Here are lithium iron phosphate (LiFePO4) battery voltage charts showing state of charge based on voltage for 12V, 24V and 48V LiFePO4 batteries — as well as 3.2V LiFePO4 cells.
Note: The numbers in these charts are all based on the open circuit voltage (Voc) of a single battery at rest. If your LFP battery manual has its own discharge curve and charging parameters, they should take precedence over the ones below.
Here’s a printable version of the above chart:
And here it is graphed out:
12V 100Ah LiFePO4 batteries are currently some of the most popular for off-grid solar power systems. They’re a drop-in replacement for 12V lead acid batteries, and a great upgrade.
They are fully charged at 14.6 volts and fully discharged at 10 volts. They are made by wiring four 3.2V LiFePO4 cells in series.
Here’s a printable version of the above chart:
And here it is graphed out:
24V lithium iron phosphate batteries are another popular option for DIY solar power projects. You can either buy a 24V LiFePO4 battery off the shelf, or get two identical 12V LiFePO4 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.
Here’s a printable version of the above chart:
And here it is graphed out:
48V batteries are more popular for larger solar systems. They rarely make sense for small-scale projects. Designing a higher voltage solar system allows you to keep amperage low, thereby saving you money on wiring and equipment costs.
48V LiFePO4 batteries 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.
Here’s a printable version of the above chart:
And here it is graphed out:
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 battery cells online. They’re best used for making your own lithium batteries. You can wire cells in series and parallel to make LFP batteries with your desired voltage and capacity combinations.
Pros: Moderately accurate
Cons: Must disconnect all loads and chargers and let battery rest
A battery’s voltage changes depending on its charge and discharge rate. 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 of LiFePO4 battery capacity 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 15-30 minutes — and then measure its open circuit 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 own the Ampere Time 12V 100Ah LiFePO4 Deep Cycle Battery (Ampere Time has since rebranded to “LiTime”). I wanted to check its capacity after having stored it for a few weeks. I brought it out of storage and measured its voltage with a multimeter. I got 13.23 volts.
To get an accurate estimate of your LFP’s battery capacity based on voltage, you need to disconnect all loads and chargers from the battery and let it rest awhile before measuring its voltage.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%.
Your LFP battery’s manual probably has a state of charge chart in it. Usually this chart is tailored to your battery, so it should take precedence over the ones above. Also, you’ll notice from the above picture how much some brands’ voltage charts can differ from the generic ones.I like this method best for estimating the state of charge of an LFP battery I’ve just received or just pulled out of storage. 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.
Measuring voltage is also a good way to check if a lithium battery (or any battery) is dead or not.Pros: Most accurate, convenient
Cons: Good battery monitors are expensive
The best way to track battery capacity is to connect a good battery monitor — such as the Victron SmartShunt or Victron BMV-712. For my recommendations, check out my review of the best battery monitors.
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.
When I tested the capacity of one of my 12V 100Ah LiFePO4 batteries, I used a battery monitor to measure the exact number of amp hours drawn from the battery.Pros: Convenient
Cons: Inaccurate
If you charge your LiFePO4 battery with solar panels, you may be thinking:
“My solar charge controller already measures battery voltage. I can just use it to check battery capacity.”
But!
This voltage reading is largely inaccurate. It suffers from all of the problems mentioned above, plus it’s done while the battery is connected to loads and chargers.
(Not to mention that some charge controllers have incorrect voltage readings.)
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 an MPPT charge controller, the controller measured 13.0 volts. That correlates to a roughly 30% state of charge — a difference of 50%! Granted, some charge controllers have much more accurate battery voltage readings than others.
I measured a battery voltage of 13.23 volts with my multimeter — roughly 80% state of charge. But the charge controller measured a battery voltage of 13.0 volts — roughly 30% state of charge. If you use your charge controller’s voltage measurement to check LiFePO4 battery capacity, you can be way off!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, its voltage would quickly jump.
Checking battery capacity this way is convenient. But beware that it can be quite inaccurate. I generally use this voltage reading just to make sure my battery isn’t close to being fully discharged.
If you use this method and want to make sure it’s as accurate as possible, you can buy a battery voltage sensor, such as the Renogy Battery Voltage Sensor or Victron Smart Battery Sense. A voltage sensor gives the controller a more accurate voltage reading, especially in solar power systems with long wire runs.
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.
The charging voltage for 12V LiFePO4 batteries is 14.2 to 14.6 volts. This works out to a charging voltage of 3.55 to 3.65 volts per cell.
Most often, you’ll see LiFePO4 battery chargers and solar charge controllers use a charging voltage of 14.4 volts for 12V lithium batteries.
Many LFP batteries recommend a charging voltage of 14.4 volts.The minimum voltage of many 12V LiFePO4 batteries is around 10 volts. The battery’s BMS should detect when the battery voltage falls to around 10 volts and trigger low-voltage cutoff. (Low-voltage cutoff is also called low-voltage disconnect, which you’ll sometimes see abbreviated LVD.)
Note: Some batteries have higher cutoff voltages, such as 10.6V. So the limit in your battery manual may not be exactly 10V.
LiFePO4 batteries in low-voltage cutoff enter a sleep mode to protect the battery cells from over discharge. LFP batteries in sleep mode can have very low voltage readings, usually less than 5 volts. You may think that the battery is dead, but really it’s just sleeping.
A LiFePO4 battery reading a voltage of less than 5 volts is probably in sleep mode. Refer to your battery manual for steps on how to wake it up.Once a battery enters sleep mode, it needs to be woken up. Refer to your battery manual for instructions on how to do this. If your manual doesn’t have instructions, check out our tutorial on how to wake up a sleeping 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.
Either disable float charging or set the float voltage to the recommended voltage in your battery manual.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% depth of discharge (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.
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