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CA742
Multiple EFI Engine Battery Precautions
Situation
Alternators: Alternators are designed to charge the battery that sup-
plies electrical power to the engine that the alternator is mounted on.
When batteries for two different engines are connected, one alterna-
tor will supply all of the charging current for both batteries. Normally,
the other engine’s alternator will not be required to supply any charg-
ing current.
EFI Electronic Control Module (ECM): The ECM requires a stable
voltage source. During multiple engine operation, an onboard electri-
cal device may cause a sudden drain of voltage at the engine’s bat-
tery. The voltage may go below the ECM’s minimum required volt-
age. 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. The engine
will now run normally. This ECM shut down usually happens so fast
that the engine just appears to have an ignition miss.
Recommendations
Batteries: Boats with multi-engine EFI power packages require
each engine be connected to its own battery. This ensures that the
engine’s Electronic Control Module (ECM) has a stable voltage
source.
Battery Switches: Battery switches should always be positioned so
each engine is running off 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.
Generators: The generator’s battery should be considered another
engine’s battery.
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