Did you know that a single voltage misreading can shorten your sealed lead acid (SLA) battery’s lifespan by up to 50%? These batteries power everything from emergency backup systems to solar storage, but most users don’t realize how critical voltage monitoring truly is.
Imagine your security system failing during an outage because the battery voltage dropped too low without warning. Voltage isn’t just a number—it’s the heartbeat of your battery’s health.
Best Tools for Monitoring Sealed Lead Acid Battery Voltage
Fluke 115 Digital Multimeter
The Fluke 115 is a top-tier multimeter for precise voltage readings on SLA batteries. Its True RMS accuracy ensures reliable measurements, while the rugged design withstands workshop conditions. The large backlit display and auto-ranging feature make it ideal for both beginners and professionals monitoring 6V/12V battery systems.
NOCO Genius5 Battery Charger/Maintainer
For maintaining optimal SLA battery voltage, the NOCO Genius5 (Model: GENIUS5) is unmatched. It features a built-in voltage meter, temperature compensation, and a repair mode to reverse sulfation. Its 5-amp output safely charges 6V and 12V batteries, extending lifespan by preventing over/undercharging.
Victron Energy BMV-712 Battery Monitor
The Victron BMV-712 offers advanced voltage tracking with Bluetooth connectivity. It logs historical data, alerts for low voltage, and calculates remaining battery capacity. Perfect for solar setups or RVs, this monitor provides real-time insights into your SLA battery’s health, ensuring you never face unexpected power failures.
How to Interpret a Sealed Lead Acid Battery Voltage Chart
Understanding your sealed lead acid battery’s voltage readings is critical for proper maintenance and longevity. Unlike simple “full/empty” indicators, voltage states reveal nuanced information about your battery’s health and remaining capacity. Let’s break down exactly how to read and apply these measurements in real-world situations.
The Science Behind Voltage and State of Charge
Voltage directly correlates with a battery’s state of charge due to electrochemical reactions in the cells. When fully charged, the lead plates contain maximum lead dioxide (positive plates) and sponge lead (negative plates), creating higher electrical potential. As the battery discharges:
- Chemical changes convert active materials to lead sulfate (PbSO4)
- Electrolyte density decreases from ~1.265 SG (full charge) to ~1.120 SG (discharged)
- Voltage drops proportionally as these reactions reduce the cell potential
For example, a 12V battery showing 12.1V (50% charge) has approximately half its lead plates converted to lead sulfate. This is why voltage measurements serve as reliable state-of-charge indicators when properly interpreted.
Critical Voltage Thresholds Every User Should Know
Three voltage benchmarks are particularly important for SLA battery maintenance:
- 12.6V+ (12V battery) indicates full charge – the ideal storage voltage that prevents sulfation
- 12.1V represents the 50% discharge point – the maximum recommended discharge depth for regular use
- 10.5V is the absolute minimum – further discharge causes permanent crystal formation
Consider this scenario: Your solar power system’s battery bank reads 11.9V after a cloudy day. According to the voltage chart, this 20% remaining charge means you should:
- Reduce load immediately to avoid deep discharge
- Prioritize recharging within 24 hours
- Consider adding battery capacity if this occurs frequently
Common Misinterpretations and How to Avoid Them
Many users make these critical mistakes when reading battery voltage:
- Mistake 1: Measuring voltage while charging or under load. This gives false readings – always test at rest (2+ hours after any charge/discharge activity). A battery showing 13V while charging might actually be at 11.8V when resting.
- Mistake 2: Ignoring temperature effects. Voltage drops about 0.003V per cell for every 1°C below 25°C. A battery reading 12.4V at 5°C is actually near full charge, not 75% as the standard chart suggests.
- Mistake 3: Assuming all 12V batteries behave identically. AGM batteries typically show slightly higher voltages (0.2-0.3V more) than flooded SLA types at the same state of charge.
For accurate assessments, always cross-reference your voltage readings with:
- Battery temperature
- Time since last charge/discharge
- Manufacturer’s specific voltage chart
Sealed Lead Acid Battery Voltage Basics
Before diving into the voltage chart, it’s essential to grasp the fundamentals of how voltage relates to your SLA battery’s state. Voltage is the electrical potential difference between the battery’s terminals, and it fluctuates based on factors like charge level, temperature, and load conditions. Here’s what you need to know:
1. Nominal Voltage vs. Actual Voltage
Most SLA batteries are labeled with a nominal voltage (e.g., 6V or 12V), but this is an average value. The actual voltage varies significantly during charging and discharging cycles. For example:
- A fully charged 12V SLA battery measures ~12.7–13.2V at rest.
