12V battery voltage chart: state of charge
A resting lead-acid battery’s voltage tells you roughly how full it is. This chart gives the state of charge for a 12 V flooded lead-acid battery from Trojan’s data sheet, scaled to 24 V and 48 V, and explains why the same trick doesn’t work well on lithium.
12 V flooded lead-acid: voltage vs state of charge
| State of charge | 12 V battery | 24 V bank | 48 V bank | Per cell | Specific gravity |
|---|---|---|---|---|---|
| 100% | 12.73 V | 25.46 V | 50.92 V | 2.122 V | 1.277 |
| 90% | 12.62 V | 25.24 V | 50.48 V | 2.103 V | 1.258 |
| 80% | 12.5 V | 25 V | 50 V | 2.083 V | 1.238 |
| 70% | 12.37 V | 24.74 V | 49.48 V | 2.062 V | 1.217 |
| 60% | 12.24 V | 24.48 V | 48.96 V | 2.04 V | 1.195 |
| 50% | 12.1 V | 24.2 V | 48.4 V | 2.017 V | 1.172 |
| 40% | 11.96 V | 23.92 V | 47.84 V | 1.993 V | 1.148 |
| 30% | 11.81 V | 23.62 V | 47.24 V | 1.969 V | 1.124 |
| 20% | 11.66 V | 23.32 V | 46.64 V | 1.943 V | 1.098 |
| 10% | 11.51 V | 23.02 V | 46.04 V | 1.918 V | 1.073 |
Read it this way: a rested 12 V battery at 12.10 V is about half full — the point Trojan recommends you don’t go below for long life. At 12.50 V it is about 80% full; below about 11.8 V it is nearly flat and should be recharged right away. Victron’s Energy Unlimited book gives a very similar table for lead-acid (12.65 V and above full, about 12.24 V at half), which is a good sign the numbers are general, not just one brand’s.
How to take a reading that means something
- Let it rest. Voltage only tracks state of charge with no charging or load for a while; Trojan says to let the battery stand on open circuit for at least an hour. A battery just off the charger reads high; one under load reads low.
- Correct for temperature. Trojan’s chart is at 80 °F. Its charger settings compensate 0.005 V per cell per °C away from 25 °C — cold batteries read a little lower.
- Hydrometer beats voltmeter. For flooded batteries Trojan calls specific gravity the most accurate state-of-charge test; a reading of 1.277 or more means full.
- AGM and gel differ slightly. Sealed lead-acid batteries have their own tables on their data sheets; the shape is similar, the exact voltages aren’t.
Charging voltages are a different thing
While charging, a battery reads well above these resting numbers. Trojan’s 27TMX data sheet lists 14.82 V bulk and 13.50 V float for a 12 V flooded battery at 77 °F, and 16.20 V for an equalizing charge. Don’t read state of charge from a battery on charge.
LiFePO4: why voltage is a poor fuel gauge
A lithium iron phosphate battery holds a nearly flat voltage through most of its discharge, then drops quickly near empty and rises quickly near full. That is good for appliances — they see steady voltage — but it means a voltmeter can barely tell 70% from 30%. Victron’s LiFePO4 manual relies on a battery monitor, not voltage, for state of charge.
| Victron 12.8 V LiFePO4 | Voltage |
|---|---|
| Nominal | 12.8 V |
| Charge (recommended) | 14.2 V (14.0–14.4 V allowed) |
| Float | 13.5 V |
| End of discharge | 11.2 V |
For a lithium bank, a shunt battery monitor that counts amp-hours in and out is the reliable gauge. Some lithium batteries also report a state of charge over Bluetooth — check how yours works it out.
Putting it to work
If a voltmeter says your 12 V lead-acid battery is at 12.10 V, you have used about half of it — the point to stop for long life. That is the 50% depth-of-discharge default the calculators on this site use for lead-acid: how many watt-hours in a 100Ah battery shows what that leaves you, how long a 100Ah battery runs a fridge how long it lasts, and how long a 100W solar panel takes to charge a 100Ah battery how long it takes to put back.
Frequently asked questions
What voltage is a fully charged 12V battery?
A rested flooded lead-acid battery reads about 12.73 V at 100% on Trojan’s chart (80 °F). A 12.8 V LiFePO4 battery is charged to about 14.2 V (Victron), and its resting voltage is not a reliable gauge of how full it is.
What voltage is a 12V battery at 50%?
About 12.10 V for flooded lead-acid at rest (Trojan). For LiFePO4, voltage at 50% is too close to its neighbours to be useful — use a battery monitor.
What is a dead 12V battery voltage?
Trojan’s chart shows 11.51 V at 10% for a rested flooded battery; Victron’s LiFePO4 batteries treat 11.2 V as end of discharge. Recharge lead-acid long before that.
What is the voltage of a 24V or 48V battery bank at 50%?
Double or quadruple the 12 V figure for lead-acid: about 24.2 V and 48.4 V at rest (Trojan’s 12.10 V × 2 and × 4).
Sources
- Trojan Battery — 27TMX data sheet (opens in a new tab) — Group 27 flooded: 85 Ah (5-hr), 97 Ah (10-hr), 105 Ah (20-hr), 117 Ah (100-hr); state of charge vs open-circuit voltage table; charger settings.
- Trojan Battery — Technical support FAQs (opens in a new tab) — “We generally recommend a discharge of 20 to 50% of the battery’s rated capacity even though the battery is capable of being cycled to 80%”; 20-hour and 100-hour ratings; freezing points by state of charge.
- Victron Energy — Energy Unlimited (book) (opens in a new tab) — Lead-acid charge efficiency 80–95% (85% suggested as a starting value); battery energy efficiency about 75% for full cycles, about 89% in partial state of charge; inverter efficiencies above 90%.
- Victron Energy — Lithium Battery Smart manual, technical data (opens in a new tab) — LiFePO4 12.8 V: cycle life 2,500 at 80% DoD, 3,000 at 70%, 5,000 at 50% (to 80% of capacity); 92% round-trip efficiency; 12.8 V 100 Ah: recommended continuous discharge ≤100 A (1C), maximum 200 A; capacity about 20% lower at 0 °C; charge only between +5 °C and +50 °C.