A battlefield is different to other archaeological sites, because there are specific conditions in contrast to conventional sites, like a settlement or a burial field.

1. A battle has only slight interference with the ground.
   Many objects like bones or equipment won't last centuries only lying on the bare ground (atmospheric condition, erosion etc.). Besides defensive positions (earth wall or trenches) and graves for the fallen combatants, there aren't many archaeological indicators (in terms of cultural/anthropogen stratum).

2. Looting.
   After a battle there was usually a systematical looting. Directly and even serveral years after the battle by residents of near settlements. Metal was an important ressource and everything useful was picked up.

3. Ground conditions.
   In combination with Number 1 archaeological objects like bones, fabrics and metal can dissolve in the ground rather quickly. For example: A chalky ground can preserve bones very good.

4. Localisation.
   We have the imagination, that battles took place on a certain, quite small place. But in reality battles could have taken place over a large area. As a result finds are even more scattered.

5. Time.
   Larger Battles in open field normally lasted only several hours or a few days. The archaeological impact to the ground is rather minimal.

In the last years Archaeologists (i can only speak for Germany) have focused more and more on Battlefield Archaeology. I will give you two popular examples, where you can get additional informations, which probably answer a few questions you may have:

1. A Bronze Age battle at "Tollense Tal"
2. "Varus Battle at Kalkriese": Weapon finds at Kalkriese, Bones and equipment of horses and mules
   Edit: formatting