The voltage of a lead-acid battery decreases as it discharges, ranging from its fully charged voltage down to a lower voltage when fully depleted.
Understanding Lead-Acid Battery Voltage
Lead-acid batteries are composed of individual cells connected in series. The voltage of the entire battery is the sum of the voltages of its cells.
- A lead–acid battery's nominal voltage is 2.2 V for each cell. This is a reference value often used to describe the battery type.
- The actual voltage varies significantly depending on the state of charge and whether a load is applied.
Voltage Range During Discharge
During discharge, the voltage of a lead-acid cell drops progressively. Based on the provided reference:
For a single cell, the voltage can range from 2.10 V in an open circuit at full charge down to 1.8 V loaded at full discharge.
This means that as the battery discharges under load, the voltage per cell will typically start slightly below the open-circuit full charge voltage and gradually drop towards 1.8 V, which is considered the point of full discharge under load.
Key Voltage Points Per Cell
Here are the typical voltage points for a single lead-acid cell:
- Nominal Voltage: 2.2 V
- Fully Charged (Open Circuit): 2.10 V
- Fully Discharged (Loaded): 1.8 V
Example: A Common 12V Battery
Most automotive and deep-cycle lead-acid batteries are 12V. A 12V lead-acid battery is made up of six individual cells connected in series (6 cells * 2.2 V/cell = 13.2 V nominal, although nominal is often simplified to 12V).
To find the voltage range for a 12V battery, we multiply the single-cell voltages by six:
| State | Voltage Per Cell | Voltage for 12V Battery (6 cells) | Notes |
|---|---|---|---|
| Nominal | 2.2 V | 13.2 V | Reference voltage |
| Fully Charged (OC) | 2.10 V | 12.6 V | Open circuit voltage when full |
| Fully Discharged (L) | 1.8 V | 10.8 V | Loaded voltage when fully discharged |
OC = Open Circuit (no load), L = Loaded
This table illustrates that a fully charged 12V lead-acid battery will measure around 12.6V when no load is applied. As it discharges under load, the voltage will drop, reaching approximately 10.8V when it is considered fully discharged.
Why Voltage Matters During Discharge
Monitoring the voltage is crucial during lead-acid battery discharge for several reasons:
- State of Charge: Voltage provides a general indication of how much capacity is left, although it's best measured under a consistent load after resting for a while.
- Performance: As voltage drops, the power available from the battery decreases.
- Battery Health: Discharging a lead-acid battery below its recommended minimum voltage (typically 1.8 V per cell or 10.8 V for a 12V battery under load) can cause damage, particularly sulfation, which can reduce the battery's capacity and lifespan.
Proper monitoring and avoiding deep discharge below the recommended levels are essential for maximizing the life of a lead-acid battery.
