Here are lead acid battery voltage charts showing state of charge based on voltage for 6V, 12V and 24V batteries — as well as 2V lead acid cells.
Lead acid battery voltage curves vary greatly based on variables like temperature, discharge rate and battery type (e.g. sealed, flooded). The voltage to battery capacity chart in your battery manual should always take precedence over the generic, averaged ones listed below.
Note: Estimating state of charge based on open circuit voltage is only accurate when batteries are at room temperature and have been resting — i.e. disconnected from all loads and chargers — for several hours.
6V Lead Acid Battery Voltage Charts
6V Sealed Lead Acid Batteries (AGM & Gel)
6V Flooded Lead Acid Batteries
6V lead acid batteries are used in some DC devices like lights, pumps and electric bikes. You can also wire two in series to create a 12V battery bank. They are made by connecting three 2V lead acid cells in series.
6V sealed lead acid batteries are fully charged at around 6.44 volts and fully discharged at around 6.11 volts (assuming 50% max depth of discharge).
6V flooded lead acid batteries are fully charged at around 6.32 volts and fully discharged at around 6.03 volts (assuming 50% max depth of discharge).
12V Lead Acid Battery Voltage Charts
12V Sealed Lead Acid Batteries (AGM & Gel)
12V Flooded Lead Acid Batteries
12V lead acid batteries are popular in solar power systems and other 12V electrical systems. They’re widely available and have a low upfront cost. Many car and marine batteries are 12V lead acid batteries. They are made by connecting six 2V lead acid cells in series.
As far as I can tell, lead acid is still the most popular rechargeable battery type for DIY solar power systems. Lithium iron phosphate (LiFePO4) batteries have become a lot more popular in recent years, though, in large part thanks to their dramatic price drops we’ve seen over the last decade.
12V sealed lead acid batteries are fully charged at around 12.89 volts and fully discharged at around 12.23 volts (assuming 50% max depth of discharge).
12V flooded lead acid batteries are fully charged at around 12.64 volts and fully discharged at around 12.07 volts (assuming 50% max depth of discharge).
24V Lead Acid Battery Voltage Charts
24V Sealed Lead Acid Batteries (AGM & Gel)
24V Flooded Lead Acid Batteries
24V lead acid batteries are another common option for solar power systems. Working with higher voltages helps keep amperage low, saving you money on wiring and equipment. They are made by wiring in series twelve 2V lead acid cells or two 12V lead acid batteries.
24V sealed lead acid batteries are fully charged at around 25.77 volts and fully discharged at around 24.45 volts (assuming 50% max depth of discharge).
24V flooded lead acid batteries are fully charged at around 25.29 volts and fully discharged at around 24.14 volts (assuming 50% max depth of discharge).
2V Lead Acid Cell Voltage Charts
2V Sealed Lead Acid Cells (AGM & Gel)
2V Flooded Lead Acid Cells
Individual lead acid cells have a nominal voltage of 2 volts (sometimes listed as 2.1 volts). You can buy 2V lead acid cells and connect them in series-parallel configurations to build a battery bank with your desired voltage and capacity.
2V sealed lead acid cells are fully charged at around 2.15 volts and fully discharged at around 2.04 volts (assuming 50% max depth of discharge).
2V flooded lead acid cells are fully charged at around 2.11 volts and fully discharged at around 2.01 volts (assuming 50% max depth of discharge).
3 Ways to Check Lead Acid Battery Capacity
1. Measure Open Circuit Voltage with a Multimeter
Cons: Must disconnect all loads and chargers and let battery rest for several hours
To properly estimate battery capacity based on open circuit voltage, first disconnect everything from your battery and let it rest at room temperature for several hours. (Battery University recommends at least 4 hours.)
Then, simply use a multimeter to measure the voltage at the battery terminals and compare the number you get to the state of charge chart in your battery manual. If your battery manual doesn’t have a chart, use the relevant one listed above.
For example, I recently wanted to test the remaining capacity of a 12V 33Ah sealed lead acid battery I own. The battery was already at rest and at room temperature — it had been sitting disconnected in my basement for the past couple weeks.
So I grabbed my multimeter, prepped it to measure DC voltage, and touched the probes to the battery terminals. I got an open circuit voltage of 12.63 volts.
I couldn’t find my battery’s manual, so I referred to the 12V sealed lead acid voltage chart above to estimate its capacity. Based on that chart, I’d estimate it had about 80% capacity left.
2. Check Specific Gravity with a Hydrometer or Refractometer
Cons: Only works for flooded lead acid batteries
Because this method requires you to open the battery to access the electrolyte solution inside, it only works with flooded batteries.
I’ve only ever used sealed lead acid batteries, so I unfortunately can’t run you through the steps on how to do this. Refer to the steps listed in your battery manual, or the product manual for your hydrometer or refractometer.
3. Use a Solar Charge Controller
If you’re using your lead acid battery in a solar power system, your charge controller probably measures battery voltage for you.
You may be thinking you can just use this reading to get an accurate estimate of your battery capacity. Unfortunately, using battery voltage to estimate capacity while the battery is connected to chargers and loads is very inaccurate.
Battery voltage varies greatly depending on factors like temperature and rate of discharge. Plus, the battery voltage reading given by some charge controllers can be inexact. Some charge controllers only display one decimal place, and others have wide margins of error. For example, one cheap PWM charge controller I tested claimed a battery voltage margin of error of ± 0.2 volts.
Still, I know most DIY solar enthusiasts will use this reading most often, if not exclusively. It’s a hassle to disconnect everything from your battery and let it rest just to measure its state of charge more accurately.
If that’s you, just keep in mind how inaccurate this number can be. Don’t think you can know the precise state of charge of your battery from it. Just use it to get a general idea of whether or not your battery is close to being fully charged or discharged.
Lead Acid Voltage FAQ
Note: To reiterate, the recommended voltages and state of charge chart in your battery’s manual should take precedence over the generic ones listed below.
What is the voltage of a fully charged 12V lead acid battery?
A 12V sealed lead acid battery will have an open circuit voltage of around 12.9 volts when fully charged.
A 12V flooded lead acid battery will have an open circuit voltage of around 12.6 volts when fully charged.
To accurately estimate a battery’s capacity based on its voltage, you must first disconnect all loads and chargers from the battery and let it rest at room temperature for several hours.
What is the minimum voltage of a 12V lead acid battery?
The minimum open circuit voltage of a 12V sealed lead acid battery is around 12.2 volts, assuming 50% max depth of discharge.
The minimum open circuit voltage of a 12V flooded lead acid battery is around 12.1 volts, assuming 50% max depth of discharge.
How much can you discharge a lead acid battery?
Many lead acid batteries can only be discharged up to 50%. Discharging them more can cause permanent damage. You should never completely discharge a lead acid battery to 100% depth of discharge. Doing so can shorten its lifespan greatly.
Here is a graph showing the relationship between depth of discharge and life cycles for non-deep-cycle lead acid batteries:
As you can see, consistently discharging a lead acid battery to 100% can severely shorten its lifespan.
What is the float voltage of a 12V lead acid battery?
The float voltage of a sealed 12V lead acid battery is usually 13.6 volts ± 0.2 volts.
The float voltage of a flooded 12V lead acid battery is usually 13.5 volts.
As always, defer to the recommended float voltage listed in your battery’s manual. Some brands refer to float as “standby.” Sometimes, the float voltage will even be listed on your battery label.
How I Got the Numbers in These Charts
To get the numbers in the voltage tables above, I looked up the datasheets for 7 popular brands of lead acid batteries. I found the state of charge charts in each and averaged them together for the final values.
Here are the datasheets I used for the sealed lead acid values (2 AGM, 2 gel), along with the page number where I found the voltage chart:
And here are the ones I used for the flooded values:
Creating these charts was far from an exact science. Only a couple of the datasheets listed open circuit voltages by capacity in table format with exact numbers. Often, they included a graph from which I’d have to infer the numbers. What’s more, the graphs often had wide bands rather than thin lines, as if to convey a margin of error or range of possible values — what I came to see as the brands hedging against providing an exact number.
Other brands provided exact numbers, but only for 0%, 25%, 50%, 75%, and 100% capacity values. From these I had to create linear functions to estimate the values between them.
I calculated all the numbers for 2V lead acid cells first, then multiplied these values by the respective number of cells in series to get the values for 6V, 12V and 24V batteries. Finally, I rounded all the values to two decimal places.