Because of the crowding, movement by one hen necessitates a major disturbance to all in a cage. Since the hens cannot exercise, there is a high prevalence of broken bones due to osteoporosis and a condition known as cage-layer fatigue. The constant contact with bare wire results in damage to the chickens' feet, which is also painful. The crowding and lack of stimulation in these cages results in destructive pecking, so the birds are debeaked. There is a high mortality rate in the battery cage system which may be as much as one per cent per week.

Hens normally go through periodic molts during which they replace their feathers. During molt, the hens do not produce eggs. If they were allowed to molt naturally, it would be a problem for the producer because it would be erratic. So after about a year of production, the hens are forced to molt. This is done by keeping the shed in total darkness, withholding water for up to three days and food for up to ten days. The severe shock of this procedure results in rapid and simultaneous loss of feathers.

Generally, after a second period of laying, the birds are pulled from their cages and taken to slaughter. There are no laws governing how birds are handled or transported nor how they are slaughtered. In their weakened condition, many suffer broken bones from rough handling. They are stuffed in crates, exposed to adverse weather and finally slaughtered, often while conscious to some degree.

Many of these problems occur in cage-free egg production as well, but battery cages represent the worst possible treatment for hens. Hens have very strong behavioral traits which are denied expression under these conditions. Wing flapping and stretching, flying, scratching, dust bathing, perching, meaningful socialization, preening and the use of a nest for laying are impossible in small cages.

More information on battery cages:
Wikipedia
HSUS No Battery Egg Campaign, including Cage Free vs. Battery Cages
COK and HSUS Expert Statements