This webpage tells you almost everything you need to know

about deep cycle batteries.

Your car battery is designed to cycle from 80% to 100%. If you discharge it below 50%, its capacity will suffer from permanent sulfation, which is the formation of large, hard lead sulfate (PbSO4) crystals on the battery's plates.

https://www.solar-electric.com/learning-center/batteries-and-charging/deep-cycle-battery-faq.html/#What%20is%20a%20Battery?