Parasitic Battery Drain Calculator
You clamped a meter on the battery and got a number. This turns that number into the thing you actually wanted to know: how many days you can leave the car parked before it will not start. Nothing uploaded.
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Published manufacturer figures ?
Learn more: turning a milliamp reading into days
What the calculator does with your number
Everything starts with capacity. If you enter reserve capacity in minutes, it converts to amp-hours by multiplying by 0.6, which is the conversion implied by the industry rule Clore Automotive states as "divide the reserve capacity of the battery by 4, or the amp hour rating by 2.4". A 100 minute reserve capacity battery becomes 60 Ah, and both halves of that rule then agree on a 25 mA limit.
Next it works out how much of that capacity you can actually spend. A starting battery stops being useful when it can no longer turn the engine over, not when it hits zero, so the default stops the clock at 50 percent charge. On a 60 Ah battery starting from full, that leaves 30 Ah to give away.
Then it adds the leak you did not measure. Your meter sees the parasitic draw, but the battery is also self-discharging, and Battery University puts lead-acid at "about 5 percent per month" with the rate doubling for every 10°C warmer. On that 60 Ah battery at 20°C that is 3 Ah a month, or another 4.1 mA sitting alongside whatever your meter read.
Worked example: 100 minutes reserve capacity, a 25 mA reading, a full battery at 68°F. Total drain is 25 plus 4.1, so 29.1 mA. Dividing 30 Ah by that gives 1031 hours, or 43 days before it stops cranking. Plain division of 30 Ah by 25 mA would have told you 50 days.
The threshold is a property of your battery, not a universal number
People trade "under 50 milliamps is fine" around like a fixed rule, but the same draw is harmless on a big truck battery and fatal on a small one. The reserve capacity rule scales it: a 100 minute battery tolerates 25 mA, a 140 minute battery tolerates 35 mA.
Carmakers publish their own numbers and they land in the same neighbourhood. Vehicle Service Pros reports General Motors quoting "40 mA as the maximum normal reading", Nissan treating 25 mA as normal, Dodge and Ram calling 5 to 35 mA typical, and Honda specifying 27 to 42 mA depending on the vehicle.
The same article makes a point worth repeating before you trust any reading: a vehicle draws far more while its modules are still shutting down, and typically stays under 75 mA during that falling-asleep period. Measure too early and you will chase a fault that does not exist.
Why the answer shrinks in winter
Cold does not remove charge from a battery, it makes the charge harder to get out. PVEducation gives the working figure as "battery capacity falls by about 1% per degree below about 20°C", and that is the coefficient this calculator uses.
At -10°C, which is 14°F, that is a 30 percent haircut. The same 60 Ah battery behaves like 42 Ah, and the 30 Ah you had to spend becomes 21 Ah. Self-discharge drops in the cold at the same time, from 4.1 mA down to about 0.5 mA, but that barely offsets the capacity you lost.
Run the earlier example again at 14°F and the 43 days becomes 34. That is the difference between a car that starts after a month away and one that does not, from nothing but the weather.
Reading the result honestly
This is a steady-current model. It assumes your draw stays flat, your battery really has the capacity printed on its label, and the discharge is slow enough that the rated capacity is a fair guide. All three assumptions are reasonable for a parked car and none of them are exactly true.
The capacity assumption is the one that breaks first. A five year old battery that has lost a third of its capacity will die far sooner than the label suggests, and there is nothing in a milliamp reading that reveals it. If the timeline here is much more optimistic than what your car actually does, a load test on the battery tells you more than another draw test will.
FAQ
What counts as a normal parasitic draw?
Most modern vehicles settle somewhere between 25 mA and 50 mA once every module has gone to sleep, which can take anywhere from a few minutes to well over half an hour after you lock the door. The reserve capacity rule scales that to your actual battery: a 100 minute RC battery tolerates 25 mA, a 140 minute RC battery tolerates 35 mA. Published manufacturer figures land in the same territory, with General Motors quoting 40 mA as the maximum normal reading and Nissan treating 25 mA as normal. If you measured before the modules finished shutting down, you will see a much larger number that means nothing.
Why is this answer shorter than drain divided by capacity?
Two reasons. The calculator stops the clock at the point the battery can no longer crank the engine rather than at zero, because a starting battery is useless to you long before it is empty. It also adds self-discharge, the charge a lead-acid battery loses on its own with nothing connected, which is around 5 percent per month. On a 100 minute RC battery at 68F with a 25 mA draw, the simple division gives 50 days to the halfway point while the calculator gives 43 days, and the gap is almost entirely self-discharge.
My draw is within spec but the battery still dies. What else could it be?
A draw that is fine on paper can still flatten a battery that has lost most of its capacity to age or sulfation, because the calculator assumes the reserve capacity printed on the label is still real. Try entering a lower capacity figure and see whether the timeline starts matching what you actually experience. Short trips are the other common cause, since a few minutes of running may not replace what starting the engine took out, so the battery drifts down over weeks regardless of what the key-off draw measures.