VOLTAGE SENSITIVE RELAY

[eidiohir]

I have the good fortune to own a Colvic Watson 34.But the bad luck to have to rewire the charging end of things.I need to buy an alternator as the present one has been washed in sea water after a cooling hose burst and I want to revamp the charging end of things at the same time.
My plan was to fit a new alternator with a split diode and a smart charger such as a Sterling so that I can have a deadicated engine start and seperate domastic.After all that my question for your combined brain power is- I have been told by a friend that a better system would be to fit a high power alternator with a voltage sensitive relay doing away with the diodes and the battery managment syatem.Is this true?and is it better?

 

[aidancoughlan]

The VSR can replace the standard 1-2-both switch. I had one fitted to my boat to replace the old system, and it is fantastic - I just switch them both on when I get on the boat, and never have to worry about charging etc. It keeps the batteries isolated (there is an emergency parallell switch) and it automatically charges the Start battery, and cuts over to the house battery when the start battery is charged enough. Check the www.BEPmarine.com website for details. Available from Merlin and/or ECSmith.

I dont understand how the VSR could do away with the "Battery Management System", if by that you mean the Steling or Adverc alternator manager though...

There was a thread a while back about a situation where VSR's are not appropriate (At least if not sized correctly)... might be worth searching to make sure it doesnt apply to you. As far as I recall, it was to do with a scenario where a heavy-drain such as a windlass was hooked to the start battery, and the battery is flat - it could draw a lot of current across the VSR from the other battery bank, melting the VSR & potentially causing a fire.

 

[eidiohir]

Thanks for that,I looked up BEP and now I'm more confused than before.
as to the VSR doing away with the advrerc, I think its the fitting of a high power marine alternator that does away with the need for it.I am new to the area of electrics and a little knowage is a dangerous thing so its a case of doing the research .I did notice that they do a Smart alternator regulator that I haven't seen before

 

[CSFenwick]

You should definitely get a multi-stage alternator controller no matter which splitting device you opt for. The high powered alternator may come with a smart controller already attached (I think Balmar do) which may be where the confusion is coming from? If not then do make sure you fit one of the external ones (Adverc, Merlin AMS, Sterling etc).

Out of interest, by high powered alternator exactly how powerful do you mean? If you only have one domestic battery plus your engine start I wouldn't have though a high powered alternator (usually considered to be in the 90A-100A and above range) would be anything other than overkill?

 

[William_H]

An alternator with attached regulator may be rated at 100 amps but will not charge your battery at any higher current (quicker) than the standard 40 amp alternaor. A battery is charged at a rate dependant on the voltage presented to the terminals. (the regulated voltage) the batteries inherent voltage ie the charge state and the size of the battery. The alternator rating is the current it can supply if required but does not usually (except in the case of completely discharged batteries) help to get a bigger current into the batteries.
Now unfortunately the inherent voltage of lead acid batteries tends to rise with charging in a non linear manner such that at low charge the voltage is relatively low but from half charge on the voltage is quite high. This means that to fully charge batteries with a 13.75 volt regulated alternator it quickly puts in the first half of charge but takes progressively longer to put in the last half.
Alternator regulator systems are primarily designed for vehicles where there is little discharge normally and long periods of charge. 13.75 volts is chosen as a voltage which will not overcharge a battery no matter how long it is on charge but will eventually reach full charge and is also suitable for the other accessories on the vehicle. (lights etc.)
This voltage is the most commonly used arrangement on boats as well. It is ideal for power boats. However sail boat owners want to be able to restore a discharged battery quickly for a short engine run period. This is where a smart charger comes in. It increases the voltage of the regulaor by perhaps another volt which causes the battery to take the higher current charge. This higher current again depends on battery charge state and battery size. The higher current can approach that of the rating of the alternator. Hence some people advocate fitting (or find it necessary) a larger alternaor. Alternators hopefully simply cannot provide the higher voltage at the higher current the voltage falls and then the current falls so should be self regulating to their current limit. However vehicle alternaors seldom approach the limit and so construction and cooling are neglected in design so alternaors in practice can overheat and fail at higher currents sometimes less than the rating.
A smart regulator provides a higher voltage to the batteries hence higher current but automatically switches to lower voltage (current) float charge when they are charged. This gives quicker charge and more complete charge with shorter engine run times.
A second advantage of the smart regulator is that they usually provide for remote battery voltage sensing so can be used with diode type isolators. The inherent volltage drop of the diodes can be allowed for in the regulaor. Wheras with a standard regulaor the loss in the diodes can not be allowed for so they are not suitable. A VSR on the other hand has no voltage drop so can be used with standard regulators. (or smart charger)

