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Use these charts to estimate a battery’s state of charge (SoC) from its resting voltage. For example, a 12 V lead-acid battery at about 12.7 V is full. It is 50% at 12.1 V. A 12 V LiFePO4 battery at about 13.4 V at rest is full. It is 50% at around 13.1 V. However, lithium voltage changes very little in the middle. So, a battery monitor is far more reliable.
Battery state-of-charge from voltage
Estimates use the formulas explained in this guide. Plan a complete system with the all-in-one calculator →
How to measure correctly
Voltage is only meaningful when the battery is at rest. This means it is not charging or discharging. Wait at least 2 to 4 hours for lead-acid batteries. Also, wait about 30 minutes or more for lithium batteries. Voltage reads low when it is under load. It reads high while it is charging. Measure it at the battery terminals. You should use a decent multimeter.
12 V lead-acid battery voltage chart at about 25 °C
| State of charge | 12 V | 24 V | 48 V |
|---|---|---|---|
| 100% | 12.70 | 25.40 | 50.80 |
| 90% | 12.60 | 25.20 | 50.40 |
| 80% | 12.50 | 25.00 | 50.00 |
| 70% | 12.37 | 24.74 | 49.48 |
| 60% | 12.24 | 24.48 | 48.96 |
| 50% | 12.10 | 24.20 | 48.40 |
| 40% | 11.96 | 23.92 | 47.84 |
| 30% | 11.81 | 23.62 | 47.24 |
| 20% | 11.66 | 23.32 | 46.64 |
| 10% | 11.51 | 23.02 | 46.04 |
Stay above 50% (12.1 V) for good lead-acid battery life. See depth of discharge. AGM batteries often rest about 0.1 V higher than flooded batteries. Therefore, check your maker’s chart.
LiFePO4, resting voltage (approximate)
| State of charge | 12.8 V (4 cells) | 25.6 V (8 cells) | 51.2 V (16 cells) |
|---|---|---|---|
| 100% (just after charge) | 13.6 | 27.2 | 54.4 |
| 100% (rested) | 13.4 | 26.8 | 53.6 |
| 90% | 13.3 | 26.6 | 53.2 |
| 70% | 13.2 | 26.4 | 52.8 |
| 50% | 13.1 | 26.2 | 52.4 |
| 30% | 13.0 | 26.0 | 52.0 |
| 20% | 12.9 | 25.8 | 51.6 |
| 10% | 12.5 – 12.8 | 25.0 – 25.6 | 50.0 – 51.2 |
| 0% | 10.0 – 12.0 | 20.0 – 24.0 | 40.0 – 48.0 |
Notice that 90% and 20% are only about 0.4 V apart. This applies to a 12 V battery. Small measurement errors or temperature changes can make a voltage reading misleading. Also, exact values vary by cell maker.
Better ways to track lithium SoC
- Shunt battery monitor: This device counts amp-hours in and out. It uses coulomb counting. As a result, it is accurate to a few percent. It also resynchronizes at full charge.
- BMS Bluetooth app: many LiFePO4 batteries report SoC directly.
- Inverter or charge controller SoC: This is useful when connected to the battery via a communications cable. However, voltage-only estimates are less reliable.
Charging voltages (12 V reference)
| Setting | LiFePO4 | AGM | Flooded |
|---|---|---|---|
| Bulk/absorption | 14.2 – 14.6 V | 14.4 – 14.7 V | 14.4 – 14.8 V |
| Float | 13.4 – 13.6 V (or off) | 13.5 – 13.8 V | 13.2 – 13.5 V |
| Low-voltage cutoff (under load) | about 12.0 V | about 11.8 – 12.0 V | about 11.8 – 12.0 V |
Multiply by 2 for 24 V lead-acid systems. Also, multiply by 4 for 48 V lead-acid systems. For 16-cell LiFePO4, multiply 12.8 V values by 4. Always follow your battery manufacturer’s settings. See inverter low voltage cutoff.
Temperature affects readings
Cold temperatures slightly drop lead-acid resting voltage. Also, capacity falls. Many chargers use temperature compensation for lead-acid batteries. However, you must not charge LiFePO4 below 0 °C. See charging LiFePO4 in cold weather.
