Parasitic Draw Time to Dead Battery Calculator

Calculate how long your vehicle battery will last with a constant parasitic drain. Essential for diagnosing electrical issues, planning vehicle storage, and preventing dead batteries. Supports Ah, CCA, RC measurements and runs entirely offline in your browser.

Battery Capacity

Typical car battery: 40-80 Ah

Parasitic Draw

Normal: 20-50mA, Problem: 100mA+

Cut-off Voltage

Voltage where battery can no longer start vehicle

Results

Enter values and click Calculate

Understanding Parasitic Draw

Parasitic draw refers to the small amount of electrical current your vehicle consumes when the ignition is off and all systems should be asleep. While some draw is normal (for clocks, security systems, ECU memory), excessive draw can drain your battery overnight.

Step-by-Step Guide

Measure your battery's parasitic draw using a multimeter (set to mA or A DC)
Identify your battery's capacity from its label (Ah, CCA, or RC)
Enter values above to calculate time until battery depletion
Compare results to your vehicle's expected storage time

Real-World Examples

Example 1: Normal Operation

60Ah battery with 35mA draw = ~71 days to complete discharge

Example 2: Moderate Issue

45Ah battery with 120mA draw = ~15 days to dead battery

Example 3: Serious Problem

70Ah battery with 500mA draw = ~5.8 days to vehicle won't start

Common Draw Reference Table

Component	Normal Draw	Problem Draw
ECU/Computer Memory	5-20mA	50mA+
Clock & Radio Memory	1-5mA	15mA+
Alarm System	15-30mA	75mA+
Aftermarket Accessories	Varies	Any constant draw

Frequently Asked Questions

What's a normal parasitic draw?

Most modern vehicles should have 20-50mA total parasitic draw. Older vehicles may be lower (10-30mA). Anything over 75mA typically indicates a problem.

How accurate is this calculation?

This provides a theoretical maximum. Real-world factors like battery age, temperature, and actual voltage curve will affect results. Consider this a conservative estimate.

When should I be concerned?

If your calculation shows less than 2 weeks until dead battery, or if you're experiencing frequent dead batteries, have your electrical system professionally diagnosed.

Formulas & Assumptions

• Time (hours) = (Battery Capacity Ah × 1000) ÷ Parasitic Draw mA
• CCA to Ah approximation: Ah ≈ CCA ÷ 7.25
• RC to Ah approximation: Ah ≈ RC × 0.6
• Assumes linear discharge to cut-off voltage
• Results rounded to nearest hour for times under 7 days

For informational and educational purposes only. Estimates depend on your inputs and simplifying assumptions. Do not use while driving. Vehicle modifications, condition, load, tyres, weather and road surface can change results. For safety critical decisions consult manufacturer documentation and qualified professionals.

Sources: Basic battery capacity and discharge calculations; Typical parasitic draw ranges from automotive service data; CCA to Ah conversion standards

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Parasitic Draw Time to Dead Battery Calculator

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