The relationship between the voltage and capacity of a lead-acid battery is complex and influenced by various factors, including the state of charge, discharge rate, temperature, and the age and condition of the battery.
1. State of Charge (SOC)
- Fully Charged: A fully charged lead-acid battery typically has a voltage of around 12.6 to 12.8 volts for a 12V battery.
- Discharged: As the battery discharges, the voltage drops. A fully discharged 12V lead-acid battery usually reads around 11.8 volts.
- SOC vs. Voltage: There is a direct correlation between SOC and voltage. Voltage decreases as the SOC decreases.
2. Discharge Rate
- High Discharge Rates: At higher discharge rates, the voltage drops more quickly due to increased internal resistance and heat generation. This can result in a lower apparent capacity because the battery reaches its cutoff voltage faster.
- Low Discharge Rates: At lower discharge rates, the voltage drops more gradually, allowing the battery to deliver more of its capacity.
3. Temperature
- High Temperature: Higher temperatures generally increase capacity and voltage. However, excessive heat can accelerate degradation.
- Low Temperature: Lower temperatures reduce capacity and voltage. Cold weather can significantly diminish a lead-acid battery’s performance.
4. Battery Age and Condition
- New Battery: A new, well-maintained battery maintains its voltage and capacity better.
- Aging Battery: Over time, sulfation, corrosion, and material shedding reduce both the voltage and capacity. The battery’s ability to hold a charge and deliver its rated capacity diminishes.
5. Voltage as an Indicator of Capacity
- Open-Circuit Voltage: When the battery is at rest (no load), the open-circuit voltage can give an approximate indication of its SOC and remaining capacity.
- Under Load: The voltage under load can also provide insights into the battery’s health. Significant voltage drops under load indicate higher internal resistance and reduced capacity.
Typical Voltage and Capacity Relationship Chart
Here’s a simplified chart for a typical 12V lead-acid battery at room temperature:
| State of Charge (%) | Voltage (V) | Capacity (%) |
|---|---|---|
| 100 | 12.6 – 12.8 | 100 |
| 90 | 12.5 | 90 |
| 80 | 12.42 | 80 |
| 70 | 12.32 | 70 |
| 60 | 12.2 | 60 |
| 50 | 12.1 | 50 |
| 40 | 11.9 | 40 |
| 30 | 11.75 | 30 |
| 20 | 11.58 | 20 |
| 10 | 11.31 | 10 |
| 0 | 11.1 | 0 |
Important Considerations
- Measurement Conditions: Voltage readings should be taken after the battery has rested for a few hours for an accurate open-circuit voltage. Readings under load provide information about performance under real conditions.
- Load Impact: The capacity available at higher discharge rates will be less than at lower discharge rates due to Peukert’s Law, which states that the capacity of a lead-acid battery decreases as the discharge rate increases.
- Maintenance: Regular maintenance, including keeping the battery fully charged and ensuring proper electrolyte levels, helps maintain voltage and capacity.
Understanding the relationship between voltage and capacity helps in assessing the battery’s state of health, determining its remaining useful life, and ensuring it is used within optimal conditions.
