Battery Life Calculator – Calculate Battery Runtime
Estimate how long your battery will last with our battery life calculator. Enter battery capacity and load to calculate runtime in hours and minutes.
Enter battery capacity and voltage
Input the battery capacity in amp-hours (Ah) and the nominal voltage. These values are usually printed on the battery label.
Enter your load current or power
Input either the current draw in amps or the power consumption in watts. If you know watts, the calculator converts using your voltage.
Set depth of discharge and calculate
Choose your battery type for automatic DoD settings, or enter a custom value. Click Calculate to see estimated runtime.
| Battery Capacity | Load Current | Runtime (80% DoD) | Runtime (50% DoD) |
|---|---|---|---|
| 50 Ah | 5A | 8 hours | 5 hours |
| 50 Ah | 10A | 4 hours | 2.5 hours |
| 100 Ah | 5A | 16 hours | 10 hours |
| 100 Ah | 10A | 8 hours | 5 hours |
| 200 Ah | 10A | 16 hours | 10 hours |
| 200 Ah | 20A | 8 hours | 5 hours |
Note: Runtimes are theoretical. Actual performance varies with battery age, temperature, and discharge rate (Peukert effect).
The Basic Formula
Battery runtime equals usable capacity divided by load current. A 100Ah battery at 10A draw gives 10 hours theoretically. But you need to account for depth of discharge limits and efficiency losses.
Depth of Discharge Explained
DoD is the percentage of battery capacity you can safely use. Lead-acid batteries should not discharge below 50% — going deeper damages them. Lithium batteries can use 80-90% safely. This is why a 100Ah lithium battery often outperforms a 100Ah lead-acid battery.
The Peukert Effect
Battery capacity decreases at higher discharge rates. A 100Ah battery might deliver 100Ah at a 5A draw but only 80Ah at a 20A draw. This is the Peukert effect. Lead-acid batteries are more affected than lithium. This calculator uses nominal capacity — expect slightly less at high loads.
Temperature Effects
Cold temperatures reduce battery capacity. At 0°C (32°F), a lead-acid battery might deliver only 60-70% of its rated capacity. Heat increases capacity short-term but accelerates aging. Battery ratings are typically at 25°C (77°F).
Avoid deep discharges
Shallow cycles extend battery life. Discharging to 50% instead of 80% can double or triple cycle life for lead-acid batteries. Plan your capacity accordingly.
Keep batteries charged when storing
Lead-acid batteries self-discharge and can sulfate if left empty. Store at full charge and recharge every few months. Lithium batteries store better at 50-60% charge.
Match battery capacity to your loads
Oversizing your battery bank reduces depth of discharge per cycle. If you need 50Ah per day, use a 200Ah battery for 25% DoD instead of a 100Ah battery at 50% DoD.
Monitor battery voltage
Learn the voltage levels for your battery type. A 12V lead-acid battery at 12.0V is nearly empty. At 12.7V it is full. Voltage monitoring prevents accidental over-discharge.
How do I calculate battery life in hours?
Divide the usable battery capacity by the load current. For a 100Ah battery at 80% DoD powering a 10A load: (100 × 0.80) / 10 = 8 hours. This calculator handles the math automatically.
What depth of discharge should I use?
It depends on battery chemistry. Lead-acid (flooded, AGM, gel) should stay at 50% DoD for maximum life. Lithium iron phosphate (LiFePO4) can safely use 80-90%. Deep cycle batteries tolerate deeper discharges than starting batteries.
Why is my battery dying faster than calculated?
Several factors reduce real-world runtime. Old batteries lose capacity. Cold temperatures reduce output. High discharge rates are less efficient (Peukert effect). Your actual load might be higher than expected. Check with a multimeter.
Can I convert watts to amps for this calculator?
Yes. Divide watts by voltage to get amps. A 120W device on a 12V system draws 10A (120/12=10). This calculator accepts either watts or amps as input and converts automatically.
How does battery age affect runtime?
Batteries lose capacity over time. A lead-acid battery might have 80% of original capacity after 300-500 cycles. Lithium batteries retain 80% capacity after 2000+ cycles. Old batteries need replacement when runtime becomes unacceptable.
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