Multiple Efi Engine Battery Precautions - MerCruiser 305 CID (5.0L) Service Manual

SERVICE MANUAL NUMBER 24

Multiple EFI Engine Battery Precautions

Situation
Alternators: They are designed to charge the battery that supplies electrical power to the
engine that the alternator is mounted on. When batteries for two different engines are con-
nected, one alternator will supply all 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 boat operation, an electrical onboard device may cause a sudden drain of
voltage at the engine's battery. The voltage may go below the ECM's minimum required volt-
age. Also, the idle alternator on the other engine may now start charging and 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 resets 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 to be con-
nected to its own battery. This ensures that the engine's Electronic Control Module (ECM)
has a stable voltage source.
Battery Switches: While engines are running, battery switches should be positioned so
each engine is running off its own individual battery. DO NOT run engines with battery
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.
NOTE: Sure Power Industries Inc., Model 32023A meets this design specification.
1. The boat may have 2 engines connected to a single Model 32023A battery isolator.
2. The Model 32023A battery isolator is connected to 2 banks of batteries.
3. Each bank contains 2 batteries with the cranking battery for 1 engine in each bank.
4. The second battery in each bank is connected in parallel to the cranking battery.
5. The Model 32023A battery isolator is designed for this type of use; 2 battery banks, 2
charging sources, 120 amps (maximum alternator output).
6. When the engines are running, either engine's alternator could be charging either bank
of batteries through the Model 32023A battery isolator.
Any other manufacturer's battery isolator that is the same type as the Sure Power Inc.,
Model 32023A could also be used.
Generators: The generator's battery should be considered in the same manner as another
engine's battery.
Index
90-861327--1 OCTOBER 1999
CHARGING SYSTEM
Page 4C-3