Example Data Table
| Case |
Capacity |
Parasitic Draw |
Accessory Load |
Reserve |
Use Case |
| Normal parked vehicle |
60 Ah |
35 mA |
0 A |
50% |
Storage check |
| Dash camera parking mode |
70 Ah |
45 mA |
0.35 A |
55% |
Accessory planning |
| Abnormal relay draw |
55 Ah |
250 mA |
0 A |
50% |
Fault diagnosis |
Formula Used
Parasitic current in amps: Parasitic A = Parasitic mA / 1000
Duty-cycle current: Duty A = Intermittent Current × Duty Cycle / 100
Total average load: Total A = Parasitic A + Accessory A + Duty A
Derated capacity: Derated Ah = Rated Ah × (1 - Temperature Loss) × (1 - Age Loss)
Usable capacity: Usable Ah = Derated Ah × (Starting Charge - Reserve Charge) / 100
Simple drain time: Time = Usable Ah / Total A
Peukert adjusted time: Time = Hour Rating × (Usable Ah / (Total A × Hour Rating)) ^ Peukert Exponent
Recharge time: Recharge Hours = Used Ah / (Charger Current × Charge Efficiency)
How to Use This Calculator
Enter the battery capacity from the label or test report.
Enter the current battery voltage for energy estimates.
Set the starting charge and the minimum reserve charge.
Add the parasitic draw measured after the vehicle sleeps.
Add constant accessory current from cameras, trackers, or chargers.
Use duty cycle for loads that turn on and off.
Add capacity losses for cold weather and battery age.
Press calculate. The result appears above the form.
Use CSV or PDF buttons to save the same report.
Car Battery Drain Guide
Understanding Car Battery Drain
A car battery stores chemical energy as direct current. The battery feeds alarms, clocks, modules, lights, chargers, and audio gear when the engine is off. Small loads are normal. Large loads can leave the vehicle unable to crank. This calculator helps compare those loads before a problem becomes a no start.
Why Parasitic Load Matters
Parasitic draw is the current that remains after the vehicle enters sleep mode. Many vehicles stay below 50 milliamps. Modified systems, stuck relays, trunk lamps, dash cameras, or weak modules can raise the draw. A higher draw shortens storage time fast. A 200 milliamp draw uses 4.8 amp hours each day.
Capacity Is Not Always Available
The label capacity is only a starting point. Cold weather reduces chemical activity. Age reduces plate performance. Deep discharge also harms many starting batteries. For that reason, the tool uses starting charge, reserve charge, temperature derating, and age derating. The result shows usable amp hours, not only rated amp hours.
Using Duty Cycle Loads
Some devices do not run constantly. Cooling fans, trackers, and pumps may cycle. Enter their running current and duty cycle. A 6 amp device at 25 percent duty becomes a 1.5 amp average load. Add that value to other loads for a better parking estimate.
Peukert Adjustment
Lead acid batteries deliver less effective capacity at heavy current. They may deliver more apparent capacity at light current. The Peukert exponent estimates that behavior. A value near 1.05 is mild. A value near 1.25 is stronger. Use 1.10 as a practical default when the exact value is unknown.
Planning Storage And Recharge
Use the result as a planning guide. Keep a safe reserve if the vehicle must start later. Recharge sooner when the battery is old, cold, or repeatedly drained. For long storage, reduce loads, disconnect accessories, repair abnormal draw, or use a maintainer. Export the report to compare tests, keep service notes, or share results with a technician.
The numbers are estimates, not a replacement for testing. Confirm suspect loads with a meter after modules sleep. Check cables, grounds, and battery health too. A weak battery can fail even with a normal draw during very cold starts.
FAQs
What is normal parasitic draw?
Many vehicles settle below 50 milliamps after sleep mode. Luxury or connected vehicles may vary. Compare with service data before judging a fault.
Why should I keep reserve charge?
A starting battery should not be drained fully. Reserve charge helps protect cranking ability and battery life. Use a higher reserve in cold weather.
Is this calculator exact?
No. It gives a planning estimate. Real results depend on battery health, temperature, sleep mode timing, cable condition, and actual measured current.
What does temperature derating mean?
Temperature derating reduces usable capacity for cold or harsh conditions. A cold battery can have less available energy than its label rating.
What is the Peukert exponent?
It adjusts battery time for discharge rate. Higher values reduce time under heavier loads. Use 1.10 when no tested value is available.
Can a drain damage my battery?
Yes. Deep discharge can shorten starting battery life. Recharge quickly after a heavy drain, especially with older lead acid batteries.
Why enter duty cycle?
Duty cycle handles loads that run part of the time. It converts intermittent current into an average current for better estimates.
What are CSV and PDF downloads for?
Use CSV for spreadsheets. Use PDF for sharing, repair records, or customer reports. Both include the calculated battery drain summary.