Here are LiFePO4 battery voltage charts showing state of charge based on voltage for 12V, 24V and 48V batteries — as well as 3.2V LiFePO4 cells.
Note: These charts are all for a single battery at 0A. Also, most lithium iron phosphate batteries follow these voltage curves, but not all. Consult the manual of your LFP battery for its specific discharge curve and voltage parameters.
12V LiFePO4 Battery Voltage Chart
12V 100Ah LiFePO4 batteries are currently some of the most popular for off-grid solar power systems. They’re a great upgrade from 12V lead acid batteries.
They are fully charged at 14.6 volts and fully discharged at 10 volts. They are made by connecting four 3.2V LiFePO4 cells in series.
24V LiFePO4 Battery Voltage Chart
24V lithium iron phosphate batteries are another popular option for solar power projects. You can either buy off-the-shelf 24V batteries or buy 12V batteries and connect them in series to make a 24V battery bank.
They are fully charged at 29.2 volts and fully discharged at 20 volts. They are made by connecting eight 3.2V LiFePO4 cells in series.
48V LiFePO4 Battery Voltage Chart
48V isn’t as popular of a voltage for solar systems, especially for small-scale projects.
They are fully charged at 58.4 volts and fully discharged at 40 volts. They are made by connecting 16 3.2V LiFePO4 cells in series.
3.2V LiFePO4 Cell Voltage Chart
Individual LiFePO4 cells have a nominal voltage of 3.2 volts. They are fully charged at 3.65 volts and fully discharged at 2.5 volts.
You can buy individual LiFePO4 cells online, though they’re harder to find than 12/24/48V LiFePO4 batteries. They’re best used for making your own lithium batteries. You can connect cells in series and parallel to make LFP batteries with your desired voltage and capacity combinations.
3 Ways to Check LiFePO4 Battery Capacity
1. Measure Battery Voltage with a Multimeter
Pros: Moderately accurate
Cons: Must disconnect all loads and chargers and let battery rest
Battery voltage changes depending on charge and discharge rates. Plus, LiFePO4 batteries have a relatively flat discharge curve from around 99% to 20% capacity. Because of these factors, it can be hard to estimate their state of charge from voltage alone.
To get an even somewhat accurate estimate based on voltage, you first need to disconnect any loads and chargers from the battery. (Don’t forget to disconnect your solar panels from your charge controller first!) Let the battery rest for a little while — I usually wait 15 minutes — and then measure its voltage with a multimeter.
Compare your measurement to the right voltage curve above, or the state of charge chart in your battery manual. Use it to get a rough estimate of your battery’s remaining capacity.
For example, I recently bought the Ampere Time 12V 100Ah LiFePO4 Deep Cycle Battery. When it arrived, I pulled it out of the box and immediately measured its voltage with a multimeter. I got 13.23 volts.
I then compared this number to the 12V LiFePO4 state of charge chart above, as well as the one in the battery manual.
Based on the charts, I’d estimate my battery’s state of charge was somewhere around 80%.
I like this method best for estimating the state of charge of an LFP battery I’ve just received. The battery is already at rest and not connected to anything. I find it too inconvenient to disconnect everything once the battery is in use.
DIY lithium battery builders will also measure the voltage of used (and new) battery cells — such as LFP cells and 18650 lithium batteries — to see which are good and which are duds.
2. Use a Battery Monitor
Pros: Most accurate, convenient
Cons: Good battery monitors are expensive
Battery monitors track the amount of amp hours consumed to accurately estimate the state of charge. They also display useful system specs such as battery voltage and current. Some connect via Bluetooth to your phone so you can check your LiFePO4 battery’s capacity in a mobile app.
3. Use a Solar Charge Controller
Cons: Very inaccurate
You may be thinking:
“My solar charge controller already measures battery voltage. I can just use it to check battery capacity.”
This voltage reading is usually very inaccurate. It suffers from all of the problems mentioned above, plus it’s done while the battery is connected to loads and chargers.
For example, recall that when I checked my battery’s voltage with a multimeter at the battery terminals, I got a voltage reading of 13.23 volts. That correlates to a roughly 80% state of charge.
But when I connected my battery to a solar charge controller, the controller measured 13.0 volts. That correlates to a roughly 30% state of charge — a difference of 50%!
After all, voltage drops under load. And a charge controller is a load. If I were to connect a solar panel and start solar charging the battery, voltage would change again.
Checking battery capacity this way is convenient. But it’s so inaccurate as to be almost useless. I generally use this voltage reading just to make sure my battery isn’t close to being under or over voltage.
You can buy a battery voltage sensor, such as the Renogy Battery Voltage Sensor or Victron Smart Battery Sense which work with some of their respective brand’s MPPT charge controllers. A voltage sensor gives the controller a more accurate voltage reading, especially in solar power systems with long wire runs.
LiFePO4 Voltage FAQ
Note: The caveat to all the following answers is that they apply to most LiFePO4 batteries. Consult your battery’s manual for its specific charging parameters.
What is the voltage of a fully charged 12V LiFePO4 battery?
A fully charged 12V LiFePO4 battery will have a charging voltage of around 14.6 volts and a resting voltage of around 13.6 volts.
How much can you discharge a LiFePO4 battery?
Many LiFePO4 batteries can discharge 100% of their rated capacity every time with no ill effects.
However, many manufacturers recommend discharging only 80% to maximize battery life. In fact, some brands state the cycle life of their batteries based on 80% DOD.
For comparison, lead acid batteries can only discharge 50% of their rated capacity. So a 12V 100Ah LFP battery has as much usable capacity as a 12V 200Ah lead acid battery.
What is the low-voltage cutoff of a 12V LiFePO4 battery?
The low-voltage cutoff of most 12V LiFePO4 batteries is around 10 volts. The BMS detects when the battery voltage falls below 10 volts and disconnects the battery to prevent over discharge.
Low-voltage cutoff is also called low-voltage disconnect, which you’ll sometimes see abbreviated LVD.
Once a battery enters low-voltage disconnect, it needs to be “woken up.” Refer to your battery’s manual for instructions on how to do this.
What is the float voltage of a 12V LiFePO4 battery?
LiFePO4 batteries don’t need to be float charged because they don’t leak charge the way lead acid batteries do.
If you can, disable float charging on your charge controller or battery charger. If you can’t, prevent the battery from entering float charge by setting the float voltage to that recommended in the battery manual — usually 13.6 volts ± 0.2 volts.