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Author Topic:   Dual Batttery Primary Wiring
Nauti Tauk posted 01-21-2007 06:23 AM ET (US)   Profile for Nauti Tauk   Send Email to Nauti Tauk  
Would like to mount a second battery in console of our 2003 Boston Whaler 170 MONTAUK. I know I need a battery switch. What is the best orientation of the battery boxes? How is the switch wired? Any help or suggestions would be helpful. Thanks
stagalv posted 01-21-2007 01:01 PM ET (US)     Profile for stagalv  Send Email to stagalv     

Jim H has written a wonderful article on this subject. Go back to the reference section of the site, look under "Electrical" and there you will find "Dual Battery". It says it is for twin engines but when you click on it there is a part on single engine / dual battery.

Additionally look for the search button on the discussion pages and type in what you are looking for. There is a great log of subjects that have been discussed.

Good luck and enjoy.


jimh posted 01-21-2007 02:44 PM ET (US)     Profile for jimh  Send Email to jimh     
Need for two batteries

The decision on using dual batteries is probably best based on the ability to start your main engine by pull starting. If you can start the main engine by pull starting then a redundant battery system may be excessive. If you cannot start your main engine without the electric starting motor then it is a good idea to have a redundant battery system.

Two battery distribution

A more modern method [than the one shown in my reference article] of dual battery operation is to maintain a separation in the vessel wiring which isolates the loads into two groups: loads related to engine starting, and all other loads. Separate batteries and separate primary distribution serve each group of loads. This approach offers several advantages over using a wiring scheme in which all loads are made in common and there are redundant batteries. A provision is made to tie the batteries in parallel in an emergency situation to help with engine starting.

The advantage of isolation of the engine starting loads from the rest of the vessel electrical loads is:

--engine starting battery cannot be drained by non-starting loads
--engine starting voltage transients do not affect other loads
--battery choice can be optimized for the load, for example, a starting battery is used for starting loads and a deep cycle battery is used for non-starting loads

The various manufacturers of electrical distribution equipment for small boats do a good job of explaining in detail how to connect their devices to obtain this sort of isolated primary battery distribution wiring.

There is some prior discussion on this topic, as recently as one day ago, which you can read to become more familiar with the various solutions.

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