Parasitic Draw Calculator
A parked vehicle can draw power from its battery even when the engine is off. Security systems, clocks, control modules, and parking-mode dash cams may all use electricity while the vehicle sits. The Parasitic Draw Calculator estimates how many days a fully charged battery can supply a constant draw before reaching a usable-capacity limit you choose.
This is a simplified amp-hour estimate, not a prediction of when your vehicle will stop starting. Battery condition, temperature, changing electrical loads, and the power needed to crank the engine can all affect the real result.
What the Parasitic Draw Calculator does
The calculator combines three inputs: your battery’s rated capacity in amp-hours, the percentage of that capacity you are willing to use, and the constant parasitic draw in milliamps. It returns an Estimated Time in days, rounded to two decimal places.
For example, a 60 Ah battery with 50% of its capacity available for the calculation has 30 Ah to supply the assumed load. At a constant draw of 50 mA, the estimated time is 25.00 days.
The usable percentage is your own planning assumption. Choosing 50% does not guarantee that the remaining battery charge will be enough to start a vehicle. The estimate also assumes that the battery begins fully charged and that its rated capacity applies to the calculation.
How to use the Parasitic Draw Calculator
- Enter battery capacity (Ah). Use the battery’s rated amp-hour capacity. The calculator accepts values from 1 to 500 Ah.
- Enter usable battery capacity (%). Choose the percentage of rated capacity you want to include in the estimate. You can enter a value from 1% to 100%.
- Enter constant parasitic draw (mA). Enter the assumed or measured continuous current in milliamps. The accepted range is 1 to 10,000 mA.
- Read Estimated Time. The result shows the estimated number of days until the selected amount of capacity has been used.
Check that your draw is in milliamps, not amps. One amp equals 1,000 milliamps, so a draw of 0.05 A is 50 mA. Entering 0.05 as though it were milliamps would describe a much smaller load and is below the calculator’s accepted range.
If you use a measured draw, remember that a vehicle’s electrical systems can take time to enter a low-power state after shutdown. A reading taken while devices are still active may not represent the steady draw you want to estimate. Follow appropriate instructions for your vehicle and measuring equipment, or ask a qualified technician to measure the draw.
How the Parasitic Draw Calculator formula works
The calculator first finds the selected usable capacity in amp-hours. It then divides that capacity by the draw in amps to estimate hours, and divides by 24 to convert hours into days. Because the input is in milliamps, the formula includes a conversion factor of 1,000.
Estimated time in days = Battery capacity (Ah) × Usable capacity (%) ÷ 100 × 1,000 ÷ Parasitic draw (mA) ÷ 24
Here is the calculation using the calculator’s default values:
- Battery capacity: 60 Ah
- Usable battery capacity: 50%, a user-selected assumption
- Constant parasitic draw: 50 mA, an assumed steady rate for this example
- Usable capacity: 60 Ah × 50% = 30 Ah
- Draw in amps: 50 mA ÷ 1,000 = 0.05 A
- Estimated time: 30 Ah ÷ 0.05 A ÷ 24 = 25.00 days
The relationship is straightforward: if all other inputs stay the same, a larger selected usable capacity increases the estimated time, while a higher draw decreases it. For instance, doubling the assumed draw halves the estimated time. These comparisons depend on the same simplified assumptions as the original result.
Use cases for the Parasitic Draw Calculator
Planning for parking-mode dash cam use. A dash cam that records while a vehicle is parked adds an electrical load. If you have a suitable measurement of the total steady draw, you can use it to estimate how quickly the selected battery capacity would be consumed. A dash cam may change its power use between recording modes, however, so one constant figure may not represent an entire parking session.
Comparing possible draw levels. Enter different assumed draws while keeping the battery capacity and usable percentage unchanged. This helps show how a lower or higher continuous load changes the estimate. Treat those figures as comparisons, not measurements of a particular camera or vehicle.
Planning around a parked vehicle. If a car will sit unused, the calculator can illustrate how a constant load consumes battery capacity over time. It does not establish a safe maximum parking period or account for every source of discharge.
Understanding a measured parasitic draw. When a technician or a properly conducted measurement provides a steady current reading, the result can put that number into the context of the battery’s rated capacity. An estimated time alone cannot identify the source of a draw or determine whether it is acceptable for a particular vehicle.
Other factors to consider when calculating Estimated Time
Starting requirements: An engine may fail to start before the selected capacity limit is reached. Starting requires a high burst of current, and the amount available depends on more than the amp-hours used in this calculation.
Battery age and condition: A worn, damaged, or partially charged battery may provide less usable capacity than its rated figure suggests. The calculator assumes a fully charged battery and applies the rated capacity as entered.
Temperature: Cold or hot conditions can affect battery performance. In particular, available starting power can differ from what a simple room-temperature capacity estimate suggests.
Changing and intermittent loads: Vehicle electronics may wake periodically, and dash cam power consumption can vary by operating mode. The formula assumes one constant draw for the entire period. If your measured value captures only one operating state, the estimate may miss other periods of higher or lower use.
Self-discharge and discharge rate: Batteries lose some charge on their own, and actual capacity can vary with how they are discharged. The calculation does not model self-discharge or adjust the rated capacity for the particular load.
For these reasons, use the result as a planning estimate. A battery monitor, an appropriate low-voltage cutoff, or a dedicated parking-mode power solution may be relevant to a dash cam setup, but their behavior depends on the equipment and installation. Check the manufacturer’s instructions for your setup.
Parasitic Draw Calculator FAQ
What is parasitic draw?
Parasitic draw is electrical current used by a vehicle’s systems while it is parked and switched off. Some standby consumption supports normal functions, while an unwanted load or fault can also drain the battery.
How do I convert amps to milliamps?
Multiply amps by 1,000. For example, 0.05 A equals 50 mA. The calculator’s draw field expects milliamps.
Does a 50% usable-capacity setting mean my car will still start?
No. It means the calculation estimates how long it takes to use an amount equal to 50% of the battery’s rated amp-hour capacity. Starting ability depends on battery condition, temperature, vehicle requirements, and other factors.
Can I use the calculator for a parking-mode dash cam?
Yes. Enter a suitable estimate or measurement of the total constant draw you want to model, including relevant vehicle and camera loads. If the camera or vehicle electronics change their power use, the result is only an approximation.
Why might the actual battery drain differ from the estimate?
The battery may not start fully charged or may have less capacity than its rating. Temperature, aging, self-discharge, intermittent loads, and changing current can also affect the outcome. The vehicle may need more remaining charge to start than your chosen percentage allows.