- Under load, this can drop to 10.5V or lower before reaching critical discharge levels.
2. Key Voltage States Explained
Every SLA battery operates within four primary voltage states, each with technical implications:
- Fully Charged (100% capacity): 12.7V–13.2V (for 12V batteries). At this stage, the battery’s lead plates are saturated with lead dioxide (PbO2) and sponge lead (Pb).
- Partial State of Charge (PSOC): 12.1V–12.4V. Prolonged use in this range accelerates sulfation, a major cause of battery failure.
- Discharged (~20% capacity): 11.8V–12.0V. Further discharge risks irreversible damage due to sulfation.
- Deep Discharge (Critical): Below 10.5V. At this point, the battery may not recover even with recharging.
Example: A 12V SLA battery powering a UPS will show ~13V after charging but drop to 12.2V after 30 minutes of use. Monitoring this trend helps predict runtime.
3. Temperature’s Impact on Voltage
Voltage readings must be temperature-compensated. For every 1°C (1.8°F) below 25°C (77°F), subtract 0.003V per cell (0.018V for a 12V battery). Conversely, add voltage in hotter conditions. Ignoring this can lead to overcharging or undercharging.
Comprehensive Sealed Lead Acid Battery Voltage Chart
Below is a detailed voltage chart for 6V and 12V SLA batteries, covering all critical states:
| State of Charge | 12V Battery Voltage | 6V Battery Voltage |
|---|---|---|
| 100% (Fully Charged) | 12.7V–13.2V | 6.3V–6.6V |
| 75% | 12.4V | 6.2V |
| 50% | 12.1V | 6.0V |
| 20% (Discharged) | 11.8V | 5.9V |
| 0% (Deep Discharge) | Below 10.5V | Below 5.25V |
Pro Tip: Measure voltage at rest (after 2+ hours without charging/discharging) for accurate readings. Loaded voltages can be misleading.
Practical Applications and Troubleshooting
Now that you understand the chart, let’s explore real-world scenarios:
1. Charging Voltage Guidelines
SLA batteries require precise charging voltages to avoid damage:
- Float Charge: 13.5V–13.8V (for 12V batteries). Ideal for standby applications like backup power.
- Bulk Charge: 14.4V–14.8V. Used for rapid recharging but must be reduced once the battery reaches 80% capacity.
Warning: Exceeding 15V causes excessive gassing and water loss, even in “sealed” batteries.
2. Interpreting Low Voltage Readings
If your battery consistently shows:
- Below 12.4V at rest: It’s likely sulfated. Try a desulfation charger or replace the battery.
- Voltage drops rapidly under load: The battery may have lost capacity due to aging or deep cycles.
3. Extending Battery Life
To maximize lifespan:
- Keep batteries at 50%–100% charge; avoid deep discharges.
- Store at 12.4V (or 6.2V for 6V batteries) in a cool, dry place.
- Use a smart charger with temperature compensation.
Step-by-Step Guide to Testing and Maintaining SLA Battery Voltage
Proper voltage testing and maintenance procedures can significantly extend your sealed lead acid battery’s lifespan. Follow this professional-grade approach to ensure accurate readings and optimal battery health.
Preparing for Accurate Voltage Measurements
Before testing, create ideal conditions for reliable readings. First, disconnect all loads and chargers for at least 2 hours – this allows surface charge to dissipate. Gather these essentials:
- Quality digital multimeter (0.5% DC voltage accuracy or better)
- Temperature probe (infrared thermometer works well)
- Battery terminal cleaner (corrosion affects readings)
- Insulated gloves and eye protection
For example, when testing a 12V UPS battery, clean the terminals with baking soda solution first. Corrosion can create resistance that falsely lowers your voltage reading by 0.1-0.3V.
The Professional Testing Protocol
Follow this exact sequence for laboratory-grade accuracy:
- Measure ambient temperature near the battery (not the battery surface)
- Set multimeter to DC volts with resolution to 0.01V
- Connect probes directly to terminals – not cables or clamps
- Hold connection for 15 seconds until reading stabilizes
- Apply temperature compensation: Add 0.003V per cell per °C above 25°C
Imagine testing a 12V battery at 35°C: Your initial 12.5V reading would adjust to 12.44V (subtracting 0.06V for the 10°C temperature difference). This corrected value is what you compare to the voltage chart.
