Adding a Dedicated Starting Battery to the C-320

Technical Articles

My dedicated starting battery is sitting on a shelf that I added in the space just forward of the forward 4D in the starboard side settee. As a slight port list is typical for many C-320 owners and the space is ample for the purpose, this is a favorite location for many later model C-320 owners where the batteries are located under the middle of the starboard settee. This location is also desirable in keeping the new jumper wires short which reduces resistance and voltage losses.




The goal is to provide an isolated emergency starting battery while making only a few wiring changes right at the battery terminals themselves.  To do this we first hardwire, in Parallel, the 2 existing 4D batteries to become a single large house bank. Then add a starting battery as the new second battery.  The existing 1-2-Both switch will now switch between Starting battery, the House bank, or combine all batteries in the Both position.


The wires that are likely to be found on the forward 4D battery terminals are: 


Forward 4D Negative (-) Terminal:

1)      A large 1/0 gauge BLACK ground wire going to the aft 4D’s Negative terminal

2)      A smaller 4 gauge BLACK ground wire going to the Windlass ground


Forward 4D Positive (+) Terminal:

3)      A large 1/0 gauge RED power going to the battery selector switch

4)      A smaller 4 gauge RED power coming from the battery Charger.

5)      A smaller 4 gauge RED power going to the Windlass breaker


Step 1:  Starting Battery Installation

1)      Mount the starting battery and strap it down.

2)      Move the large 1/0 gauge Red (#3 above) from the Forward 4D to the Positive terminal on the Starting Battery.

3)      Move the battery charger cable (#4 above) from the Forward 4D to the Positive terminal on the Starting Battery.

4)      Connect the Starting Battery’s Negative terminal to the Forward 4D’s Negative terminal using a short Black 1/0 gauge pre-tinned cable.


Step 2:  Wire both 4Ds in Parallel 

The Negative terminals of the 4D batteries are already connected together by a short cable.  To complete the process of wiring the 4Ds in Parallel, add a new short cable to directly connect both 4D Positive terminals together.  This must be a Red 1/0 gauge cable.


The Result:

  • A single large house battery bank that will provide a few more usable Amp hours than when using them individually.  The battery selector position that used to select the AFT 4D is now the "House" battery.


  • A dedicated Starting battery that can be selected by using the selector position that used to be the FORWARD 4D.  This is now the "Starting" battery.


  • Normal operation is to leave the battery switch selected to "House" and use the house bank to start the engine, and run the world.  If the house bank won't crank the engine because it's too run down, just switch to the Starting battery position. 


  • Switch to ALL when motoring to recharge the house bank and top off the starting battery if necessary but remember to switch back to the “House” position when the engine is off.


  • The Shore Power Charger will charge both the house and starting battery automatically whenever you're plugged in regardless of what you do with the selector switch since it connects directly to the House and Starting battery terminals.


Safety Concerns:

  • When working around these batteries, please wear safety glasses, tight fitting gloves and long sleeves. Remove any watches, rings or metal.
  • To be extra safe, temporarily disconnect all the cables connected to the negative terminals of all 3 batteries and reconnect them last.  You can put a zip-tie through the cable lugs to keep them together and away from the terminal posts until you’re ready to reattach them.
  • Be careful with wrenches, so that you don’t accidentally short any terminals.



  • Consider adding a Blue Seas ACR (7600 or 7610) that will automatically keep the starting battery topped off whenever there is a charge source on the house battery.  Then, for all intents and purposes, you just leave the selector set to "House" and the starting battery is automatically maintained both when motoring and when plugged into shore power.
  • Always use tinned marine grade wire of the same or larger gauge than the ones being replaced. Make sure all connectors are properly crimped and then sealed from moisture.
  • Consider adding a Link 2000 monitor or digital volt meter on the panel to keep an eye on battery voltages.  I have found that the analog battery meter on the panel is not accurate enough to provide reliable information on the status of the batteries.
  • Consider an AGM automotive starting battery.  It will work well regardless of which type of house batteries you have.  Do not get a combination Deep Cycle/Starting battery.  It makes a poor starting battery.

For more photos, click on this image to visit the photo album..


Jeffrey Hare, Catalina 320 International Association Webmaster

Woodbine II, Hull # 809


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