First off a flooded lead acid battery is not designed to be used or charged in any other position other than upright. They should never be charged on the side as the acid may leak out and the plates may fall out of their brackets and short out.
There are two other types of batteries AGM (absorbed glass mat) may be used horizontal or at an angle but still it is recommended to be charged upright. Gel batteries are better than flooded since the plates are suspended in a jello like liquid helping hold the plates apart and perform better since the gel acts as a dampening agent.
We have transitioned over to AGM batteries for the safety of being sealed and being designed to be used at other orientations. They are also a bit more environmentally friendly since they have nominal lead in them.
Each battery has its pros and cons.
You may want to read this link on basic batteries.
http://www.batterystuff.com/kb/artic...ry-basics.html