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Electrics, Electronics & Hardware

Cabling in a second battery

by ColinR » 07 Sep 2015, 13:16

Looks simple enough and there are many sample wiring diagrams available but I have what is probably a noddy question.

On my existing setup I have a black cable that runs to earth on the engine.

I have a heavy red cable that runs to the isolator switch.

I also have two thinner cables off the positive side that I don't know where they run to. What would they be for and how do I accommodate those in a twin battery setup, or do I just leave them on one battery and use that as the house one and wire the other battery in for starting only.

Are they, for instance, a feed for items that are always live, bilge pump / CO monitor for instance?
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by Ianfs » 07 Sep 2015, 14:02

Hi Colin

Not a stupid question. One might be for the Hydraulic Trim Pump, it might draw 110 -130 amps, you often get a positive feed to the up and down solenoids to a common terminal. Its a tough old bird, put a screwdriver across the terminals for a tester and watch the sparks.

The other may be if you have a Slave Starter Solenoid.

Depends how thick they are?

No expert but if they are stiffish copper red sheathed cables OD about 6mm, I doubt they would be used for a bilge pump, an 1100gph only draws about 6 amps.

As for how you to wire in the other battery will depend upon what you want it for. If you want it to be a second starter that's one way, if you want it to run auxiliaries that's another, or you may want it to do both and put in a remote charging system. :? :D

If you are not going to overnight at anchor, then I doubt you'll need a "real" Deep Cycle Battery, like a Rolls or a couple of Trojans. Therefore you could get a pretend Deep Cycle & Starter as a back up.

There's bound to be some sort of high tech wizzy thing these days which does the lot and you just plug it in. :shock:

James, Ed, Gaz.....your turn. :D :?:
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by ColinR » 07 Sep 2015, 14:21

Hi Ian

6mm sounds about right, there are two cables of that size.

wiring.jpg
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What I want is to be able to isolate the electrics completely (maybe leaving the bilge on)

or

have the feed from a battery that is saved for starting (Number 1)

or switch to battery two for sitting around at anchor so that the starter battery is kept fully charged (this would be the one with the bilge pump attached) (Number 2)


or both when under way so that both batteries are charging.

Just a thought, would the thinner cables be from the alternator?
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by _Ed_ » 07 Sep 2015, 16:10

For anything that may be used to start an engine you want at the very least 4 AWG cable. If you can find tinned copper cable that is 4AWG like this: http://www.ebay.co.uk/itm/4-AWG-Gauge-T ... 0437603525 then even better.

If you had a big V8, and the battery is not close I would probably consider 2AWG. AWG is 'American wire gauge' and 4AWG is around 20mm^2 cable ~5mm OD (excluding rubber insulation) and 2AWG is around 33mm^2 or 6.5 OD.
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by Ianfs » 07 Sep 2015, 16:33

Just a thought, would the thinner cables be from the alternator?


No I don't think so. Again, I think, the battery is charged via the large cables from the alternator connected to the starter solenoid then to the battery via the large red (positive) cable and the large black (negative) cable.

I know what you are looking for, a pretty standard set up. Most bigger boats have battery isolators and combination switches to allocate what you are looking for. There are isolators which work by measuring voltage drop and then isolating a battery. Consequently if you have a drain on one to almost flat the isolator will keep them separate but you can then use combination switch to connect them.
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by Xboatboyx » 07 Sep 2015, 17:38

Ianfs wrote: James, Ed, Gaz.....your turn. :D :?:


Are you joking me :roll: . I'll join Matt & Mark in the i haven't got a clue section if that's OK with them .
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by Ianfs » 07 Sep 2015, 17:57

Are you joking me :roll: . I'll join Matt & Mark in the i haven't got a clue section if that's OK with them


Sorry James, now there's me thinking you will be our next Marine engineering guru, you certainly know a lot about types of boat and how they handle! :)

Oh and Mark actually wrote a post about Hydraulic Motors! :shock:

Mind you come to think about it Dads a bit wizzy with electronic and electrical stuff.........Martin, your turn???? :D
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by mark101 » 07 Sep 2015, 18:07

I only repeat information I have learned from experience, otherwise I'm a joke when it comes to tech


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by ColinR » 07 Sep 2015, 22:34

Well I've communicated with one who should know, Tompamarine. The thinner wires are bilge pump and radio for the memory. We have a cunning plan :D
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by Gazjen » 08 Sep 2015, 01:55

The easiest way is to fit a 1 or 2 or both battery switch is that your plan?

Put everything you've got now on the output of the switch then link the battery negative with the same gauge cable then run a new red positive cable from the new batt to the 2nd batt feed on the switch again in the same gauge.

This way you can use each battery independently and either as start or house battery. Or both if they are low on charge.

Something like this:

http://pages.ebay.com/link/?nav=item.vi ... ID=EBAY-GB
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