It very well could be a bad batt or simply the way the Perko switch is utilized. One crucial element to extended battery lif is making sure it has a charge throughout the off season. Placing them on a automatic trickle charger is the only way to ensure that. A battery that sits for months on end that is not recieving a charge will loose capacity to the point of what is happening here. I pull my batts when the season is over and place them on trickle chargers on the bench until the first season outing every year and have never had battery issues on any of my boats.