Engine DC Electric System

May 20, 2014


dc electric system 

Engine DC electric system

Each engine has its own separate 12 V DC system used to power the engine starter motor and the engine instrumentation.

The ZF Micro Commander electronic engine controls are powered by this system and described in the Engine page.
The FloScan flow meters are powered by this system and are described in the Fuel System page.
The Marsili starboard rudder position indicator is powered by this starboard system and is described in the Steering Mechanism page.

Basically each system consists of the engine alternator and the engine starting battery. There are no connections to the general DC system, except from the engine alternators to charge the service batteries.

The idea behind the separate engine DC electric systems is to isolate the engines as much as possible from the general electric system and to minimize the possibility of not being able to start an engine if there is a malfunction in the general electric system or if the service batteries have been completely discharged.



In Harbor and Navigation

Always leave the engine panel breaker of each engine on.  You may never know when you have to start the engines.

Never turn the engine panel breaker off when the engine is running.  Damage to the alternator may occur.

Periodically check the electrolyte level of the engine batteries.  Add distilled water if necessary.

Boat inactive or Hauled out

Turn the engine panel breaker of each engine off.

Dead Engine Battery

The engine batteries should never discharge completely since they carry no electrical load when the engines are turned off.  It is extremely important if a battery is dead to determine the cause before recharging the battery.  There might be a problem with the alternator.

To charge a dead engine battery make sure that the engine battery charger is connected to the discharged battery and recharge it.



Starting Batteries

Since the engine DC system is only used to start the engines, the batteries are large 4D size, standard truck batteries, not necessarily marine batteries.

The Cummins recommended battery size is 250 A but they seem too large for our climate.  So the new ones have 180 A each.

Each battery is enclosed in a size 4D battery box.

Purchased in February 2005 2 starting batteries of 12V 240 Ah from Atom accumulatori.
Purchased on March 16, 2010 two new batteries, since the old ones almost died, especially the starboard one, from Atom accumulatori.
Purchased in May 2014 two new RHIAG Exide heavy duty batteries from Motonautica Angerese after the case of the port battery broke and acid was spilled on top of the fuel tank.  What a mess!


RHIAG Exide, port  12V  180 Ah Serial # 46396235 Motonautica Angerese
1 RHIAG Exide, stbd  12V  180 Ah Serial # 46389989 Motonautica Angerese
2 4D size battery boxes Osculati

Engine Wire Harnesses

To the original Cummins wire harnesses that go from the engine to the instrument panel the following wires were added:

  Load A Size mm2 Color
Positive, after engine room circuit breaker - 10 Red
Positive, key switched - 10 Blue
Muffler exhaust temperature probe - 1.5 Purple

The additional positive wires were added to accommodate the higher loads generated by the MicroCommander engine controls. The key switched wires actuate a relay in each engine room.

The original Cummins wire harnesses that go from the engines to the instrument panels were too short and additional length was spliced into the middle.  The original color coding was maintained at both ends.


cat_14_e dc engine.htm