The best way to approach this cache is from either campsite 5 or 6. If there are people camping in both campsites ask them if it is okay to cross through to get to the cache site. Not many people know about this Battery because it doesn't lie directly off of the perimeter road that circles the island. If you are going around the island in the counter clockwise direction you should look for a dirt trail on your left just after you pass Battery Ledyard and the trail that leads to Perles Beach. Head up this dirt trail, if you are riding your bike (which I recommend) you will probably have to walk it up the hill and park it at the top. If you can't access the battery from either campsite there is a small steep trail in between the two campsites that you can use. Make sure the cache is not visible when you re hide it.
There is a small informative sign near campsite 6 that talks about the history of the Battery. My sisters love exploring this place and can entertain themselves for hours. The unlit rooms can be a little spooky. If you are interested in exploring the rooms and hallways bring a flashlight so you can check them out. A flashlight won't be required to find the cache.
Angel Island SP Brochure & Map
UPDATE 8-20-06 Cache container is now a metal tin and should prevent the raccoons from chewing on it. The battery has been locked up for cleaning and maintenance. This won't affect the finding of the cache but you won't be able to explore the unlit rooms