when one side is loosing the battle was surrendering as drafted peasant and/or soldier an option ? how did the winning side stop their own force ? i guess they just didnt keep chasing the enemy forever. how did the loosing sides officers deal with fleeing peasants of their own army ? how much effort into capturing "cowards" ?
after the battle was there effort into looting enemy arms and armor and make it useful for your own troops ? was that organized or everyone picks what he needs ? what about injured people ? how to get rid of the dead ? care about the dead at all ?
Corroborating the earlier response, the battles usually ended in a rout. The actual hand to hand fighting didn't kill that many people, but eventually one side's formations or morale would weaken to the point where the soldiers broke and fled. While in the formation of two masses attacking each other casualties stayed relatively low, once an army's formation dissipated, it was game over.
Especially in the later Middle Ages when it became apparent that peasants armed with the cheapest of all weapons-a spear or pike-could reasonably fend off mounted knights, the cavalry took on more of the role of shock troop. So, often they would blast in from the sides or they'd be the ones cleaning up the mess of the rout. There are reports of men fleeing for miles while infantry and mounted knights hunt them down across the countryside.
Some of the best examples of this that I've come across are during the War of the Roses, so check out Alison Weir's War of the Roses for some good descriptions of this. For example, during the Battle of Towton, the rout resulted in a tremendous number of royal Lancastrians being rundown by horses, or drowning in their flight. This is the bloodiest battle fought in England after the Battle of Watling Street, where Roman soldiers reportedly killed over 200,000 native Britons under Boudicca-true casualty numbers are certainly a toss up.
Other ways a battle would end could be nightfall. Once the lights go out, play time was over, and often one side would retreat with the others taking on the mantle of victor. Another possibility is the capture of the army's leader. Think of the Battle of Poitiers when the French King John II was captured (much to French embarrassment). The battle ended shortly thereafter with a general slaughter of fallen/wounded French soldiers. The wealthy knights and lords were often ransomed, though in some of these battles there was little mercy shown. A great read for the Hundred Years War and Medieval life in general is Barbara Tuchman's A Distant Mirror. Seriously one of my favorite books ever, everyone should check it out! Let me know if you have any more questions