Parasitic Draw Battery Drain Calculator

Parasitic draw battery drain calculator results estimate how quickly a steady key-off current uses battery capacity. Enter a settled sleep-current reading, not the higher current seen while vehicle modules are still shutting down.

What the Estimate Includes

• Accepts current in milliamps or amps.
• Uses the battery's amp-hour rating and a chosen starting and stop state of charge.
• Shows amp-hours and the percentage of rated capacity used each day.
• Compares the reading with a vehicle-specific maximum if you enter one.

The result assumes a constant draw and the battery's full listed capacity. It is a planning estimate, not a prediction of the exact time until a no-start.

Use the Vehicle's Sleep-Current Limit

There is no single pass or fail number for every vehicle. Factory accessories, alarm systems, telematics, battery size, and module strategy can change the allowed draw. Find the maximum current and required sleep time in service information for the exact vehicle. General Motors service material shows that opening a door, using passive entry, or unlocking the vehicle can keep different modules awake for different times. See its vehicle timeout guidance.

What the Drain Time Means

The calculator multiplies rated amp-hours by the percentage of capacity between the starting and stop points, then divides by current in amps. Real time can be shorter because an older, cold, partly charged, or damaged battery may hold less energy than its label. Current can also change when a module wakes, a relay cycles, or an aftermarket device transmits.

Measure Current Without Damaging the Meter

Follow the vehicle and meter instructions. When a meter is connected in series, do not crank the engine or switch on a high-current load. Start on the meter's highest protected current range, use the correct lead socket, and allow the vehicle to return to sleep. Fluke's parasitic drain test guide covers meter setup and safety.

Isolate the Circuit Without Losing the Pattern

First record the settled draw and any timed wakeups. On a modern vehicle, pulling a fuse can wake modules or change the condition you are trying to find. Measuring voltage drop across installed fuses or using a suitable low-current clamp can preserve module state. Once a circuit is identified, use its wiring diagram to test the devices, relays, wiring, and module inputs on that circuit.