P38 Battery Drain Calculator – Days Until No-Start
Convert parasitic draw (mA) and battery capacity (Ah) into a practical estimate: how long you can park before “no-start territory”.
| Draw | Time |
|---|
How to use it
- Measure parasitic draw (mA) with the vehicle locked and asleep.
- Enter battery capacity (Ah) and a realistic usable percentage.
- Use the output to understand urgency, then isolate the circuit with fuse-by-fuse testing.
Typical P38 drain reality
The P38 can be sensitive to wake-ups. RF receiver behavior, door/tailgate inputs, BECM logic, HEVAC/radio feeds and aftermarket wiring can all keep the car awake.
What is considered normal battery drain on a P38?
On a healthy Range Rover P38, a fully asleep vehicle typically draws below 40–50 mA. Values above this are not automatically a fault, but they do reduce parking time significantly.
A draw of 100–150 mA may allow the vehicle to sit for several days, while 200–300 mA often leads to no-start situations within 24–72 hours, depending on battery condition and usable capacity.
What does “no-start” actually mean?
“No-start” does not mean a dead battery. On the P38, voltage dropping below critical thresholds can cause BECM confusion, immobiliser issues, and starter inhibit even while some capacity remains.
This calculator therefore uses usable capacity, not total battery capacity, to better reflect real-world behaviour rather than laboratory discharge curves.
What to do after using this calculator
If the calculated time is shorter than your normal parking period, the next step is not replacing the battery but isolating the cause.
Start by measuring current draw correctly, then pull fuses one by one to identify which circuit keeps the vehicle awake. This is where the P38 Fuse Finder becomes the logical next diagnostic step.
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