Advanced Maintenance Based on Voltage Readings
Different voltage ranges require specific maintenance actions:
- For batteries reading 12.4-12.6V: These are in the ideal maintenance range. Apply a float charge at 13.5V for 2 hours monthly to prevent self-discharge. This is perfect for seasonal equipment like marine batteries.
- For batteries at 11.8-12.2V: Immediately recharge using a 3-stage charger. First bulk charge at 14.4V until current drops, then absorption at 14.0V, finishing with float at 13.5V. This multi-stage approach prevents stress on weakened batteries.
- For persistent low voltage (below 11.8V): Try a desulfation cycle with a pulse charger before replacement. Many “dead” batteries recover 80% capacity after proper reconditioning. However, if voltage won’t hold above 12.2V after charging, replacement is advised.
Pro Tip: Maintain a voltage log – tracking weekly readings helps identify gradual capacity loss before failures occur. A healthy battery should never lose more than 0.1V per week when properly stored.
Advanced Voltage Analysis for Different SLA Battery Applications
Sealed lead acid batteries serve diverse purposes, each requiring specialized voltage management strategies. Understanding these application-specific voltage profiles ensures optimal performance across different use cases.
Voltage Characteristics by Battery Type
Not all SLA batteries behave identically. The three main types exhibit distinct voltage patterns:
| Battery Type | Full Charge Voltage | Recommended Discharge Cutoff | Float Voltage |
|---|---|---|---|
| Standard SLA (Gel) | 12.8-12.9V | 11.8V | 13.6-13.8V |
| AGM (Absorbent Glass Mat) | 13.0-13.2V | 12.0V | 13.2-13.4V |
| Deep Cycle SLA | 12.7-12.8V | 10.8V | 13.5-13.7V |
For instance, an AGM battery in a car audio system should never drop below 12.0V during operation, while a deep cycle marine battery can safely discharge to 10.8V for trolling motor applications.
Application-Specific Voltage Management
Backup Power Systems (UPS):
Maintain float voltage between 13.5-13.8V with monthly equalization charges at 14.4V. These systems require tighter voltage control – allowing voltage to drop below 12.4V during outages significantly reduces battery lifespan.
Solar Energy Storage:
Implement voltage-based charge control with these thresholds:
- Bulk charge until 14.4V (temperature compensated)
- Absorption phase at 14.2V until current drops to 3% of C20
- Float at 13.5V with weekly equalization
Advanced Diagnostic Techniques
Beyond simple voltage measurements, these professional methods provide deeper insights:
- Load Testing: Apply 50% CCA load for 15 seconds while monitoring voltage drop. Healthy batteries shouldn’t drop more than 0.5V from resting voltage.
- Voltage Recovery Test: After full discharge, measure how quickly voltage rebounds when load is removed. Slow recovery indicates sulfation.
- Cyclic Voltammetry: For laboratory-grade analysis, this measures voltage response to controlled current variations, revealing detailed electrochemical health.
For example, a solar battery bank showing 12.2V at rest but dropping to 10.1V under minimal load likely has severe sulfation requiring immediate attention.
Pro Tip: Always cross-reference voltage readings with specific gravity measurements (when possible) and internal resistance tests for complete battery health assessment.
Voltage Optimization Strategies for Maximum Battery Lifespan
Proper voltage management can extend SLA battery life by 2-3 times compared to typical usage patterns. These advanced techniques go beyond basic maintenance to optimize performance across different operational conditions.
Precision Charging Techniques
The charging algorithm you use significantly impacts battery longevity. Follow this professional three-phase approach:
- Bulk Phase (14.4-14.8V): Apply maximum current until voltage reaches 75% capacity. This high-voltage stage should never exceed 4 hours to prevent overheating.
- Absorption Phase (13.8-14.2V): Maintain elevated voltage while current tapers. For AGM batteries, limit this phase to 2-3 hours to avoid electrolyte dry-out.
- Float Phase (13.2-13.6V): Compensates for self-discharge without overcharging. In hot environments (35°C+), reduce float voltage by 0.003V/°C/cell.
