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Author Topic:   Moving Batteries
Rockrock posted 07-13-2013 09:40 AM ET (US)   Profile for Rockrock   Send Email to Rockrock  
Hi all,

I originally ask about this in the wrong section. I am in the process of moving my batteries to the center console from the transom area on my 1989 17 Montauk. I have two group 27 AC Delco Voyager sealed batteries and a 1989 Yamaha 90 2 stroke engine. Does anyone know how many amps my outboard draws during start? I can not find the amperage draw anywhere! I also have a 55lb thrust Motor Guide on transom. I am planning on running 6 AWG wire to the trolling motor through the rigging channel. For the 90 Yamaha starter and alternator, I am planning on running either 4 or 6 AWG wire with no splicing. I would like to get by with 4 AWG if possible. Not knowing the amp draw of my 90 makes wire size hard to select. I will probably need approximately 14 feet one way or 28 feet round trip. I will also be wiring in parallel through a Perko battery selector. I have most of the work completed and am down to purchasing the wire. Thanks in advance for any input you may have.

jimh posted 07-14-2013 08:28 AM ET (US)     Profile for jimh  Send Email to jimh     
According to Hebert's Rule, a conductor's rated power distribution current for a three-percent drop means the maximum length for that current and drop is equal to the system voltage. Since we are discussing 12-Volt system, that means 12-feet is the maximum length for the conductor to have its rated three=percent drop. I suspect that on a 17-foot boat the distance to the outboard engine from the center console will be about 12-feet or less. This means we can look at the wire table for current for power distribution and find the conductor size based on its rated current.

Using the table at

http://www.powerstream.com/Wire_Size.htm

as a reference for the rated current for power distribution, we see the conductors you have mentioned have the following ratings:

6-AWG = 37-Amperes
4-AWG = 60-Amperes

If you want to maintain a three-percent voltage drop or less, the maximum distance you can use either of those conductors in a 12-Volt system is 12-feet.

Hebert's Rule is described in
Cf.: http://continuouswave.com/ubb/Forum6/HTML/003384.html

jimh posted 07-14-2013 08:47 AM ET (US)     Profile for jimh  Send Email to jimh     
Choosing a conductor size for the engine electrical starter motor is more difficult than just selecting a certain voltage drop tolerance and current flow to be anticipated. Typically the current to an electrical starter motor is extremely high at the initial moment the starter motor is activated. The starter motor is in a stalled condition, and a very high current will flow. This causes a very high voltage drop. The power delivered to the starter motor must, at this point, still be great enough to permit the starter motor to begin to rotate. As soon as the starter motor begins to rotate, the current demanded will drop, the voltage drop in the power distribution cables will decrease, and more power will be available to the starter motor. The starter motor will accelerate, spinning the outboard engine over, and starting it.

The larger the conductors used to connect the starter motor to the battery, the more likely the starter motor will be able to overcome the initial stalled condition and start the outboard engine. The conductor size used is typically chosen not for the absolute peak current, which can be perhaps 300-Amperes or more, but for a compromise--that it can get the starter motor rotating and handle the more steady-state current of the starter motor without much drop.

I do not have any data about the peak starting current of a Yamaha 90-HP outboard, nor for the steady-state starting current. I would make an inference from the existing engine starting cables. For example, if the existing engine starting cables are made with 4-AWG cable and are 6-feet long, if you want to extend the distance to 12-feet you should use a conductor with twice the current rating.

From the table we see 4-AWG is 60-Amperes. To get 120-Amperes we have to move to 1-AWG. This would be for the entire run, not just making an extension of the existing cables. Approaching the design this way will give you the same margin as the original system had.

Rockrock posted 07-15-2013 09:44 PM ET (US)     Profile for Rockrock  Send Email to Rockrock     
Thanks Jim! You are obviously very knowledgeable in all areas of boat stuff. I appreciate your time concerning my project. My current wires appear to be 4 AWG and run 8 feet one way or 16 feet round trip. This was the original wiring Yamaha used. I had my current set up wired in parallel through a Perko switch. I had the 4 AWG + lead from the starter connected to the COM Perko and the - lead directly to the battery. Not knowing any better, I used 8 AWG wire to connect both batteries to the 1 and 2 Perko positions. I also used 8 AWG wire to connect the - posts together on both batteries. I probably had a total length of 5 or 6 feet of 8 AWG involved. This set up started the engine with no problems at all. I have used this set up for over 8 years. I would really like to get those batteries out of my way in the transom area! I understand they put the batteries in the center console on the current Montauks. I would like to see there rigging? My local boat dealer claims they use 6 AWG wire to rig all there battery set ups. (Not a whaler dealer) They claim they have no electrical issues concerning engine starts! Makes me wonder how they get by doing that! One reason I hoped to use 4 AWG is cost! In addition, I am not sure larger wiring will fit through the engine cowling on my 90. I am absolutely sure 1 AWG wont! Any way, thanks for your expert input.

Appreciatively,

Rockrock

jimh posted 07-16-2013 11:14 PM ET (US)     Profile for jimh  Send Email to jimh     
Smaller wire means more voltage drop. More voltage drop means less margin in the system for when the battery voltage is low to begin with. Configurations that work with fully-charged batteries on a warm day may not work with batteries that are partially discharged on a cold day.
DVollrath posted 07-17-2013 01:36 AM ET (US)     Profile for DVollrath  Send Email to DVollrath     
Rockrock,
I moved the battery of my 1988 Montauk to the console. I have a newer Yamaha F70 on the transom, and a single group 24 West Marine battery of unknown age (it came with my boat when I bought it 2 years ago). The one-way length of starting cable turned out to be about 17', starter to battery. I used 6 gauge, and it starts just fine, but this is for my starter. I have no idea what the draw of your older, larger motor might be. My motor came factory rigged with about 8' of 8 gauge. You may want to see what the recommendation of the Yamaha rigging guide is, if you can find one appropriate for your outboard.

As JimH points out, larger wire will give you more margin and will add to your ability to start under less than optimal conditions. As you realize though, there is extra cost in the larger cable, both in terms of dollars as well as workability.

Dennis

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