Why would a car battery go out if the car hasn’t been used for a long time?


Why would a car battery go out if the car hasn’t been used for a long time?

In: Engineering

A body in motion stays in motion. A body in rest stays in rest. The battery basically gets stale and doesn’t work anymore.


Batteries self-discharge over time, as the chemical reactions that generate the electricity slowly proceed even though you’re not drawing any power or using it. The typical car battery, a lead-acid battery, loses about 5% of its charger per month.

Batteries discharge over time. Alkaline batteries have expiration dates for a reason.

And automotive lead-acid batteries are no exception, especially considering that most cars have small “phantom” draws on the battery most of the time, even when not turned on. The electrical system can draw tiny amounts of power from the battery constantly and over a long time it adds up to be a significant amount. And thar’s in addition to the gradual discharge that is a function of the battery’s internal chemistry.

Even the weather can have an impact.

To put it plainly, lead-acid batteries like your car battery, work by acid wanting to react with plates of metal. And for that reaction to happen, we make the little particles that want to be transfered run through wires to power stuff for us on the way through. If we take that option away by turning the circuit off, the reaction still wants to happen, and it will slowly eat away at itself overtime until it no longer needs to run through the wire.

A car battery sitting on a shelf will have its own slow rate of self-discharge, unavoidably, due to the internal chemistry.

Separate from that:

Modern cars require a steady small battery current at all times, for its computer, for preserving the radio settings, for anti-theft systems, and so on. Some cars activate separate electric motors and sensors, hours after the owner exits, as part of their emissions system tests.

If cars aren’t driven for many weeks, these small currents add up to a major total current drain, leaving batteries in a discharged state.

(So, if owners can’t drive their cars every week, and if they park in a garage, they should invest in a modern microcontroller-based battery charger/maintainer, to preserve their battery health)

A chemical battery (like in a car) is two chemicals that want to react with each other separated by a barrier. The chemical reaction that wants to happen involves little energy balls (electrons) going from one chemical in the battery to the other. BUT, because there’s a barrier between the battery’s two components, the energy balls can’t transfer directly – they have to go out one end of the battery, through whatever circuit we have hooked up, and into the other end of the battery to get at the chemical inside that half and complete the reaction.

So why do batteries “leak” or “self-discharge” over time? Well that barrier is a physical thing, and it’s not completely 100% perfect seal at the atomic level. This question compares pretty well with “why does a basketball go flat if you leave it for months after being inflated”. Isn’t it sealed by the valve? Yes, but every seal leaks at least a little and in time it adds up. In the basketball, air pressure forces air out through the valve. In battery, there’s a sort of “chemical pressure” from the reaction trying to happen that eventually leaks through/around the barrier.

Plus in the case of car batteries, one of the two chemicals is strong sulfuric acid, so it can physically eat a hole right through the battery and completely wreck it after a long time rather than just slow self-discharge leaking.

Parasitic draw.
Most cars have small, always on devices… Like the centery key proximity sensor, alarm, etc