Battery SOC From 85% to 65%: Battery in a Parked Car

Disclaimer:
This post is based on my personal experience and intended for informational and educational purposes only. Procedures, tools, and parts may vary depending on the vehicle and situation. Always consult your service manual or a professional before performing any maintenance.

For more information, please read the full disclaimer here.

Battery SOC (State of Charge) From 85% to 65%

On November 24, I parked my car in the garage and left it untouched until December 14, giving me a chance to see how my BMW behaves when it sits idle for weeks.The garage stayed around 10°C, keeping the car safe from extreme cold and weather swings.

Before starting the engine, I checked the battery with the BimmerLink app.
It showed a State of Charge of 65%, which surprised me because just a few days earlier, on November 15, it was around 85%.
The battery had lost roughly 20% charge while the car sat still, even without any warning signs.

When I finally turned the key, I noticed something else: the engine took a few extra seconds to start.
It wasn’t dramatic, but I could clearly feel the difference compared to normal daily use, even in mild temperatures.

Impact on Other Car Components

The battery is usually the first component to show signs of inactivity, but it is not the only one affected. When a car stands still for several weeks, engine oil gradually drains back into the oil pan, which means the engine needs a little more time to build proper lubrication during the first start.

Fuel pressure also drops when the system remains unused for a longer period, so the fuel pump needs extra seconds to restore normal pressure levels before combustion becomes stable.
This combination explains why the first engine start felt slightly slower and less immediate than usual.

Other Components React more Subtly

Brake discs can develop a thin layer of surface corrosion, tires can lose a small amount of pressure, and rubber seals may stiffen slightly until they warm up again.

None of these effects cause immediate damage, but they clearly show that cars prefer regular use over long periods of standing still.

How Often Should I Start a Car that Stands Unused?

If a car has to remain parked for an extended time, starting it occasionally makes a noticeable difference. In general, starting the car once every 7 to 10 days helps keep the battery in a healthier state and prevents excessive discharge.

However, simply starting the engine for a few minutes is not enough.
The car should ideally be driven for 20 to 30 minutes, allowing the alternator to properly recharge the battery and all mechanical components to reach operating temperature.

Conclusion

This short period of inactivity provided a clear and realistic insight into how a modern car behaves when it stands unused. In less than three weeks, the battery lost around 20% of its charge, even though the car stayed in a garage with relatively mild temperatures.

The slightly longer first engine start confirmed that both electrical and mechanical systems need time to recover after long inactivity.