This is a vague question and has many different points that can and should be considered. But in all, I will do my best to provide an answer to a general battle during the early Medieval period. First and foremost, open battles happened quite infrequently. I mean, very rarely did to opposing forces march to an open field, size each other up and then proceed to give them the business. With that being said...

Any armed force that moves about is going to comprise a long chain of soldiers, wagons, tents and various supporters and retainers. Squires and horses for nobility. Each noble or knight could have a couple horses. Blacksmiths. Fletchers. Cooks and pages to prepare food. Some form of surgeon would be present for those of the upper echelon. And the best part is that almost none of these people would see a battle. They would be at a camp that was nearby. Also, we have to remember that a majority of the forces would be levies from the feudal estates of various lords, barons, knights, aka the landed gentry. The levies typically are only there fighting after the planting season and before the harvest (between spring and fall). They are more often conscripted to fight for their lord or face punishment. They don't have retainers or squires to help carry stuff or their equipment. They carry it themselves. They end up feeding themselves. They also typically walked everywhere without the chance to ride a horse.

As for a battle, it depends in large part to location, weather and position. Battles do and don't last very long. Some battles are very fast and over quickly due to a complete breakdown in morale if heavy cavalry is involved or if an ambush is sprung. Battles could last days if there were an open field siege (Hussite Wars where the Bohemians created wagon forts to defend from with spears, crossbows and early gun powder weapons).

I digress, so I will list some assumptions - this battle is part of a larger conflict between two European crowns; the armies are comprised of the typical feudal arrangement of relatively few nobles and knights in comparison to the levies that make up a large portion of spear troops that are lightly armed; the armies themselves are being led by a semi-important to important general with some relation to the royal family (blood or political); last, we will assume this was a straightforward battle without much in regard to trickery, flanking or betrayal - good ole fashioned battle between two armies and only a few nearby villages or towns.

In medieval battle, the goal was not to destroy the enemy completely, although that would have been a bonus if you were invading or defending lands. It was much more important to route or break an enemy army and then take prisoners of worth for ransom. A member of the royal family or someone important in the aristocracy could be ransomed and pay for an entire campaign or fighting season. Lesser lords and nobles would obviously bring less in terms of ransom, but their household or the crown would actually pay the ransom. The peasant levies would still be captured as they pose a potential threat if left behind enemy lines. The levies carried no real worth for ransom except to their feudal lord - the lord would still need to have people available for harvest after the fighting season. As a battle ends, one army is broken and retreats. There will be a time during the evening that people are sent forward to locate anyone important that might have fallen (see articles regarding Charles of Burgundy being located after his last battle and identified because of his scars). Both sides of the conflict would send out these people, also comprised of monks or priests to provide last rights to the dying. Each army would have returned to their respective camps after the battle to assess damage, losses, gains, prisoners, potential ransoms, etc. This would then play into the next parlay between the forces - basically a way to say..."I have this person and these lords, what will you trade me for them?"

Quick and dirty version - the armies will withdraw from the field to their own camps, prisoners have been captured and their ransom value assessed. People will check for others on the battlefield that have fallen but have not yet died.