Catalina 34

General Activities => Main Message Board => Topic started by: Stu Jackson on September 21, 2011, 12:48:27 PM

Title: Basic Battery Wiring Diagrams
Post by: Stu Jackson on September 21, 2011, 12:48:27 PM
This interesting topic keeps reappearing all over the internet boating forums.  Basically, there are two very simple ways to wire your house and reserve banks for a solid electrical system.  BOTH OF THEM require that you "rethink" the simple 1-2-B switch, as follows.  The ONLY connections to the C post of the switch are to the starter and the distribution panel.  The alternator output is REMOVED from the C post and goes directly to the house bank.

OPTION 1 - Single 1-2-B Switch Feeds BOTH Starter and House Loads (Distribution Panel (DP))

1. Take the alternator output off the starter and run a new heavy gauge wire to the house bank positive - from the alternator output (AO) with a fuse near the battery bank.  Some installations had a small wire between the AO and the starter.  Remove this wire.  Run the power to the starter FROM the C post of the 1-2-B switch.  Simple.  It's already there!

2. Take the shorepower charger's output off the reserve/start battery and leave it only on the house battery positive - again fused near the battery. If you have solar panels or a wind gen the output also goes direct to the house bank.

3. Buy either an Echo Charger or an ACR (automatic charge relay) and wire it to the house +, start + and ground.

What this does is change your 1/2/both switch to a "use" switch and not a charge switch because the AO no longer goes TO the switch, it goes directly TO the house bank. The 1-2-B switch still lets you decide which battery to use for any load but the charging of the reserve bank is automatic.  ANOTHER advantage is that IF your house bank COMPLETELY DIES, you can use your reserve bank for LIMITED house loads

The easiest way to use this system is to use the house bank for everything and keep the reserve battery for emergency use. When you arrive on the boat turn the switch to 1 (house bank) and turn it to off when you leave. Engine starting is easily accomplished by the house bank.

And there is no chance of error.  And if your echo charger or ACR breaks, then, ONLY WHEN CHARGING,  simply use the B position on the switch!  Backup, backup, backup.


OPTION 2 - Two Switches:  A 1-2-B Switch Feeds the House Loads (DP)) and a SECOND On/Off Switch Feeds the Starter

The two switches are connected. 

1 - If the starting battery goes bad simply turn the 1/2/BOTH/OFF to BOTH and flip the ON/OFF to OFF you can now start your engine and do house loads off the house bank yet still 100% isolate the starting battery.

2 - If the House bank goes bad simply flip to position #2 and leave the ON/OFF turned to ON. This will allow powering house and starting loads from the starting battery and allows it to also act as a reserve.

3 - For normal everyday use flip the 1/2/BOTH/OFF to #1 and the ON/OFF to ON.

Some boaters prefer this arrangement because they feel the need to separate their electronics from the starting power requirements.


Please note that in BOTH cases the 1-2-B switch is used as a "USE" switch, only "deciding" which battery bank to take power OUT of.  The AO goes TO the house bank, NOT to the C post of the battery switch.  The advantage is that the 1-2-B switch operation can NEVER blow the diodes out of the alternator.  You can even turn the danged switch OFF with NO damage!!!

Important ConsiderationOnce you move the alternator output to the house bank, that wiring will be live.  Either remove the fuse or add a simple on/off switch (Alternator Service Switch - make SURE it is closed before you start your engine) to turn the positive power off when working on the alternator or the engine.

The Blue Seas Dual Circuit Switch, in my opinion, is NOT an option that should be considered, although many do so.  The SERIOUS DEFICIENCY in this concept is that it can only do two things: keep the reserve and house banks separate OR combine them.  When a bad bank and a good one are combined the bad bank drags the good bank down, which is exactly what keeping banks separate tells us should not be done.

Here's how they work, from the Blue Sea website: (

Here is an excellent discussion bout the DANGERS associated with the dual circuit concept:  by Maine Sail called "Darn AGMs" --

Once you read both, you'll realize the serious implications of that concept, regardless of the type of batteries that you have.


Option 1 diagram courtesy of Dave Mauney, Option 2 courtesy of Maine Sail

PLEASE NOTE for Option 2: The wire from the 2 post of the 1-2-B switch is shown going to the starter; it can also just as easily be run to the lower post of the on/off switch, which is just the other end of the same wire.  The switches are usually closer to each other, resulting in less wiring.  Plus, it is hard to get two lugs onto the starter post.  

