Battles in eu4 is about snowballing, the more damage you deal to the enemy the less they deal to you.
This is not the problem however, the problem is that you regiments stay in the field as long as their moral allowes, which may sound well but what if you have a much larger army then the frontage, instead of allowing your fresh regiments to take their place the ones in the fight will continue to fight, take damage, do little and just cost you manpower.
By rotating your army and the enemy army would at, lets say each 6 days, just after the shock phase swap out its regiments with fresh ones if it can if not it will keep the ones it got left in the field.
This is not the problem however, the problem is that you regiments stay in the field as long as their moral allowes, which may sound well but what if you have a much larger army then the frontage, instead of allowing your fresh regiments to take their place the ones in the fight will continue to fight, take damage, do little and just cost you manpower.
By rotating your army and the enemy army would at, lets say each 6 days, just after the shock phase swap out its regiments with fresh ones if it can if not it will keep the ones it got left in the field.