Electrical Cables and Wiring for your boat [Archive]

Dave

04-02-2010, 11:34

Pirate Pete...
Hi all out there.

Last night on the chat

http://www.polyboatowners.com/chat.htm

We got onto the subject on what size cable should you run, in this case from the Main Cranking Battery to a VSR Relay & then to 2 AUX Batteries for a 24V electric. About 5m away.

The VSR charges 1 Aux Battery for 1 min & then the other once the Main is charged. (13.7 V according to the web site )

http://www.bepmarine.com/Trolling-Ba...-166-1471.html (http://www.bepmarine.com/Trolling-Bank-VSR-Module-166-1471.html)

The motor used on this boat is a Suzuki 80HP 4 St with an Alternator output of 27A.

http://www.hainessuzukimarine.com.au...ID=24&mainID=3 (http://www.hainessuzukimarine.com.au/content.aspx?pageID=24&mainID=3)

I dont know the size of the Main Cranking battery but mine is a Century N70ZN with a CCA of 600A.

From what I can find the recommended charge voltage for valve-regulated-lead-acid batteries (VRLA) are typically charged to between 2.26 and 2.36V/cell. So a 6 cell 12 volt battery should be charged at 13.56 to 14.16 Volts.

Im a spaarkie by trade & all my paper work & calcs are based on AC & 230 / 400V Cable.

I had a quick look on the net last night & found this site.

http://www.rpc.com.au/products/efn/efn133.pdf

That gives a chart to use to determine cable size allowing for a 5% voltage drop. ( 5% of 14.16v is 0.708v ) If I use this chart I will get 13.45V to the VSR so I would need to use a larger size than what is recommended.

http://i233.photobucket.com/albums/ee312/Polyboatowner/12VoltCableSize.jpg

The chart allows for the out & back current path so it is the distance the cable has to run ie. 5m not cable length 5 out & 5 back.

I hope that so far I have made sense.

Now here comes the part that Im a bit scratchy on.

HOW MUCH CURRENT does the AUX Battery Draw to charge ??????

Is it 27A the max output from the Alternator or some what higher because the Main Battry will also be trying to put charge into the AUX. ( The VSR’s wiring diagram wants 70A Fuses )

If I say its 27A the alternator can put out then looking at the chart 30A (next closest to 27A) needs a 13.6mm Cable to run that current that distance. ( again here is where Im in trouble as in Australia the closest to that is 16mm )

Most of the people on the site seemed to think that this was way too big a cable & thought 6mm was heaps big enough.

If I say its 70A the max current set by the fuse. ( This is what my training tells me, the fuse is there to protect the cable & stop FIRES ) then I need to look at the chart at 80A (next closest to 70A ) then you would need to run a 25.7mm cable ( again here is where Im in trouble as in Australia the closest to that is 25mm )

So what is the right cable to use???

Mcgyver...

Pete is 100% correct, load must be less than the rating of the fuse which must also be smaller than the current carrying capacity of the cable. Pete i would not and have never used less than 16mm2 cable when linking batteries, which is basically happening when your voltage sensitive relay is energised. I say this because if you have a big stint on the electric and pull significant charge out of the aux batteries when you do start motoring again, and your main battery reaches the required voltage and energises the vsr you will have a large potential difference between the batteries drawing a high current.

sorry for draging on but i cant really shorten it down. :)

Ultimately your outboard never charges your aux batteries your main battery does, though your outboard charges your main battery. And that is why i think 16 or 25mm2 cable is not a bad idea(higher ccc, less voltage drop and lower conductor temp).

I have set up a few dual and tri battery systems in 4x4's with larger alternators (70-110Amp) i am unsure how this set up will work for you with only a 27Amp alternater i think it might be a little ambitious, but i really hope that i am wrong.

Andy...

Ok, I will try and explain this the bast I can. I will do it over multiple posts in an attempt to help people understand a bit better. Once everyone gets it, I will move on.

The amount of current your battery will draw from any systems depends on a few things, but the primary reasons are:
1) State of charge of the battery.
2) Charging Voltage.

If you hook a fully charged battery to a flat battery, they will tend to equalize, that is the charged battery will lose some charge, and the flat battery will gain some charge. A fully charged battery will not charge a fully flat battery. The reason for this is that the fully charged battery does not have a high enough voltage, and hence cannot supply enough current to charge the flat battery.

If we increase the voltage, then the flat battery will draw more current. This is basic ohms law. If for the exercise we pretend that a battery at one give state of charge is a fixed resistance we can prove this theory. Pretend the battery is exactly 1ohm when its flat. Ohms law states current = voltage / resistance.

12V / 1ohm = 12 amps

Increase charge voltage from 12V to 13.8:

13.8 / 1ohm = 13.8 amps

This is why it is so important to factor in cable size when charging batteries. If the cable is to small, you get a large voltage drop, and not enough voltage charging your batteries.