Dual battery set up help

[stevemid]

There should be NO electrical connections to the alloy skin of your boat. To do so is to beg for electrolysis and holes in the skin.

"Grounding" on a boat means all the negative connections tied together. In a boat with a shunt on one set of the batteries, it is the side of the shunt NOT connected to the batteries that is tied to all the other negative terminals on the boat.

If your shunt is the only thing attached to the negative terminals of the Aux battery, then there should be no way for any current to avoid being measured by the meter attached to the shunt. See discussion on shunt following

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[stevemid]

It sounds like your alternator is OK. It is putting out a measured 35 amps and it is rated at 59 (I think) so that's pretty good IMO.

If I may ask, what is the age, make, type and state of your batteries? It may be worthwhile having them tested. I have assumed they are new and in good condition and will take and hold a full charge. If they are old and haven't been kept fully topped up or if they have continuously been over discharged (more than 50% depleted) they could be cactus.

You have a 240 w solar panel but the boat sits in a shed? So the purpose of the solar is to provide additional charge when you are out and anchored up? If the boat sits in a shed for extended periods it would be worthwhile to keep a trickle charger or two on the batteries

The shunt system. The purpose of the shunt is to allow you to measure current into and out of the aux battery setup. That's how you are using it, right? This is taken from here: http://hobohome.com/news/?p=537

A shunt is often listed as an optional extra when you buy your solar regulator. Plasmatronics sell theirs in two parts – the shunt and something called the shunt adapter. In my experience very few people understand the function of this device and more importantly why it is an important part of your motorhome or caravan solar setup.



From this photo of a shunt you can see that it is a fairly simple device, no moving parts and no electronics. How can this lump of brass actually perform any useful function?

Simply put, a shunt translates “current flowing” into a voltage signal. The voltage produced by a shunt is exactly proportional to the amount of current flowing through it. For example if 10amps is flowing from your solar panels to the battery (through your shunt) – this will produce a voltage of 1 millivolt at the small terminals of the shunt.. The same shunt will produce 2 millivolts when 20 amps is flowing through the shunt. This same setup will produce negative 3 millivolts when the batteries are discharging (at night for example). This is a really useful thing when it comes to reporting what is going on with your solar power and battery system.

We could of course measure the current directly (with a ammeter)– but to do this we would need to use heavy cables (capable of carrying the load and charge current) all the way to and from the measurement device. It is far easier to run tiny wires to carry a voltage signal that is proportional to the current; then just measure this voltage.

When is a shunt required?
If you want to accurately track the state of your batteries (how charged they are), the best way to do this is to count all the energy that is removed from the battery and count all the energy that is added (by the solar panels). This involves measuring the current flowing in the wire going to the battery. Remember – a shunt cannot report energy that it does not see. This is one of the most common mistakes I see in motorhome and caravan wiring. If your system has a shunt, one side must be connected DIRECTLY to the battery terminal and there must be NO other cables attached to that battery terminal. To attach cables to the battery terminal that the shunt is connected to is to by-pass the shunt.



The PL40 solar controller can measure and track a small load directly (without a shunt) when it is attached to the load terminal. This terminal is only rated at 5 amps (20 amps on the PL20). Thus, if you need to measure the current being used by larger loads (inverters etc) you need to fit a shunt.

The state of charge (SOC) display on the PL series of regulators is meaningless unless a shunt is correctly fitted.
Shunts are rated with two sets of figures written as “200A = 200mV”. The first number is the maximum current that the shunt can handle – this is typically 100 or 200 amps. The second figure tells us how much voltage will be produced when the shunt is carrying this maximum current. So a shunt marked 200A = 200mV will carry a maximum of 200 amps and when it is doing so, it will produce 200milli-volts across the measurement terminals.
It is important to make sure that the regulator is expecting to see and understands how to read the voltage that your particular shunt produces. The shunt adapter supplied as an optional extra for the PL series of regulators has three settings that allow it to work with most common shunts.



Some tips in installing your shunt.

• Make sure you are using the correct shunt for your setup. See the information that came with the regulator.
• Make sure you do not exceed the rating of the shunt – it will get very hot very quickly.
• Be absolutely sure that ALL the current (both from charge sources AND discharge sources) passes through the shunt. It cannot measure what it can not see.
• Some regulators require the shunt to be installed in the negative battery cable while other regulators require the shunt to be installed in the positive side of the battery. The Plasmatronics PL series do not have any preference. Consult you manual – getting it wrong can damage the regulator.