I would suggest that you fit firstly a VSR then if you think you need to a smart charger. (usually this is because you have a 12v fridge) Lastly depending on actual charge current with the smart regulaor you may find benefit in fitting a larger alternator.
If you mostly have shore battery charger then you probably seldom need to charge the batteries and for those rare occasions when you need to recharge away from shore power a longer engine run time would be OK. it depends on what you want to do. You maynot need a smart charger. olewill

 

[Ben_e_Toe]

Have recently fitted a VSR to my boat, works very well andis simple to install and use!

 

[andy_wilson]

I trust you are not shy to start the engine in your motor sailor, in which case first question how much you need to spend on all this. If you do not have a dedicated engine start and associated switching, then I agree that this is worthwhile to separate the domestic load.

As you need to buy an alternator anyway, it's size is a function of Amp Hours required and running time available to put them in. As you make no mention of having capacity or charge problems previously, the obvious recommedation is to replace the alternator like for like (though when mine rusted up for similar reasons I went from 60Amps to 80Amps for the extra tenner).

A 1,2,Both,Off switch is totally out of fashion and perfectly servicable to manage split banks, though some human intervention is required. Slightly more fashionable would be separate engine and domestic isolators, with a bridging switch. Switch 1 feeds the engine circuits, switch 2 feeds the domestic switch panel, switch 3 is used when under power to share charge with the domestic bank, or to bridge the batteries if the starter battery is flat and needs assistance form the domestic bank. Human intervention still required, particularly to switch off the bridge to prevent flattening all the lot at anchor.

The next development of this would be to add a VSR. I particularly like the look of the Merlin stuff, and come the day this is what I will buy...

http://www.merlinequipment.com/prod_range.cfm?cat_id=85

Note you can buy the switches and then the VSR separately later if you wish, or buy a cluster. With this you switch on the engine and battery switch and forget about it all. When you start the engine, for the first few minutes the engine battery gets all the charge. Once it's voltage hits the pre-determined level the VSR shares the charge with the domestic side. When you switch off the engine the battery banks become isolated again, protecting the engine battery from discharge by the domestic system. Good practice suggests that this arrangement still requires a way of bridging the battery banks in case of emergency starting.

Blocking diodes present a slight voltage drop and are only suitable when used in conjunction with battery sensing alternator controllers. VSR's do not suffer this and are therefore suitable with or without a controller.

Final improvement would be to add an alternator charge controller, which will optimise the output of the alternator. Normally alternator output tails off dramatically shortly after starting as battery voltage (charge) rises. The law of diminishing returns means that it would take an infinite time to reach 100% charge. With a controller the charge rate is kept high - just below gassing point - for longer to reduce charging time, and ensure a more complete charge.

For my money - which is not inexhaustible - in a motor sailor, I would first replace the alternator with something similar, then separate the battery banks with appropriate switching arrangements.

Then I would spend money to fit a solar panel to maintain batteries when the boat is not used. If you are in a marina with shorepower this may not be an issue for you.

If cash is still a plenty I would then fit an alternator charge controller (mine is more of a sailor so my engine is not on much). Finally I would fit a VSR so I wouldn't have to think about the process.

In your case the latter two priorities could be reversed perhaps, according to taste.

