Advanced DC Battery Runtime Calculator
Formula Used
Bank voltage: single battery voltage × series batteries.
Bank capacity: single battery Ah × parallel strings.
Nominal energy: bank voltage × bank capacity.
Usable delivered energy: nominal Wh × efficiency × depth of discharge × temperature factor × age factor × usable reserve factor.
Simple runtime: usable delivered Wh ÷ load watts.
Peukert runtime: rating hours × (bank Ah ÷ (battery-side current × rating hours))Peukert exponent, then adjusted by discharge, temperature, age, and reserve factors.
Battery-side current: load current ÷ system efficiency.
How to Use This Calculator
- Choose the closest battery chemistry.
- Enter the voltage and amp hour rating for one battery.
- Enter how many batteries are in series and parallel.
- Select watts, amps, or ohms as the load input method.
- Add system efficiency, discharge limit, reserve, and battery condition factors.
- Use Peukert exponent 1.00 for ideal behavior. Use higher values for lead acid batteries.
- Press Calculate Runtime to view the result above the form.
- Download the CSV or PDF report for saving or sharing.
Example Data Table
| Battery |
Bank Voltage |
Capacity |
Load |
Efficiency |
DoD |
Reserve |
Approx Runtime |
| 12 V 100 Ah LiFePO4 | 12 V | 100 Ah | 300 W | 92% | 80% | 10% | 2.65 h |
| Two 12 V 100 Ah in series | 24 V | 100 Ah | 300 W | 92% | 80% | 10% | 5.30 h |
| Four 12 V 100 Ah, 2S2P | 24 V | 200 Ah | 600 W | 90% | 80% | 15% | 4.90 h |
| 12 V 200 Ah lead acid | 12 V | 200 Ah | 200 W | 90% | 50% | 10% | 4.86 h |
Understanding DC Battery Runtime
DC battery runtime is the expected time a battery bank can supply a load before it reaches a chosen discharge limit. The value is not only capacity divided by current. Real systems lose energy in wiring, converters, controllers, and protection circuits. Battery age, temperature, and discharge rate also change useful capacity.
Why Runtime Changes
A battery rating is usually measured under controlled test conditions. Many lead acid batteries are rated over twenty hours. If you pull current faster, available capacity falls. This is described with Peukert’s law. Lithium batteries have a lower Peukert effect, yet they still lose runtime at high current, low temperature, or old age. That is why a practical calculator needs more than volts and amp hours.
Designing a Safer Battery Bank
Start with the true load. Use watts when you know appliance power. Use amps when the equipment label gives current. Use resistance for simple DC heaters or resistive loads. Then add system efficiency. A converter that is 90 percent efficient makes the battery deliver more power than the load receives. Depth of discharge protects the battery from damage. A reserve margin keeps energy available for alarms, startup surges, and unexpected delays.
Reading the Results
This calculator gives nominal energy, usable energy, load current, simple runtime, Peukert adjusted runtime, and remaining reserve. The simple runtime is useful for quick estimates. The Peukert result is better for batteries affected by discharge rate. The target runtime section helps size a future bank. It shows the approximate amp hour capacity needed at the chosen voltage and settings.
Good Planning Tips
Use conservative values for critical loads. Check manufacturer data when available. Measure real load with a meter if possible. Include cable voltage drop for long runs. Do not design around complete discharge. Batteries last longer when they are kept within healthy limits. Recheck the estimate after adding new equipment, changing chemistry, or operating in hot or cold locations.
Maintenance
Maintenance matters as well. Clean terminals, tight connections, and balanced parallel strings reduce waste. Fuse each string correctly. Keep ventilation suitable for the chosen chemistry. Log every test result. A small record helps you notice capacity loss early and plan replacement before a backup system fails again.
FAQs
What is DC battery runtime?
It is the estimated time a battery bank can power a direct current load before reaching the selected discharge limit.
Why is my real runtime lower than the estimate?
Real runtime can drop due to old batteries, high current draw, cold temperature, wiring loss, converter loss, or inaccurate load ratings.
What Peukert exponent should I use?
Use 1.00 for an ideal estimate. Lithium batteries are often close to 1.05. Lead acid batteries may need 1.10 to 1.30.
What does depth of discharge mean?
Depth of discharge is the allowed portion of capacity you plan to use. A lower value usually improves battery life.
Should I enter watts or amps?
Use watts if the device label gives power. Use amps if the label gives current at the same battery bank voltage.
What is system efficiency?
It accounts for losses in converters, wiring, controllers, and protection devices. Lower efficiency means the battery supplies extra power.
Why add a reserve margin?
A reserve protects against aging, startup surge, weather changes, and longer operation than expected. It improves planning safety.
Can this size a solar battery bank?
Yes. Use the target runtime result as a first estimate. Then confirm charging needs, solar input, and manufacturer limits.