A car parked for a weekend starts right up on Monday. Let it sit for a month or two and you may hear nothing but a click when you turn the key.

The difference between those two mornings comes down to a slow chemical drain that never stops, even when the engine is off. Knowing how that drain works helps you decide how long your car can safely sit.

What drains a parked battery

Lead-acid batteries lose charge on their own through a process called self-discharge, roughly 1 to 5 percent per month at room temperature. That sounds small until you add the parasitic loads, the small amounts of power the electronics draw while the car is off. Your clock, alarm system, keyless entry receiver, and the memory in the engine computer all pull a few milliamps continuously.

On a modern car that adds up to something in the range of 20 to 50 milliamps, which works out to a meaningful chunk of the battery's capacity over weeks. A typical 45 to 60 amp-hour car battery can sit two to four weeks before cranking gets sluggish. Cold weather shortens that window because the chemical reactions slow down and the battery has less usable power.

Why two weeks is the soft limit

Most automakers design for a car that gets driven regularly, not one that sits. A battery that's already a few years old holds less reserve, so a car that started fine at three years old might struggle after ten days at five years old.

If your car has an aftermarket dash camera wired to a constant power source, that device alone can pull enough to flatten a healthy battery in a week. Treat the two-week mark as a rough guideline, not a hard rule.

Signs the battery is fading early

Slow cranking is the first clue. So is a battery that needs a jump every time the car sits for more than a few days. Dim headlights at idle and a battery light that flickers point the same way.

You can check the resting voltage with a cheap multimeter: a fully charged battery reads about 12.6 volts with the engine off, and anything under 12.4 volts means it's partly discharged. Below 12.0 volts, you're looking at a battery that may not recover well.

How to park it safely

Drive it. A 20 to 30 minute highway run once every week or two keeps the battery topped up and burns off moisture in the oil. Short trips around town do less, because starting the engine takes a big burst of power and the alternator needs time to put it back.

If you can't do that, a battery maintainer is the clean fix; a smart trickle charger costs $30 to $80 and keeps the battery at full charge without overcharging. Disconnecting the negative terminal works too, but you'll lose radio presets and the computer may need to relearn idle.

Park on a flat, dry surface, inflate the tires to the pressure on the door sticker, and leave the fuel tank at least half full to limit condensation.

The age factor you can't ignore

A car battery typically lasts three to five years. If yours is past four and the car sits often, plan on replacing it before it strands you. Testing is free at most auto parts stores, and they'll tell you the cold cranking amps left, a measure of the power the battery can deliver to start the engine in the cold.

That number tells you more than the calendar. If your car lives outside and sits a lot, a maintainer pays for itself faster than a tow. Next time you leave it parked for a stretch, check the voltage before you turn the key.