Other versions of these same diagrams have appeared elsewhere on our website and are available at the "Electrical 101" topic:,5977.0.html (,5977.0.html)

Maine Sail has also written something similar to this, with more written details:

This is another very good basic primer for boat system wiring:  The 1-2-B Switch by Maine Sail (brings together a lot of what this subject is all about)


Important Reminder:

Once you do either of these changes, make sure that the BATTERY SENSE wire from your regulator goes to your house bank.  The instructions with external regulators gives you an option to connect it to the back of the alternator.  This will NOT work, since it will be reading almost if not more than a volt LESS than if it was properly placed at your house bank.

1/13/2015 - Maine Sail just published this:   Alternator Regulator Voltage Sensing (


Battery Fuse Sizing with tables (from Maine Sail) (

USING THE 1-2-B SWITCH  added 1/6/2015

This is another good explanation from Maine Sail:

Question:  is there really a need for an emergency parallel circuit when you have a 1 ..both..2 switch installed?

Not unless you use it as a charge directing switch..

With a dead bank your best option is to simply switch to the good bank and not to combine them, before starting the motor..

When you combine a bad battery with a good one all it does is make the good battery work harder trying to both start the motor and to charge the bad battery. It is kind of like saying I can easily run a 100 yard dash and you are then asked to give someone a piggy back for the 100 yards. Think you'll perform the same...?

What if the dead bank has suffered an internal short, this happened just two days ago to member Capta on his generator starting battery. Your bad bank could now be a 10V battery as opposed to a 12V battery. Ouch!!

This is NOT a good time to be using BOTH as the battery voltages will never equalize and the shorted bank will continue to heat up and suck the good bank dead too..

Best practice with a 1/2/BOTH is to simply switch entirely to the good bank until you have a charge source on-line.

The BOTH portion was really intended for charge directing but is most often misunderstood because boat builders used to supply very small batteries and grossly undersize the wiring. With two batteries the boat started better so the myth that you need to use BOTH to start got a foot hold. If you need to use BOTH to start a motor there is something DRASTICALLY wrong with the batteries, system or wiring.

-Maine Sail / CS-36T

Shorepower Charging

Once you make this change and install an ACR (BlueSea or Yandina combiner) or an Echo Charger, simply run your shorepower charger to your house bank, too.  The ACR will take care of the reserve (or start) bank).

Title: Re: Basic Battery Wiring Diagrams
Post by: Stu Jackson on October 18, 2011, 04:49:25 PM
Here's one courtesy of "ctj1950" from sailnet.  About as simple a way to convey the basic wiring as I can find, just like Dave's Option 1 drawing above - it's exactly the same thing but this one shows the bits and pieces.  (

Maine Sail has noted:  if you have the same size wire for the ACR as you do the main battery bank cables then the fuses on the banks are already sized for the ACR wire. The solar panel fuse should be on the bus bar side of the controller.


The purpose of posting this wiring diagram is to show the kinds of details not shown on the more simplistic ones, for example, Dave's, above.  It is essentially the same thing, but shows the interconnecting wiring and bus bars.

It is NOT to be used for wire SIZING.  The wire sizes are based on YOUR OWN loads and wire distances.

You and you alone are responsible for the size of the wires and, hence, the fuse sizes

Title: Re: Basic Battery Wiring Diagrams
Post by: Stu Jackson on May 08, 2014, 01:54:04 PM
Here's another good, simple one, an example of the three switch version described above, but could easily be done with a simple 1-2-B switch.

Please note that we are NOT recommending using the wire gauges or fuse sizes shown on your boat.  That is both up to you as well as the need for those to be based solely on YOUR loads, alternator size, wire run distances, etc.

The purpose of posting this is to show a reasonable alternate switching arrangement.
Title: Re: Basic Battery Wiring Diagrams
Post by: Stu Jackson on May 12, 2014, 09:35:25 AM
Three switch wiring diagrams from Maine Sail (originally posted on

If you are going to use the three switch method be sure it is wired PROPERLY. This is one of the most miswired switch scenarios I see.

Alt goes to house...

The parallel switch is wired to the LOAD SIDE of switch 1 & 2......
Title: Re: Basic Battery Wiring Diagrams
Post by: Stu Jackson on May 12, 2014, 09:35:45 AM
Title: Re: Basic Battery Wiring Diagrams
Post by: Stu Jackson on May 12, 2014, 09:36:05 AM
Title: Re: Basic Battery Wiring Diagrams
Post by: Stu Jackson on December 23, 2015, 04:46:29 PM
We have been fortunate enough to have a lot of sailboat skippers visit our website to learn about boat electrical systems.

Many thanks to jonelli, who is very active on the forum.

He has a Catalina 30 MarkII - 1987 in Long Beach.


I sent jonelli this pm:

Thanks, much appreciated.  Your effort to portray the main components of the system are very clear and should/will be useful to so many skippers who are literally struggling with the basic concepts.

Thanks again.