Welcome to the forum savash.
The reason are probably that the fuse / socket you use for powering the camera are powered on for a while after you turn off the car.
But the 20 minutes are not anything for your battery unless it is very close to dying anyway, i personally use parking guard in one of my cameras, so it record for 3 hours after i am parked.
That cover all my shopping, and at home or other places i visit there is CCTV so not really a need to do parking guard there.
Also my car battery are very small ( small car, small battery ) even if i have replaced the original 45 Ah battery with one that is 50 Ah
The cameras i have tested parking guard in, and that do have a build in timer, well the timer start at 1 hour, and most often 48 hours is the MAX, though before that low voltage would probably kick in.
I use low bitrate recording for parking guard myself, this mean the camera use a lower bitrate to create smaller files and generate less internal heat when parked, so in my case 3 minutes of parking guard footage might be 130 MB where as 3 minutes of regular recording are 3-400 MB in size
You say there are options for longer times. this confuse me a little ( not familiar with this brand / model )
But with that i assume that either the camera have a build in battery, or it is powered using the OBD port. ( or a hard wire kit )
But the cameras i have tested and t hat did come with a hard wire kit, well the fact you use that do not force you to use parking guard at all, there is always a OFF option too.
Your car battery only start to suffer if it get to really low voltages, but if it is over 12.2 volts you should be fine, i also use a 12.2 volt cut off on my system even if the timer stop the recording long before the camera drop to 12.2 volts.