Worked example: estimating SoC on a lead-acid bank
A cabin owner checks a 24 V AGM bank at 7 am, after no charging or loads overnight except a small fridge:
- Reading: 24.45 V
- Rested? Mostly; the fridge draws a little, so the reading is slightly low.
- Chart: 24.48 V is about 60%; 24.20 V is 50%.
- Estimate the charge at about 55 to 60%. The owner should limit loads. Wait until solar recharges the bank. Also, fully charge it within a day or two. This avoids sulfation.
Why lithium voltage alone misleads
A 12 V LiFePO4 battery might read:
- 13.30 V at 85%
- 13.25 V at 60%
- 13.20 V at 40%
That is only 0.1 V across 45% of capacity. Therefore, a meter error of 0.05 V can change your estimate. Also, a small load during measurement changes it by 20% or more. So, use the BMS app or a shunt monitor for lithium.
Voltage under load vs at rest
| Situation (12 V LiFePO4, about 50% SoC) | Typical voltage |
|---|---|
| At rest | 13.1 – 13.2 V |
| Light load (5 A) | about 13.1 V |
| Heavy load (100 A) | about 12.6 – 12.8 V |
| Charging at 20 A | about 13.4 – 13.6 V |
For AGM the drop under heavy load is larger, often 0.5 to 1.0 V. That is why low-voltage cutoffs need careful setting. See inverter low voltage cutoff.
Setting up a shunt battery monitor
- Install the shunt on the battery negative, so every load and charger passes through it.
- Enter battery capacity (Ah), chemistry and charged voltage.
- Set the “charged” detection (for example 14.0 V and tail current 4% for LiFePO4).
- Fully charge once to synchronise.
- Read SoC, current, time-to-go and history from the display or app.
Common mistakes
- Measuring while charging or under load and treating it as resting voltage.
- Using a lead-acid chart for lithium or vice versa.
- Ignoring cable voltage drop when reading voltage at the inverter rather than at the battery.
Checking a 48 V LiFePO4 rack battery
Rack batteries usually report SoC directly to the inverter. However, if you only have voltage, approximate resting values for a 16-cell (51.2 V) pack are:
| SoC | Approx. resting voltage |
|---|---|
| 100% | 53.4 – 53.6 V |
| 70% | 52.8 V |
| 50% | 52.4 V |
| 20% | 51.6 V |
| 10% | about 50 – 51 V |
As with 12 V packs, the middle range is flat, so rely on the BMS data where possible.
FAQ
What voltage is a dead 12 V battery?
A resting lead-acid battery is nearly empty below 11.8 V. Also, this damages the battery. A LiFePO4 battery’s BMS may shut off. Then it might read 0 V or around 10 V. You can reset it with a charger.
Is 12.4 V a good battery voltage?
For lead-acid, this means roughly 75% charged. In contrast, 12.4 V at rest indicates under about 10% for LiFePO4.
Why does my battery voltage jump after charging stops?
It doesn’t jump. Instead, it relaxes downward from the charging voltage to its resting voltage over minutes to hours.
What voltage is 50% for a 12 V LiFePO4 battery?
Roughly 13.1 V at rest, but use a monitor; voltage is not precise for lithium.
What voltage should a 12 V AGM battery be when fully charged?
About 12.8 to 12.9 V at rest a few hours after charging, depending on the manufacturer.
Related guides
- Lead-Acid Battery Maintenance for Solar (Flooded and AGM)
- 12V vs 24V vs 48V Solar System: Which Voltage Should You Choose?
- How to Wire Batteries in Series and Parallel (12V, 24V, 48V)
- LiFePO4 vs AGM Batteries for Solar: Which Is Better?
Sources and further reading
- Battery University: BU-409b Charging Lithium Iron Phosphate: LiFePO4 charging voltages and behaviour.
- Battery University: BU-415 How and When to Charge: charging habits for lead-acid and lithium batteries.
- Battery University: BU-205 Types of Lithium-ion: how lithium chemistries, including LiFePO4, compare.