LiFePO4 Voltage Calculator: Estimate Your State of Charge
LiFePO4 Voltage Calculator: Estimate Your State of Charge
With old lead-acid batteries, voltage dropped linearly. You always knew how much gas was in the tank. With Lithium Iron Phosphate (LiFePO4), the voltage stays almost exactly the same from 90% down to 20%.
This makes guessing your battery level extremely difficult. Use this tool by ShockTrail to interpret your voltmeter readings.
The SoC Estimator
Select your system voltage and input the current reading to estimate the remaining percentage.
LiFePO4 SoC Estimator
Chemistry Logic by ShockTrail--
The “Flat Curve” Problem
Look at the discharge curve of a Lithium battery. It is flat. Extremely flat.
- At 99% Full: ~13.4 Volts
- At 30% Full: ~13.1 Volts
A difference of just 0.3 Volts covers nearly 70% of your battery’s capacity. If your multimeter is cheap or your wiring has voltage drop, your reading could be off by 50%.
Resting vs. Load Voltage
The chart and calculator above only work for Resting Voltage. This means the battery must have no load (nothing running) and no charger connected for at least 30 minutes.
According to Power Sonic, reading voltage while the fridge is running will give you a “False Low” due to voltage sag. Reading it while solar is charging will give you a “False High.”
The Only Real Solution: A Smart Shunt
Because voltage is so unreliable for Lithium, ShockTrail engineers strongly recommend installing a Battery Monitor with a Shunt (like the Victron SmartShunt).
A shunt counts the actual electrons flowing in and out (Coulomb Counting), giving you a 100% accurate reading regardless of voltage fluctuations.
LiFePO4 Voltage Chart (Reference)
Save this table for quick reference. Note how steep the drop-off is at the very end.
| State of Charge | 12V System | 24V System | 48V System |
|---|---|---|---|
| 100% (Charging) | 14.4V – 14.6V | 28.8V – 29.2V | 57.6V – 58.4V |
| 100% (Resting) | 13.6V | 27.2V | 54.4V |
| 50% (The Plateau) | 13.2V | 26.4V | 52.8V |
| 20% (Low) | 12.9V | 25.8V | 51.6V |
| 0% (Dead) | 10.0V | 20.0V | 40.0V |
Frequently Asked Questions (FAQ)
Why does my battery read 14.4V but drops to 13.6V immediately?
This is normal. 14.4V is the “Charging Voltage” needed to push energy in. Once the charger stops, the battery settles to its natural “Resting Voltage” of 13.6V. It is still 100% full.
Is 13.0V bad for Lithium?
No. 13.0V is roughly 30-40% charge. However, if it drops below 12.8V, you are entering the “knee” of the curve where voltage crashes rapidly. You should recharge immediately.
Relevant Search Tags
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