Guide

LiFePO4 voltage chart: 12 V, 24 V and 48 V batteries

What your lithium battery’s voltage says about its charge, for 4, 8, 15 and 16 cells. And why the voltage alone is a poor fuel gauge for LiFePO4.

The chart

These are resting voltages: the battery has had no charge and no load for at least half an hour, at about 25 °C. A 12 V battery has 4 cells, a 24 V battery 8, and a 48 V battery 16 cells (51.2 V). Some batteries sold as 48 V have 15 cells.

Approximate resting voltages for LiFePO4. Cells and brands differ a little.
Charge	One cell	12 V (4 cells)	24 V (8 cells)	48 V (15 cells)	48 V (16 cells)
100%	3.40 V	13.6 V	27.2 V	51.0 V	54.4 V
90%	3.35 V	13.4 V	26.8 V	50.3 V	53.6 V
80%	3.32 V	13.3 V	26.6 V	49.8 V	53.1 V
70%	3.30 V	13.2 V	26.4 V	49.5 V	52.8 V
60%	3.28 V	13.1 V	26.2 V	49.2 V	52.5 V
50%	3.26 V	13.0 V	26.1 V	48.9 V	52.2 V
40%	3.25 V	13.0 V	26.0 V	48.8 V	52.0 V
30%	3.22 V	12.9 V	25.8 V	48.3 V	51.5 V
20%	3.20 V	12.8 V	25.6 V	48.0 V	51.2 V
10%	3.00 V	12.0 V	24.0 V	45.0 V	48.0 V
0%	2.50 V	10.0 V	20.0 V	37.5 V	40.0 V

Why the voltage is a poor fuel gauge

Look at the middle of the chart: from about 20% to 90%, a LiFePO4 cell only moves from 3.20 V to 3.35 V. That’s 0.15 V for 70% of the battery. On a 12 V battery, 13.1 V and 13.2 V can be 20% of charge apart, and a cheap voltmeter can’t tell the difference.

Load and charging make it worse. With the kettle on, the voltage sags. With the solar charging, it rises. Neither tells you the real charge.

That’s why your battery’s board (the BMS) counts the amp-hours going in and out instead. Its charge percentage is a much better gauge than any voltage chart, as long as it gets a full charge now and then to reset its count.

CellBoss shows the board’s own charge percentage, the amp-hours left and the time to full or empty, not a guess from the voltage. And it shows every cell, so you see the whole picture.

Charging voltages

The most a LiFePO4 cell should see is 3.65 V: 14.6 V for 12 V, 29.2 V for 24 V and 58.4 V for a 16-cell 48 V battery. The board cuts the charge off around there.
Many chargers and inverters charge to a little less, for a longer life. Use the settings your battery’s maker gives you.
Straight after charging, the voltage sits higher than the chart and settles down over the next hour.

Low voltages

Below about 3.0 V a cell is nearly empty, and most boards switch the discharge off somewhere between 2.5 V and 2.8 V a cell. Don’t leave a battery flat: charge it soon after a cut-off.

The lowest cell matters more than the total. If one cell reaches its limit first, the board stops the whole battery, even when the others still have charge. That’s why it pays to watch every cell, not only the battery’s voltage.

Cold weather

In the cold, the voltage reads lower and the battery gives less. And don’t charge LiFePO4 below 0 °C: most boards block it. On a frosty morning in the Karoo, let the battery warm up first. CellBoss Pro can give you a heads-up when it’s too cold to charge.

Questions

What voltage is a fully charged 12 V LiFePO4 battery?

About 13.6 V when it has rested after charging, and 14.2 V to 14.6 V while it’s still on the charger.

What voltage is a 48 V lithium battery at 50%?

About 52.2 V for a 16-cell (51.2 V) battery, or 48.9 V for a 15-cell one, at rest. Your board’s charge percentage is more reliable than the voltage.

What voltage is a flat 12 V LiFePO4 battery?

Around 12.0 V is nearly empty, and most boards cut off between 10 V and 11.2 V (2.5 V to 2.8 V a cell). Charge it soon after that.

Why does my battery show 13.2 V but the charge says 40%?

The voltage curve is very flat in the middle, so a small change in voltage is a big change in charge. Trust the board’s count over the voltage, as long as the battery gets a full charge now and then.