Mercury Mercruiser Tow Sport 5.7 MPI Manual page 63

Section 5 - Maintenance
Batteries: Boats with multiengine EFI power packages require each engine be connected to its own battery. This ensures that
the engine's ECM has a stable voltage source.
Battery switches: Battery switches should always be positioned so each engine is operating off of its own battery. Do not
operate engines with switches in both or all position. In an emergency, another engine's battery can be used to start an engine
with a dead battery.
Battery isolators: Isolators can be used to charge an auxiliary battery used for powering accessories in the boat. They should
not be used to charge the battery of another engine in the boat unless the type of isolator is specifically designed for this
purpose.
Generators: The generator's battery should be considered another engine's battery.
90-8M0076545 NOVEMBER 2012 Page 55
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