Lead Acid Battery Runtime Calculator
A lead acid battery runtime calculator helps estimate how long a fully charged battery can support a steady electrical load. It accounts for rated battery capacity, voltage, discharge rate, and the portion of capacity you intend to use. This can be useful when planning power for vehicle accessories, backup equipment, camping gear, or other battery-powered devices.
The result is an estimate in hours, not a guarantee. Lead acid battery performance changes with temperature, age, discharge rate, and operating conditions. Use the estimate as a planning starting point and check the battery manufacturer’s specifications when a reliable runtime is important.
What the Lead Acid Battery Runtime Calculator does
The Lead Acid Battery Runtime Calculator estimates runtime for a constant-power load connected to a lead acid battery. It uses the battery’s rated capacity and capacity rating period to establish a baseline, then applies a Peukert exponent to approximate how the discharge rate affects runtime.
Lead acid batteries generally deliver less usable capacity at higher discharge rates. For example, a battery rated at 100 amp-hours over 20 hours should not be expected to deliver the same amp-hour capacity at every load. The calculator also lets you specify a usable depth of discharge, so you can estimate runtime while leaving some capacity unused.
The displayed result is labeled Estimated Runtime and is rounded to two decimal places. The calculation assumes the battery starts fully charged and that the load remains constant. It uses nominal voltage to estimate current, so it does not follow the battery’s changing voltage during discharge.
How to use the Lead Acid Battery Runtime Calculator
Enter values that describe your battery and the load it must supply. The calculator has six inputs:
- Rated battery capacity (Ah): Enter the battery’s stated capacity in amp-hours. For example, enter 100 for a battery rated at 100 Ah. Use the capacity associated with the rating period you enter below.
- Nominal battery voltage (V): Enter the battery system’s nominal voltage in volts, such as 12 V for a typical 12-volt battery. Use the voltage of the complete battery system if multiple batteries are connected together.
- Battery-side load (W): Enter the power drawn from the battery in watts. Include inverter and other system losses in this figure rather than entering only the power consumed by an AC appliance.
- Capacity rating period (hours): Enter the discharge period used for the stated amp-hour rating. A 20-hour rating means the capacity was specified over a 20-hour discharge period. Check the battery label or manufacturer’s documentation.
- Peukert exponent: Enter the exponent applicable to the battery if it is available. The illustrative default is 1.2. Different battery designs and conditions can have different values, so an assumed exponent introduces uncertainty.
- Usable depth of discharge (%): Enter the percentage of the battery’s capacity you intend to use. For example, 50% means the estimate is scaled to use approximately half of the full-discharge runtime.
Once the values are entered, read the Estimated Runtime in hours. If your device has a changing power draw, a single constant load may not describe its operation well. Consider its typical operating pattern and leave a suitable margin in your planning.
How the Lead Acid Battery Runtime Calculator formula works
The calculator first approximates battery current by dividing the battery-side load in watts by nominal battery voltage. It then applies Peukert’s equation to estimate full-discharge time using that current, the rated capacity, the rating period, and the selected exponent. Finally, it scales the full-discharge estimate by the chosen depth of discharge.
Estimated runtime = Rating period × [Rated capacity × Nominal voltage ÷ (Rating period × Battery-side load)]Peukert exponent × Usable depth of discharge ÷ 100
In this formula, capacity is measured in amp-hours, voltage in volts, load in watts, and the rating period in hours. The resulting runtime is expressed in hours. An exponent of 1 represents a simple linear relationship; an exponent above 1 adjusts the estimate for the effect of discharge rate.
The depth-of-discharge step is a simplifying approximation. Actual partial-discharge behavior depends on the battery and the voltage at which your equipment stops operating. The calculation also does not model voltage changes during discharge, battery recovery, temperature, aging, or manufacturer-specific discharge curves.
