Workshop calculator
Parasitic battery drain calculator: how many days until no start?
After the modules go to sleep, a modern car should draw only a few tens of milliamps. More than that slowly empties the battery; the calculator shows how many days it takes to reach a state of charge that may no longer start the engine.
Ah
%
mA
%
About 50 % in winter for most cars; lower in summer.- Charge available
- 35.0 Ah
- Time until no start
- 29.2 days (700 h)
Within the usual range once the modules are asleep.
How to use it in a diagnosis
- Let the car sleep (often 20–45 minutes, doors closed and latched, key away).
- Measure the draw with a clamp meter or an ammeter in series without waking the modules.
- Enter the battery capacity (Ah), its current state of charge and the measured draw.
Questions mechanics ask
What is a normal parasitic draw?
Once all modules are asleep most cars draw roughly 20–50 mA. Above about 50–80 mA look for the cause; above a few hundred mA the battery can go flat in a few days.
How do I find what is draining the battery?
Measure the voltage drop across each fuse with the car asleep instead of pulling fuses (pulling fuses can wake modules). The fuse showing a drop feeds the circuit with the draw.