For example, a telecom backup battery bank in a 40°C equipment shelter should float at 13.1V instead of the standard 13.5V to prevent accelerated aging.
Advanced Discharge Management
Discharge profiles should adapt to your specific application:
| Application | Recommended Depth of Discharge | Voltage Cutoff | Recovery Protocol |
|---|---|---|---|
| Emergency Lighting | 30% (shallow) | 12.4V | Immediate recharge within 8 hours |
| Solar Storage | 50% (moderate) | 12.1V | Next-day full recharge |
| Wheelchair/Mobility | 80% (deep) | 11.6V | Equalization charge within 4 hours |
Professional Maintenance Protocols
Implement these expert-recommended practices:
- Monthly Equalization: Apply 15.0-15.5V for 2-4 hours to balance cells (only for flooded SLA types)
- Quarterly Capacity Testing: Discharge at C/20 rate to 10.5V while logging voltage drop patterns
- Annual Internal Resistance Check: Compare to manufacturer specs – 20% increase indicates aging
For critical systems like hospital UPS backups, we recommend installing automated battery monitoring systems that track:
- Voltage trends over time
- Charge/discharge cycle counting
- Temperature-compensated voltage thresholds
- Predictive failure alerts
Safety Note: Always wear acid-resistant gloves when working with batteries showing voltage below 11V – this indicates possible cell reversal and hydrogen gas risk. Never attempt to charge batteries reading below 8V as this may cause thermal runaway.
Long-Term SLA Battery Voltage Management and Sustainability
Effective voltage management extends beyond daily operations to encompass lifecycle optimization, environmental responsibility, and future-proofing your battery investments. This section explores the comprehensive strategies for maximizing value while minimizing ecological impact.
Lifecycle Voltage Profiling and Predictive Maintenance
Tracking voltage patterns over time reveals critical aging indicators:
| Battery Age | Expected Voltage Characteristics | Maintenance Action | Remaining Useful Life |
|---|---|---|---|
| 0-12 months | Stable voltage, <1% drop under load | Standard charging cycles | 100-80% |
| 13-36 months | 5-8% voltage drop, slower recharge | Monthly equalization | 80-50% |
| 37+ months | 10-15% voltage drop, rapid self-discharge | Capacity testing every 2 months | 50-0% |
For instance, a 3-year-old forklift battery showing 12.1V at rest (previously 12.6V when new) with 25% faster discharge rates indicates approximately 40% remaining capacity – signaling the need for replacement planning.
Environmental and Safety Considerations
Proper voltage management directly impacts sustainability:
- Overcharging prevention (keeping below 14.8V) reduces gassing and electrolyte loss
- Deep discharge avoidance (>10.5V) minimizes lead sulfate crystallization
- Temperature compensation decreases thermal stress and energy waste
Modern best practices include:
- Implementing adaptive charging algorithms that adjust voltage based on usage patterns
- Using smart battery monitors that track cumulative amp-hour throughput
- Adopting hybrid charging stations that combine solar with grid power
Cost-Benefit Analysis of Voltage Optimization
Investing in proper voltage management yields significant returns:
| Investment | Cost | Benefit | ROI Period |
|---|---|---|---|
| Smart Charger | $150-$400 | 30-50% longer battery life | 8-12 months |
| Battery Monitoring System | $200-$800 | Predictive maintenance savings | 14-18 months |
| Temperature-Controlled Storage | $500-$2000 | 2-3x cycle life extension | 24-36 months |
For example, a data center using 20 SLA batteries could save $8,000-$12,000 annually by implementing voltage-based maintenance instead of calendar-based replacement.
Future Trends: Emerging technologies like AI-powered voltage prediction and self-healing battery chemistries promise to revolutionize SLA maintenance, potentially extending useful life beyond 10 years with proper voltage management.
System Integration and Advanced Voltage Monitoring Techniques
Modern SLA battery systems rarely operate in isolation – their voltage management must integrate seamlessly with power systems, monitoring networks, and operational protocols. This section explores professional integration strategies for optimal performance across complex installations.