[scottar]

AsSteve has mentioned above there should be no electrical connections to the hull. There will however always be a circuit between negative and the hull via the outboard and sometimes via radios and gauges depending on the mounting mechanisms.

[Noelm]

I kind of have this strange doubt in the back of my mind about the validity of not grounding devices to the hull, in reality the motor is gounded, and the negative battery cable is grounded, the radio antenna is grounded and so it goes, so, whats th difefrence in grounding everything to the hull (like a car) and running a seperate wire to the negative? or to be more confused, doing both? which in a way, you are. Just to clear the air, I am not saying what is right and wrong, and I come from a very long elecrical background, it is just something that I need to think about a bit more, and perhaps get some other thoughts while I do.

[myusernam]

I kind of have this strange doubt in the back of my mind about the validity of not grounding devices to the hull, in reality the motor is gounded, and the negative battery cable is grounded, the radio antenna is grounded and so it goes, so, whats th difefrence in grounding everything to the hull (like a car) and running a seperate wire to the negative? or to be more confused, doing both? which in a way, you are. Just to clear the air, I am not saying what is right and wrong, and I come from a very long elecrical background, it is just something that I need to think about a bit more, and perhaps get some other thoughts while I do.

yeah but no I think it's real. there will be resistances between each bonding point and you will get all sorts of small currents flowing.

[Noelm]

hhmm, maybe, but the motor is bolted to the transom, with conductive bolts, and little earthing wires, the battery earth cable is bolted to the motor, the control cables are all conductive, and so it goes, I am not 100% sure that everything is earthed like a car, and then double earthed to make us feel good by a seperate cable back to the same negative battery terminal (maybe)

[Noelm]

Just to make things "interesting" I have twin motors on a fibreglass boat, so no funny earthing problems, sepearte batteries, the usual setup, however, one set of anodes (on the port motor) get eaten away 5 times quicker than the starboard motor... why? how?

[scottar]

I would put a meter between the two motors Noel and check to see if there is any potential between them. If there is, then as soon as she hits the water there will be electron flow and anode wear. Do you have a heavy negative wire between the two start batteries?

[Noelm]

Nope, no connection between the two motors, done the meter thing too, it is a strange thing, but the difference in the anodes is very noticable, and always has been.

[scottar]

It could possibly be like the old trick of two metals into a lemon. It might be worth fitting a "bonding" wire between the two negative battery posts or between the motors themselves like they do on the larger boats between all the various through hull fittings. There are so many different opinions on this sort of stuff it can make your head spin.

[Mischief Maker]

I have link between the negative posts on aux battery and a link back to the neg start battery as well as the shunt neg which goes to bus bar and picks up redarc., solar and final sub circuits

[Mischief Maker]

My opinion of a bond on a boat is to do just that "bond" it, it's not to be used to run return current through the chassis and create the circuit, is this way of thinking correct

[scottar]

That is correct. It is effectively just creating a short circuit between the two or more items so that they are at the same potential and thus it should be impossible for current to flow between them. It prevents them from effectively setting up as a cell. Bonding wires should definately not be used as a "negative".

[stevemid]

It could possibly be like the old trick of two metals into a lemon. It might be worth fitting a "bonding" wire between the two negative battery posts or between the motors themselves like they do on the larger boats between all the various through hull fittings. There are so many different opinions on this sort of stuff it can make your head spin.

A fibreglass boat with 2 engines sticking in the water is exactly like two metals stuck in a lemon. You have a complex formula to sum the total resistance of everything attached to each motor down to the water. Thus each "metal" is bound to be different (even if they are bonded together) and there is a difference in potential. One side will therefore be the plus the other the negative. Electrons will flow (from port to stbd) and the lead will do it's job.

[stevemid]

yeah but no I think it's real. there will be resistances between each bonding point and you will get all sorts of small currents flowing.

Having conductivity is one of the great big negative compromises you make in using an alloy boat in a salt water environment. The way you manage the risks of that in the 1st place is to never intentionally use the boat to carry current and secondly, as much as possible, to isolate everything that does carry current from the alloy, and thirdly to use anodes on the motors.