State of charge and resting voltage
| Charge | Battery | Range in the charts | Per cell |
|---|---|---|---|
| 100 % | 12.73 V | 12.65–12.73 V | 2.122 V |
| 90 % | 12.62 V | 12.57–12.62 V | 2.103 V |
| 80 % | 12.50 V | 12.49–12.50 V | 2.083 V |
| 70 % | 12.37 V | 12.37–12.41 V | 2.062 V |
| 60 % | 12.24 V | 12.24–12.32 V | 2.040 V |
| 50 % | 12.10 V | 12.10–12.24 V | 2.017 V |
| 40 % | 11.96 V | 11.96–12.17 V | 1.993 V |
| 30 % | 11.81 V | 11.81–12.10 V | 1.969 V |
| 20 % | 11.66 V | 11.66–12.03 V | 1.943 V |
| 10 % | 11.51 V | 11.51–11.96 V | 1.918 V |
Resting voltage at about 27 °C. Typical: Trojan (10–100 %); range: Trojan and Battery University. Readings taken hours after the last charge or discharge.
What to know
Measure after 6–24 hours with no charge or load (Trojan): right after charging, surface charge makes the voltage read high, and under load it reads low. The voltage falls almost in a straight line, about 0.14 V for every 10 %, so a rested reading is a fair guide.
Charging and limits
| Charge (absorption) | 14.4–14.8 V |
|---|---|
| Float | 13.2–13.5 V |
| Equalize | 15.5–16.2 V |
| Discharge cut-off | 10.5 V under a 20-hour load: rated capacity is measured down to here |
| Storage | recharge when it falls to 70 % |
| Charging temperature | never charge a frozen battery; use temperature compensation |
From the datasheets and manuals in the sources: the datasheet of your battery or charger comes first.
Tips
Measure after 6–24 hours with no charge and no load (Trojan). Right after charging, surface charge makes the voltage read high.
On a flooded battery, a hydrometer reading of every cell is more precise than voltage and shows a weak cell; Trojan’s table gives both.
A discharged lead-acid battery can freeze below 0 °C (Trojan): keep it charged in winter, and never charge a frozen battery.
Open in the calculator →Enter the voltage you measured and get the charge with its range.
Other batteries
12 V LiFePO4 battery (4S) 24 V LiFePO4 battery (8S) 48 V LiFePO4 battery (16S) LiFePO4 cell (3.2 V) 18650 / 1S LiPo cell (3.7 V) 2S Li-ion / LiPo (7.4 V) 3S Li-ion / LiPo (11.1 V) 4S Li-ion / LiPo (14.8 V) 6S LiPo (22.2 V) 36 V Li-ion battery (10S) 48 V Li-ion battery (13S) LiHV cell (4.35 V) 12 V AGM battery 12 V gel battery 6 V flooded lead-acid battery NiMH cell (1.2 V)
Frequently asked questions
What voltage is a 12 V flooded lead-acid battery at 50 %?
About 12.10 V at rest; the charts we used range from 12.10 V to 12.24 V.
What voltage is a 12 V flooded lead-acid battery at 100 %?
About 12.73 V after 6–24 hours of rest. The charger holds it at 14.4–14.8 V during absorption and 13.2–13.5 V on float.
At what voltage is a 12 V flooded lead-acid battery empty?
At rest, about 11.51 V means only about 10 % is left. Rated capacity is measured down to 10.5 V under a 20-hour load: don’t leave the battery there.
Sources: the makers’ charts and datasheets listed on the calculator page.