Voltage Monitoring in Integrated Power Systems
When connecting SLA batteries with other power components, consider these critical voltage thresholds:
| System Type | Ideal Operating Voltage | Voltage Tolerance | Integration Requirement |
|---|---|---|---|
| Solar Hybrid Systems | 13.2-13.8V (float) | ±0.5V | MPPT controller synchronization |
| UPS Backup Arrays | 12.4-13.2V (standby) | ±0.3V | Automatic voltage compensation |
| Telecom Power Plants | 13.6-13.8V (constant) | ±0.2V | Redundant monitoring circuits |
For example, integrating a 48V SLA battery bank (four 12V units) with a solar inverter requires individual cell monitoring to prevent voltage imbalance – no single battery should deviate more than 0.2V from others in the string.
Advanced Voltage Monitoring Methodologies
Professional installations employ these sophisticated monitoring techniques:
- Dynamic Voltage Profiling: Records voltage response to load changes at 100ms intervals to detect early degradation
- State-of-Charge Algorithms: Combines voltage readings with coulomb counting for ±1% accuracy
- Predictive Failure Analysis: Tracks voltage recovery rates after discharge to forecast remaining cycles
In marine applications, we recommend installing three-tier monitoring:
- Basic voltage display at helm station
- Data-logging monitor at battery bank
- Cloud-connected gateway for remote alerts
Troubleshooting Complex Voltage Issues
When facing persistent voltage problems in integrated systems:
- Check for ground faults – Even small 0.5V leaks can cause chronic undercharging
- Verify charger compatibility – Some “smart” chargers conflict with battery management systems
- Test under realistic loads – Voltage drops that only appear at 50%+ load indicate connection issues
A common integration mistake is using different cable lengths in parallel battery banks – even 2ft differences can create 0.1-0.3V imbalances. Always use identical cable specs for all parallel connections.
Pro Tip: For critical systems, implement a voltage validation protocol that cross-checks readings from at least two independent monitoring points to eliminate single-point failures.
Mastering SLA Battery Voltage: Professional-Grade Optimization and Risk Management
Reaching peak performance with sealed lead acid batteries requires a holistic approach that combines precise voltage control with comprehensive system oversight. This final section synthesizes advanced techniques used by industry professionals to achieve maximum reliability and efficiency.
Advanced Voltage Optimization Framework
Implement this four-tier optimization strategy for industrial-grade results:
| Tier | Voltage Parameter | Optimization Technique | Expected Improvement |
|---|---|---|---|
| 1 | Static Voltage Levels | Temperature-compensated charging | 15-20% lifespan increase |
| 2 | Dynamic Response | Load-dependent voltage profiling | 12% efficiency gain |
| 3 | Cyclic Behavior | Adaptive discharge thresholds | 30% more usable cycles |
| 4 | Long-term Drift | Predictive voltage trend analysis | 50% reduction in failures |
For mission-critical applications like hospital power backups, implementing all four tiers can extend battery string life from 3-5 years to 7-10 years while maintaining 95%+ reliability.
Comprehensive Risk Assessment Protocol
Professional battery managers evaluate these five voltage-related risk factors:
- Voltage Imbalance: >0.2V difference between parallel batteries indicates developing faults
- Charge Acceptance: Failure to reach 14.4V within specified time reveals sulfation
- Resting Voltage Decay: >0.1V/day drop suggests internal leakage
- Load Response: >1V drop at 50% load signals connection issues
- Temperature Correlation: Abnormal voltage/temperature ratios warn of thermal runaway risk
For example, a battery bank showing perfect 13.2V at rest but dropping to 11.8V under 30% load likely has corroded interconnects needing immediate attention.
Quality Assurance and Validation Procedures
Implement these professional validation checks:
- Weekly: Full voltage profile under three load levels (0%, 25%, 50%)
- Monthly: 24-hour voltage hold test (fully charged batteries shouldn’t drop >0.3V)
- Quarterly: Comparative analysis of all parallel battery voltages (max 0.15V deviation)
- Annually: Full discharge test with voltage logging every 5 minutes
Data centers typically automate these tests using battery monitoring systems that generate compliance reports meeting IEEE 1188 standards. Modern systems can detect developing issues 6-8 months before failure through subtle voltage pattern changes.
Final Recommendation: Combine these advanced techniques with the fundamental voltage chart knowledge from earlier sections to create a complete SLA battery management system. Remember that voltage is both the most accessible and most informative parameter for assessing and maintaining battery health across all applications.