Final thought. without all the fancy stuff, batteries are far easier to charge if you have more of them. My first step was to double my domestic bank. This way it is far less likely to hit the critical 50% discharge point when batteries start to get damaged, and secondly, they accept more charge more quickly. definitey a low tech. alternative strategy.

 

[roger]

VS relays have been frowned on in the past as being mechanical they might be less reliable than splitting diodes which have no moving pparts.
With a smart alternator controller you would normally have a battery sensing connection so that the voltage loss through splitting diodes is unimportant. ( the control circuit compensates)
With a tree way diode splitter you could have three entirely separate battery banks - including one just for the fridge as I do.
The 1,2, both off rotary switch is a relic of the past because it gives you a very easy way, in a second of inattention, sickness, or inexperienced crew, to totally destroy your alternator. Of course you may be always in full control and fresh as a daisy. I'm not. I use separate switches for each battery and only switch off when I leave the boat. I think thats much safer.
The usual recommendation is that the alternator rating in amps should be at one third of the total battery capacity in ampere hours.

 

[eidiohir]

good info,I should maybe give a little more about "Jerod".She is a motorsailor fitted with a 90hp Thornrycroft which I don't normaly stop below force 3 to 4(anything below force 3 just doesn't do anything) ,so the running time is not a ploblem.The battery bank consists of three large 120amp truck type batteries connected together.There is a small solar pannel which just about keeps the batteries from losing charge on the mooring.Jerod has a electric windless which I use quite a bit,I think that it should be connected to the starter as it is only used with the engine running?I want to keep one battery for the engine and two for domestic.The alternator I was recomended indeed has some kind of smart regulator fitted (balmar) but going from what has been said the ideal solution for me is to fit a replacement alternator (much cheaper) with a VSR, a battery management unit maybe the Adverc and a new type battery switch,at the monment I have a simple on or off switch.The VSR I was looking at has a switch which allows you to combine all batteries for starting if needed and seperates the charging to allow the engine battery to be charged first.
I wouldn't say that money is no object but I am prepared to spend a bit to get the best.

 

[roger]

Sizing the alternator

With 360 amp hrs of battery the usual recommendation would be 120 Amp alternator which in turn will probably require a new pulley system as the load would be very high for an ordinary V belt. This will mean fresh pulleys for crankshaft and water pump too and possibly space problems.

You have various different types of battery requirement :-
short duration very high amperage for engine start
medium duration high amperage (I'm guessing here ) for windlass
long duration low loads for ordinary services.
In addition your windlass will usually require very large and long cables from the urdinary batteries. A clever way round this is to put a separate battery up near the sharp end with a separate charging lead for it.
I have chosen to fit a specialist high output short duration battery purely for engine start. It has very small physical size and claimed max op current of 1100 amps for 5 secs.
Fittting that would save you space if nothing else.

 

[pvb]

But the real answer is...

You can safely ignore all the "usual recommendations" on alternator size. With a 360Ah battery bank, you'll be unlikely ever to see much more than 80A charge, so a 120A alternator is just a waste of money (and may well be difficult to fit). Best answer is a good quality 90A or 100A alternator controlled by a "smart" regulator such as Adverc. I have a 660Ah domestic bank, with a 90A alternator and Adverc regulator - works fine.

Also ignore the idea of a separate battery in the bows for a windlass. It's not a clever solution - it introduces lots of potential problems. It's actually cheaper to run big cables back to the starter or domestric batteries.

 

[eidiohir]

Re: But the real answer is...

I do already have two large welding cable type wires going from the batteries to the windless and in a Colvic watson 34 space is not a problem for batteries.The old alternator was rated at 60amps and I can get a direct replacement one rated at 80.I think that my mind is made up at the moment,I intend to fit a normal 80 amp alternator, VSR,with a Adverc control unit.the only thing that I haven't nailed down yet is the battery switches I will do a bit of research to find the best arrrangement