Worked example using the default inputs
Suppose you have a fully charged lead acid battery and use the following illustrative assumptions:
- Rated capacity: 100 Ah
- Nominal voltage: 12 V
- Battery-side load: 120 W
- Capacity rating period: 20 hours
- Assumed Peukert exponent: 1.2
- Usable depth of discharge: 50%
The estimated current is 120 W ÷ 12 V, or 10 amps. The formula then gives:
20 × [100 × 12 ÷ (20 × 120)]1.2 × 50 ÷ 100 = 4.35 hours
The calculator displays an Estimated Runtime of 4.35 hours, or about 4 hours and 21 minutes. This is an illustrative estimate, not a promise that every 100 Ah battery will run a 120 W load for that duration.
Use cases for the Lead Acid Battery Runtime Calculator
The calculator can help compare battery and load combinations before you depend on them. Common uses include:
- Vehicle accessories: Estimate how long a battery might support a steady accessory load when the engine is off. Consider the capacity you need to retain for starting the vehicle separately.
- Camping and recreational equipment: Plan for lights, fans, or other devices powered by a lead acid battery. Combine simultaneous loads and include any conversion losses in the battery-side wattage.
- Backup power: Get an initial estimate for equipment connected to a battery and inverter during a power interruption.
- Comparing loads: Change the battery-side wattage to see how a lower or higher steady load affects the estimate.
- Comparing discharge limits: Try different usable depth-of-discharge values to see how reserving more capacity changes the estimated runtime.
For intermittent devices, estimate their operating pattern carefully. A refrigerator compressor, for example, cycles on and off, while startup demand and inverter behavior can also matter. The calculator accepts one constant-power load, so it cannot reproduce every part of that cycle.
Other factors to consider when calculating Estimated Runtime
Battery condition and charge level matter. A battery that has aged, has been damaged, or starts below a full charge may deliver less runtime than the result suggests. The calculator assumes a fully charged battery and uses the rated capacity you enter.
Temperature affects lead acid battery performance. Cold conditions can reduce available capacity, while operating conditions that differ from those used for the rating can make a real-world result diverge from the estimate.
Inverter and system losses also affect the load. If an appliance consumes 100 W on the AC side, the battery-side load will generally be higher because the inverter uses some power. Use a measured battery-side draw or an estimate that includes those losses. Wiring and other connected equipment can add to the draw.
Cutoff voltage can determine when your equipment stops working. A device or inverter may shut down before the battery reaches the depth of discharge you selected, particularly under a heavy load. Manufacturer discharge curves and equipment specifications can provide more useful guidance for a particular setup.
Finally, choose a depth of discharge that fits your battery and intended use. Discharging deeply can affect battery life, and a starting battery has a different job from a battery designed for repeated deep cycling. Consult the manufacturer’s guidance when deciding how much capacity to use.
Lead Acid Battery Runtime Calculator FAQ
How long will a 100 Ah lead acid battery last?
It depends on the battery voltage, load, rating period, Peukert exponent, usable depth of discharge, and operating conditions. With the illustrative default inputs of 12 V, a 120 W battery-side load, a 20-hour rating, an exponent of 1.2, and 50% usable depth of discharge, the calculator estimates 4.35 hours.
What does the Peukert exponent mean?
The Peukert exponent is a coefficient used to approximate how discharge rate affects a battery’s available capacity. A value of 1 gives a linear result in this calculation, while a higher value changes the estimate to reflect the effect of discharge rate. Use a manufacturer-provided value when available; otherwise, treat the entered value as an assumption.
Should I enter the appliance’s wattage or the battery-side load?
Enter the battery-side load. If the appliance runs through an inverter, include the inverter’s losses and any other relevant system loads. Entering only the appliance’s stated AC wattage can make the runtime estimate too optimistic.
Why is the capacity rating period important?
The rating period tells you the discharge duration associated with the battery’s amp-hour capacity. The calculator uses that period as part of its Peukert calculation. The capacity and rating period must refer to the same battery specification for the result to be meaningful.
Does a 50% depth of discharge guarantee that half the battery’s capacity remains?
No. The calculator scales its full-discharge runtime estimate by 50%, but actual partial-discharge behavior depends on the battery and cutoff voltage. Treat the result as an approximation and use the battery manufacturer’s guidance and suitable monitoring for your setup.