Many “dead” car batteries are actually perfectly good batteries. They just can no longer provide the hundreds of amps needed to start a car. Many “dead” sealed lead acid batteries are actually un-dead batteries that can no longer reliably provide a couple of hundred watts of power needed to keep a computer running in a power outage.
A couple of years ago I decided to add another small solar panel to the collection I have on my roof. I have a 5 and 10 watt. This new one is a 20 watt. It is dedicated to providing emergency power for lighting, a small fan and other misc. small low voltage devices. For this setup I needed a battery since it would need to be able to provide power 24 hours a day. I decided an un-dead car battery would be perfect since the largest load it would need to power for any extended period of time would be less than one half an amp. There is quite a difference between 200 – 600 amps and a half an amp.
The battery pictured was one I replaced when It would no longer start my car.
I could have used a linear regulator to charge the battery with the solar panel. The cost of a linear regulator is typically less than a dollar. I decided to go with a charge controller instead because they more efficiently use the available power to charge the battery. You get more run time at night when you use a charge controller.
There are cheaper charge controllers but after reading about several of them I went with this one. The 20 watt solar panel is currently (9-24-12) $52.99 plus shipping at Ul-solar.com