circuit voltages. In hot temperatures, the voltages are lower.

The ideal temperature range for voltage state of charge measurements is 60°F to 80°F (15°C to 26°C). The voltage charts we listed earlier will be the most accurate in this range.

In cold weather, below 60°F (15°C), the open circuit voltages for a given state of charge will read 0.1 to 0.2 volts higher than the room temperature values.

In hot weather, above 80°F (26°C), the open circuit voltages will read 0.1 to 0.2 volts lower.

Always factor temperature in when estimating the state of charge from the battery voltage. Adjust your voltage readings up or down if the temperature is outside the ideal 60-80°F range.

Charging Voltage Requirements for Lead Acid Batteries

When charging lead acid batteries, proper voltage levels are critical. Here are some key charging voltage requirements to be aware of:

• Apply a charging voltage of 2.30V to 2.45V per cell, depending on the battery type. Gel and AGM batteries need voltages at the higher end.
• Reduce the voltage by 3mV per cell for every 1°C above 25°C. Increase by 3mV per cell for every 1°C below 25°C.
• Finish the charge with a lower float voltage around 2.25V to 2.30V per cell. This maintains the full charge and compensates for self-discharge.
• For wet cell batteries, carefully monitor the voltage at the end of charging. Stop charging if the voltage remains at 2.4V per cell for an extended period to prevent overcharging.

Refer to your battery specifications for the optimal charging voltages. Staying within the recommended ranges will maximize the performance and life of your lead acid batteries.

Charging Methods for Lead Acid Batteries

There are a few different methods used to charge lead acid batteries:

• Constant Voltage – Charges at a set voltage level, typically around 2.45V per cell. The current drops off towards the end as the battery reaches full capacity.
• Constant Current – Charges at a set current level. Reliable but requires monitoring voltage to prevent overcharging.
• Multistage – Uses different voltages and currents at various stages to speed up charging but prevent overcharging.
• Float – Keeps the battery at a lower 2.25V to 2.30V per cell to maintain full charge. Compensates for self-discharge.

Most lead acid battery chargers today use a multistage approach with bulk, absorption, and float phases to provide the fastest, safest charging.

Always follow your battery and charger manufacturers’ recommendations for optimal charging methods and voltages. Proper charging is essential to maximizing the performance and lifespan of your lead acid batteries.

Using Lead Acid Batteries with Solar Power Systems

Given their affordability, lead-acid batteries remain popular for solar energy storage. Here are some tips for using them with solar power systems:

• Use battery voltage charts to check the state of charge periodically. Ensure batteries aren’t undercharged or overcharged.
• Choose deep-cycle batteries designed for solar rather than starter batteries. Opt for sealed VRLA batteries for reduced maintenance.
• Keep batteries cool and prevent freezing. Battery life decreases rapidly above 30°C.
• Limit the depth of discharge to 50% or less for the greatest longevity. Charge from PV regularly.
• Use a charge controller with temperature compensation and multi-stage charging capabilities.
• Periodically equalize flooded batteries to prevent sulfate buildup. Follow manufacturer guidelines.

With proper battery selection, installation, and maintenance, lead acid batteries can provide many years of reliable energy storage for solar systems. Just be sure to follow the voltage guidelines and recommendations provided by reputable manufacturers.

Frequently Asked Questions

What Is a Healthy Voltage for a 12V Lead Acid Battery?

For a fully charged 12V lead acid battery at rest, a voltage around 12.6V to 12.8V indicates full capacity.

11.8V is considered fully discharged for most lead acid batteries. The voltage will vary under load and charge.

How Can I Tell if My Lead Acid Battery Is Bad?

Consistently low voltages, an inability to hold a charge, corrosive buildup, bulging, cracking, and low specific gravity readings can all indicate a bad lead acid battery that may need replacement.

Why Does Lead Acid Battery Voltage Drop Under Load?

The internal resistance of the battery causes voltage drops under load. The greater the load, the larger the voltage drop will be.

This is normal, but very large voltage drops under load may indicate worn-out batteries.

What Should the Charging Voltage Be for a 48V Lead Acid Bank?

The optimal charging voltage for 48V flooded lead acid batteries is typically around 58V to 62V at the start of charging.

Sealed batteries may need slightly higher voltages. Refer to the battery specifications.

How Can I Revive a Dead Lead Acid Battery?

Try slow charging at a low voltage of around 2.1V per cell. If voltage won’t hold above 10V per 12V battery, or specific gravity is below 1.1, the battery is likely too far gone to be revived.

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