Parasitic Battery Drain Calculator
Battery and drain inputs
Drain estimate
Enter battery details and a measured draw to estimate discharge time, or switch modes to estimate average current from elapsed time.
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Parasitic drain formulas
An amp-hour is current multiplied by discharge time. This calculator first adjusts the rated capacity for the condition factor and the selected charge window:
It then applies the constant-current relationship in either direction:
Nominal power is watts = volts × amps. This simple model does not treat voltage as constant in the real battery; voltage is used only for the displayed power and energy equivalents.
How to use the calculator
- Find the Ah rating. Check the battery data sheet. CCA and reserve-capacity minutes are different ratings; use the related RC calculator if amp-hours are unavailable.
- Set a realistic charge window. Enter the likely starting charge and the remaining charge at which you want the estimate to stop.
- Allow for condition. Reduce the capacity factor when age, cold, partial degradation, or uncertainty means the battery may not deliver its full label rating.
- Use sleeping current. A recently switched-off vehicle may keep control modules awake. Let the vehicle reach its specified sleep state before treating a current reading as its steady key-off draw.
- Read the result as an estimate. Intermittent loads, self-discharge, changing voltage, battery chemistry, and temperature can shift the real time substantially.
Worked example: 70 Ah battery and 50 mA draw
Suppose a 70 Ah battery is estimated at 80% of rated capacity, starts at 100% charge, and the calculation stops at 50%. The usable capacity within that window is 70 × 0.80 × 0.50 = 28 Ah.
A 50 mA draw is 0.05 A, so the linear estimate is 28 Ah ÷ 0.05 A = 560 hours, or about 23 days 8 hours. This is not a promise that the vehicle will start at that time. Cold weather, an aging battery, wake events, and a different true starting charge can shorten it.
Reverse estimate: If the same assumed 28 Ah was used over 10 days, the implied average draw would be 28 Ah ÷ 240 h = 0.1167 A, or about 117 mA. That estimate combines all loads and unmodeled losses during the interval.
Parasitic battery drain FAQs
How is parasitic battery drain time calculated?
The linear estimate divides usable capacity in amp-hours by average current in amperes. Usable capacity is the label Ah rating multiplied by the condition factor and the difference between starting and remaining state of charge.
Can I calculate parasitic current from discharge time?
Yes, as an average estimate: divide the assumed amp-hours used by elapsed hours. The result can include sleeping loads, intermittent module wake-ups, lights, accessories, battery self-discharge, and errors in the charge or capacity assumptions.
Is CCA the same as amp-hour capacity?
No. Cold cranking amps describe short-duration cold-starting capability. Amp-hours describe charge capacity over time. Do not enter a CCA value in the Ah field.
What parasitic draw is normal?
There is no single limit for every vehicle. Equipment and module behavior differ, and some vehicles take time to enter sleep mode. Fluke gives broad reference points in its test guidance, but the service information for the exact vehicle should control the diagnosis.
Why might the real battery die sooner?
The battery may begin below the assumed charge, have less capacity than estimated, be cold or degraded, or experience intermittent loads. Capacity also changes with discharge rate, and the vehicle may be unable to crank long before the battery is fully discharged.
Does a 50% remaining-charge setting guarantee a restart?
No. It is only a planning endpoint. Starting ability depends on battery condition, temperature, engine requirements, cable losses, and voltage under cranking load.
Are my inputs stored or tracked?
No. The calculation runs locally in your browser. This tool does not upload, store, or attach your battery or current values to analytics events.
Limits and electrical-safety disclaimer
- The model assumes constant average current and proportional Ah capacity. It does not model Peukert behavior, self-discharge, recovery, temperature, voltage cutoff, charger activity, or individual module wake cycles.
- The capacity-condition factor and charge percentages are user assumptions, not measurements made by this tool. Estimating current from elapsed time is especially sensitive to errors in those assumptions.
- A starting battery is designed for high-current cranking, not routine deep cycling. Repeated deep discharge can damage it.
- This calculator cannot identify the faulty circuit or determine whether the battery, charging system, wiring, or vehicle electronics are healthy.
Meter safety: Measuring current in series can open the battery circuit, wake vehicle modules, erase settings, create sparks, or damage a meter if a high-current load operates. Never crank the engine with an ammeter connected in series. Use correctly fused, properly rated equipment and follow the meter, battery, and vehicle manufacturer’s procedures; consult a qualified technician when uncertain.
Methodology and sources
Last reviewed: August 2, 2026. The calculation uses the amp-hour relationship and treats real vehicle behavior as a limitation rather than an implied diagnosis: