Re: Mercury 150 saltwater hard starts and stalls at idle
Battery is for cranking, and accessories, EFI models use the battery for fuel supply, otherwise, the ignitions are all stator/alternator driven, with a minimum cranking RPM of 300, slower than that the motor may not spark or start at all, but once started idling 700 or better, each cyl should have a hot blue arc it least 7/16 inch in free air.
Batteries dying often turn out to be the contant drain of the radio/cdplayer, they have a constant 12 V to keep station presets in memory, and a separate switched 12 V line to turn on with the ignition switch.
Disconnect either battery cable, rig a test light between the battery post and the disconnected cable, if the light is on, something is drawing power and will kill the battery. Start pulling fuses, one at a time until the light goes out to isolate where the problem is.