Skip to Content

Test it, charge it, or replace it — with numbers, not guesswork

Car Battery Hub

Car Battery Voltage to State of Charge Calculator

Battery Voltage to State of Charge

Enter the resting voltage from your own multimeter. The calculator looks it up in a published open-circuit voltage table for that chemistry and interpolates between rows.

Engine off, no charger, nothing drawing power, rested at least 4 to 12 hours (BCI's table assumes 24 h).

Shown for the record only. The BCI table is stated at 26 °C (78 °F); none of the cited tables publish a voltage correction, so none is applied.

Estimated state of charge ~63% Charge soon

12.35 V sits between the 12.24 V (50%) and 12.45 V (75%) rows of the BCI starter-battery table; linear interpolation gives 63%.

No temperature entered.

Surface charge after driving or charging reads high. If you have just driven, switch the headlights on for a minute, then rest the battery and measure again.

Bands used

Full: 90% and up. Good: 75 to 89%. Charge soon: 50 to 74% (OPTIMA notes sulfation begins below 12.4 V on lead-acid, and Trojan recharges stored batteries at 70 to 75%). Low, charge now: 25 to 49%. Discharged: under 25%. A reading above the 100% row is flagged as probable surface charge; a reading below the last row is flagged for a load test.

Tables used (12 V column drives the result)

BCI standard, flooded starter battery (26 °C / 78 °F, 24 h rest)
State of charge12 VSpecific gravity2 V cell6 V8 V
100%12.651.2652.106.328.43
75%12.451.2252.086.228.30
50%12.241.1902.046.128.16
25%12.061.1552.016.038.04
0%11.891.1201.985.957.72
Trojan flooded/wet deep-cycle: state of charge vs specific gravity and open-circuit voltage
State of charge12 VSpecific gravityCell6 V8 V
100%12.731.2772.1226.378.49
90%12.621.2582.1036.318.41
80%12.501.2382.0836.258.33
70%12.371.2172.0626.198.25
60%12.241.1952.0406.128.16
50%12.101.1722.0176.058.07
40%11.961.1481.9935.987.97
30%11.811.1241.9695.917.88
20%11.661.0981.9435.837.77
10%11.511.0731.9185.757.67
Trojan AGM: state of charge vs open-circuit voltage
State of charge12 VSpecific gravity*Cell6 V8 V
100%12.841.2952.146.428.56
75%12.541.2452.096.278.36
50%12.241.1952.046.128.16
25%11.941.1451.995.977.96
0%11.641.0951.945.827.76
Trojan gel: state of charge vs open-circuit voltage
State of charge12 VSpecific gravity*Cell6 V8 V
100%12.841.2952.146.428.56
75%12.661.2652.116.338.44
50%12.361.2152.066.188.24
25%12.001.1552.006.008.00
0%11.821.1251.975.917.88

*Trojan: specific gravity cannot be measured in a sealed AGM or gel battery; the value is approximate and used for freezing-point estimates.

Renogy LiFePO4 voltage chart (resting, open circuit)
State of charge12 V battery1 cell24 V battery48 V battery
100% charging14.63.6529.258.4
100% rest13.63.4027.254.4
90%13.43.3526.853.6
80%13.33.3226.653.1
70%13.23.3026.452.8
60%13.13.2726.152.3
50%13.03.2626.152.2
40%13.03.2526.052.0
30%12.93.2225.851.5
20%12.83.2025.651.2
10%12.03.0024.048.0
0%10.02.5020.040.0

6 V and 24 V readings are scaled to a 12 V equivalent before lookup; the tables' own 6 V columns agree with that scaling to within 0.01 V. Where two LiFePO4 rows share a voltage (13.0 V covers 40 to 50%), the calculator reports the midpoint of that span. Voltage is a weak state-of-charge signal for lithium between roughly 20 and 90%; a shunt-based monitor is the manufacturer's recommendation.

Sources: Battery University BU-903, Table 3 (BCI standard) · Trojan Battery User's Guide TRJN0109 (2019), Table 7 and section 9.4 via Trojan guides and manuals · Renogy LiFePO4 voltage chart · OPTIMA: basic information on battery voltage

How It Works

A rested lead-acid battery’s open-circuit voltage tracks its state of charge closely enough that manufacturers publish lookup tables. The calculator takes the voltage you read with your own multimeter (engine off, no charger, nothing drawing power, rested at least 4 to 12 hours), scales a 6 V or 24 V reading to a 12 V equivalent, and finds where it falls in the table for your chemistry.

Five tables are used, and each is reproduced above: the Battery Council International standard for flooded starter batteries (as printed in Battery University BU-903), Trojan’s flooded deep-cycle, AGM and gel tables from its User’s Guide, and Renogy’s resting-voltage chart for 12 V LiFePO4. Between rows the calculator interpolates in a straight line and says so in the result; it never adds rows of its own.

A reading above the 100% row is flagged as probable surface charge, the temporary high voltage a battery shows right after driving or charging. A reading below the last row is flagged for a load test rather than turned into a number. Temperature is recorded if you enter it, but no correction is applied: the BCI table is stated at 26 °C (78 °F) and none of the cited tables publish a voltage adjustment.

Worked Example

A 12 V flooded starter battery reads 12.35 V after sitting overnight. On the BCI table that sits between the 12.24 V (50%) and 12.45 V (75%) rows: (12.35 − 12.24) ÷ (12.45 − 12.24) × 25 + 50 = 63%. The band is “Charge soon”, since OPTIMA notes sulfation begins once a lead-acid battery sits below 12.4 V.

FAQ

Why does my battery read 12.9 V or more?

Surface charge. Right after driving or charging, the plates hold a temporary excess that reads higher than the true resting voltage. Switch the headlights on for a minute, let the battery rest, then measure again.

Which chemistry should I pick?

A standard car battery is a flooded starter battery, so use the BCI table. Deep-cycle flooded batteries (marine, RV, golf cart) use Trojan’s flooded table. AGM and gel batteries have their own Trojan tables because their fully charged voltage is higher. LiFePO4 12 V batteries are four cells in series and use the Renogy chart.

Does temperature change the reading?

The tables are stated at about 26 °C (78 °F). Trojan publishes a temperature correction for specific gravity, not for voltage, so this calculator records your temperature and applies nothing. Treat readings on a very cold or hot battery as approximate.

Why is the LiFePO4 result so coarse?

Lithium iron phosphate has a flat discharge curve: on Renogy’s chart 13.0 V covers 40 to 50% and the whole 20 to 90% span fits inside 0.6 V. Renogy and Battle Born both recommend a shunt-based battery monitor rather than voltage for lithium state of charge.

Is this a battery tester?

No. It converts a voltage you measured yourself into a table lookup. It cannot detect a failed cell, low cranking amps, or a battery that holds voltage but collapses under load; a load test or conductance tester does that.

Sources