i have a 1986 merc 35 on a pontoon. it wont keep the battery charged on long rides especially if we run depth finder, sterio, etc. is the charging system too small or do i have a problem? if the charging system is inadequate, are there any solutions?

 

This post might be usefull.
http://www.outboardrepairs.com/topics/000078.html#0

Other than that how old is the battery? Do you start off with it fully charged?
I use my battery to run a fish finder, FM radio, VHF radio, downriggers, and GPS. I have never used enough juice to even notice the battery going lower on charge. All this is without the engine running. I use a separate trolling 10hp that does not charge the battery.