Conclusion: Mastering SLA Battery Voltage for Optimal Performance
Throughout this comprehensive guide, we’ve explored the critical relationship between voltage readings and sealed lead acid battery health. From interpreting basic voltage charts to implementing advanced monitoring systems, you now understand how proper voltage management can double or triple your battery’s lifespan while preventing unexpected failures. Remember these key takeaways:
- Voltage is the most reliable indicator of state-of-charge and overall battery health
- Temperature compensation and proper measurement techniques are essential for accuracy
- Different applications require tailored voltage management strategies
- Advanced monitoring can predict issues months before failure occurs
Armed with this knowledge, we encourage you to implement these voltage monitoring practices with your SLA batteries today. Whether you’re maintaining a simple backup system or managing an industrial battery bank, consistent voltage tracking will save you time, money, and frustration.
Keep your voltage chart handy, invest in proper monitoring tools, and enjoy the peace of mind that comes with truly understanding your battery’s condition.
Frequently Asked Questions About Sealed Lead Acid Battery Voltage
What is the ideal voltage range for maintaining a sealed lead acid battery?
The optimal voltage range depends on the battery’s state. For a 12V SLA battery, maintain 12.6-12.8V when fully charged at rest.
During operation, never let voltage drop below 10.5V. For storage, keep between 12.4-12.6V to prevent sulfation. AGM batteries typically show 0.2-0.3V higher readings than flooded types at the same charge state. Always check manufacturer specifications as some deep-cycle variants have different optimal ranges.
How often should I check my SLA battery’s voltage?
For critical applications (UPS, medical equipment), check voltage weekly. For seasonal equipment, test monthly during storage. Always measure:
- Before and after charging cycles
- After extended periods of inactivity
- When noticing performance issues
Use a quality digital multimeter and measure at the terminals after 2+ hours of rest for accurate readings. Log results to track trends over time.
Why does my battery show 13V when charging but drops to 11.8V when disconnected?
This indicates either severe sulfation or capacity loss. The charging voltage is superficial, while the rapid drop reveals the battery can’t hold charge. Test capacity by:
- Fully charging to 12.6V+
- Applying known load (e.g., 5A for 1 hour)
- Measuring voltage drop rate
If voltage plummets below 12V within 30 minutes under light load, the battery likely needs replacement. This symptom often appears in batteries over 3 years old.
Can I revive a deeply discharged SLA battery reading below 10V?
Possibly, but with limitations. First, attempt a slow charge at 0.1C (e.g., 2A for 20Ah battery) for 24 hours. If voltage rises above 12V, complete a full charge cycle. However, expect:
- 20-40% permanent capacity loss
- Reduced cycle life
- Higher self-discharge rates
Batteries below 8V are usually unrecoverable. For critical applications, replacement is safer than attempting revival.
How does temperature affect SLA battery voltage readings?
Temperature significantly impacts voltage – for every 1°C below 25°C (77°F), subtract 0.003V per cell (0.018V for 12V battery). Conversely, add voltage in hotter conditions. Example: A battery reading 12.6V at 15°C is actually at ~12.7V when temperature-compensated. Always measure ambient temperature near the battery (not case temperature) for accurate compensation calculations.
What’s the difference between float voltage and equalization voltage?
Float voltage (typically 13.2-13.8V for 12V batteries) maintains charge without overcharging. Equalization voltage (14.4-15V) is a periodic higher charge that:
- Balances cells in multi-battery systems
- Breaks down sulfate crystals
- Should only be done monthly for 2-4 hours
Warning: Never equalize sealed VRLA batteries – only flooded types with removable caps can safely handle equalization.
Why do my parallel-connected SLA batteries show different voltages?
Voltage differences in parallel configurations indicate:
- Unequal cable lengths/resistance (keep within 5% length difference)
- Varying battery ages/capacities
- Poor terminal connections
Acceptable variance is ≤0.1V. For imbalances >0.3V:
- Disconnect and charge batteries individually
- Clean all terminals and connections
- Ensure identical cable specs
- Consider adding a battery balancer
How can I accurately estimate remaining runtime using voltage?
For reasonably accurate estimates:
- Fully charge battery and note resting voltage (should be ~12.6V)
- Apply typical operating load
- Create a discharge curve by logging voltage every 15 minutes
Example: If voltage drops from 12.6V to 12.1V in 2 hours under 5A load, you can estimate ~4 hours total runtime to 10.5V cutoff. For precise measurements, combine voltage readings with amp-hour counting for ±5% accuracy.