With chargers and timers it is important to locate them where they will not be exposed to the gasses from the battery. The arc from the switch contacts could ignite them. Also, the charger should be switched off before attempting to clamp or un clamp the leads from the battery. I guess it goes without saying the charging area must be well ventilated with no nearby ignition sources. I think in the OP situation I would buy the sealed gel cell batteries, if they are available in a deep cycle.
