Multiple Efi Engine Battery Precautions - MerCruiser 5.7 MPI User Manual

Section 5 - Maintenance

Multiple EFI Engine Battery Precautions

Alternators: Alternators are designed to charge the battery that supplies electrical power
to the engine that the alternator is mounted on. When batteries for 2 different engines are
connected, one alternator will supply all of the charging current for both batteries. Normally,
the other engine's alternator will not be required to supply any charging current.
EFI Electronic Control Module (ECM): The ECM requires a stable voltage source. During
multiple engine operation, an onboard electrical device may cause a sudden drain of
voltage at the engine's battery. The voltage may go below the ECM's minimum required
voltage. Also, the alternator on the other engine may now start charging. This could cause
a voltage spike in the engine's electrical system.
In either case, the ECM could shut off. When the voltage returns to the range that the ECM
requires, the ECM will reset itself, and the engine will operate normally. The ECM shuts off
and resets itself so quickly that the engine may only seem to have an ignition miss.
Batteries: Boats with multi‑engine 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 generators battery should be considered another engine's battery.
Page 72 90-864197072 NOVEMBER 2007