Alternator to split charge diode wiring + grounding

[SvenglishTommy]

Hello fellow boat owners, I can say that now as I recently purchased my first boat (although I wont dare call myself practical just yet)

My first attempt at DIY boat maintenance is replacing the rather dated electrical system from a 1,2,Both selector switch to seperate house and started banks charged through a split charge diode. I have a couple of questions regarding the alternator wiring in this new setup though, maybe someone can advise.

In the current setup there is just one pair of cables running to the engine for both starting and charging, so I assume the alternator positive is just hooked up to the starter cable at some point. With the diode though this will obviously not work as the diode will only allow current to flow one way. Would the standard way to install this type of setup be to run a seperate wire from the alternator positive to the diode and leave the negative common?

My second question is to do with grounding. As far as I can tell, the boat is currently not grounded at all. I think the original setup intended the boat to be grounded through the prop shaft, but a Volvo MD2010 was fitted in '97 which is electrically isolated from it's prop shaft(with specific instructions in the manual not to use the prop as a ground). I guess this leaves me ungrounded and with no option other than to ground the the negative through the keel, my problem is I have no idea how to go about doing this, can anyone help?

Mnay thanks,

Tommy

 

[VicS]

You'll probably find the alternator output is connected to the main battery cable at the starter motor. You will have to replace this with a connection from the alternator to the split charge device. BUT you will have to consider what to do about the "sensing" as the doides will intoduce a 0.7 volts drop. Some split charge devices include an extra diode and a connection for the alternator sensing otherwise you have to decide which battery to use for sensing or introduce a compensating diode. In either case you will have to convert the alternator to battery sensing, unless it already is, which is inlikely.

Have you considered a voltage sensing relay instead?

As it is all new to you why not keep a 1,2, off, both switch or just use separate isolating switches on each battery. You will still need an isolating switches anyway.

A bit theoretical but you will find some useful info in the electrics sections here: http://www.tb-training.co.uk/ (Previously Reading College)

Why do you want to "ground" the installation?

 

[SvenglishTommy]

Thanks for the tips VicS,

I wish I had kept the 1,2,both setup now, but I've past the point of no return. It's actually almost complete, just trying to find some crimpers to put the ends on the 35mm cables. I bought a 4-step electronic alternator regulator to compensate for the voltage drop over the diode, although I discovered afterwards that my current alternator is(or can be) battery sensed anyway.

As for the grounding, I was under the impression that you had to ground the electrical system to seawater unless it was completely isolated, not just isolated on the positive like mine is. Is that not right? I have also installed AC shorepower circuits which I'd like to be grounded as well. My main concern is safety (corroded, unprotected wires sitting in bilge water is what promted the overhall) so I want to get it right.

 

[andy_wilson]

Regulations

I have yet to install shorepower sockets, but on a similarly sized yacht as yours, my intention is to keep wiring as short as possible, as high as possible and trunked all the way. AC inlet close to breaker, breaker close to battery charger, and just a couple of cabin sockets also close by.

It will be fully isolated from anything metal on the boat, and the wiring will be nowhere near the engine, tanks, sinks, seacocks or winches.

There is a considerable risk of problems if you earth everything metal together to a ground plate, or even just the battery -ve, both from corrosion risk, or electrucution risk in the event of a short going to metal on the yacht where the shorepower connection is not up to transmitting it to earth.

It is not inconceivable that all your on-board earth could become live if the shorepower cable earth connection is damaged in some way.

Design it neat, and completely isolated from everything on board except the battery charger.

 

[redhot]

I changed my relay to a diode. I just wired the feed from the alternator to the centre pin and the +'ve terminal of each bank of batteries to one of the other pins, making sure that the starter battery was wired as #1. This just stops any power going from one bank to the other and thus avoiding draining you starter whilst using lights, or damaging you leisure batteries by starting on them.

At the same time as doing this, I wired in a Sterling alternator controller, a gizmo I would well recommend.

Thanks
Mike

 

[SvenglishTommy]

My shore power setup is just like your plan Andy. The AC inlet is within a few inches of the breaker unit which has an RCCB, a 6 amp breaker going to the charger and a 10 amp breaker going to a pair of sockets (again within a few inches). The thought of wiring this setup directly into the DC system just seems crazy to me, but everything I've read says that you should ground the shore power along with the DC to seawatwer. To be honest, I'm not that concerned about grounding the already grounded(twice) AC earth, but I'm confused about grounding the DC, do people generally ground the DC circuits?

I'm glad you recommend the Sterling alternator regulator redhot as that's the one I have too, I have a Sterling charger, split charge diode and galvonic isolator as well so I hope they make good stuff. Their prices are way below the rest, but their products do look a bit 'home-made'

 

[boatmike]

There are many questions raised by your post and answering directly would cause controversy as there are plenty of experts on here with different opinions (many valid) I would advise you buy a little book from Adlard Coles called Boat Electrical Systems by Dag Pike. It's very simple and covers wiring of diodes etc with diagrams. One other thing. DONT ground your ships electrics to the keel or anodes until you have read the advantages and disadvantages of doing so. Ideally the battery negative should be isolated from ground wherever possible and anodes seperately connected to whatever they are protecting like propshaft etc. This may not be possible to achieve if there is a direct connection between your engine block (which is connected to negative anyway) and the propshaft. But it is desirable. Read up on the subject first.

 

[markdj]

The standard way is to ground the dc/ac earths but put the ac through a galvanic isolator to stop any dc leakage from high earths. You can test if you have a problem in a marina by measuring the voltage between your boat earth and shore power earth. That will tell you the direction of voltage and current usually through water into your boat via anodes if you don't use a galvanic isolator. If you only go into marinas occasionally then a galvanic isolator is not really required. I think voltage via earths should be less that 1.2 volts to reduce premature corrosion of your anodes.


Mark