How to measure resting voltage
Voltage only reflects state of charge when the battery is at rest. Switch off all loads and chargers (including solar) and wait at least 2–4 hours for lead-acid, or 30 minutes or more for lithium. A battery that's charging reads high; one under load reads low.
12 V lead-acid voltage chart
| State of charge | 12 V | 24 V | 48 V |
|---|---|---|---|
| 100% | 12.73 | 25.46 | 50.92 |
| 90% | 12.62 | 25.24 | 50.48 |
| 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 |
AGM batteries usually read slightly higher (around 0.05–0.1 V per 12 V block) than flooded batteries at the same charge.
12 V LiFePO4 voltage chart
| State of charge | 12.8 V | 25.6 V | 51.2 V |
|---|---|---|---|
| 100% (charged, settling) | 13.6 | 27.2 | 54.4 |
| 99% | 13.4 | 26.8 | 53.6 |
| 90% | 13.3 | 26.6 | 53.2 |
| 70% | 13.2 | 26.4 | 52.8 |
| 40% | 13.1 | 26.2 | 52.4 |
| 30% | 13.0 | 26.0 | 52.0 |
| 20% | 12.9 | 25.8 | 51.6 |
| 17% | 12.8 | 25.6 | 51.2 |
| 14% | 12.6 | 25.2 | 50.4 |
| 9% | 12.5 | 25.0 | 50.0 |
| 0% | 10.0 | 20.0 | 40.0 |
Figures vary a little between manufacturers and cell temperature. Treat both charts as guides, not exact values.
Frequently asked questions
What voltage is a fully charged 12V battery?
A rested, fully charged lead-acid battery reads about 12.7 V. A LiFePO4 battery settles at about 13.4–13.6 V after charging.
What voltage is a dead 12V battery?
A lead-acid battery below about 11.8 V at rest is badly discharged. LiFePO4 batteries drop sharply below about 12.5 V and the BMS usually cuts off around 10–11.5 V.
Why can't I tell LiFePO4 charge from voltage?
Lithium iron phosphate has a very flat voltage curve: from 20% to 90% the voltage changes by only about 0.3 V. A shunt-based battery monitor counts amp hours in and out instead.