Battery isolator wiring

[JimC]

I've been investigating the wiring of my new (to me) boat. The engine and domestic batteries each have their own isolator switch, but these only control the load circuits i.e. the starter motor and domestic loads respectively. The alternator output is permanently connected to the batteries via a diode splitter. Therefore the batteries are not truly isolated when the switches are open because their charging circuits are still connected.

On the face of it I can't see anything wrong with this arrangement: the alternator diodes should prevent any current leakage from the batteries when the boat is not in use and the diodes can't be blown due to having an open circuit on the alternator output.

On the other hand the wiring diagram for the Yanmar 2GM20F engine shows the alternator output joined to starter motor and then going to the battery via an isolator switch, which thus controls both the load and charging circuits. My installation obviously has separate wires from starter and alternator.

My system seems to work well and I'm reluctant to disturb it unless a more knowledgeable forumite can point out its shortcomings.

 

[halcyon]

[*]]My system seems to work well and I'm reluctant to disturb it unless a more knowledgeable forumite can point out its shortcomings.

Before deciding if you need to / or want to change anything, you need to check the voltage at the alternator and both batteries, with the engine running with a few revs on.

Diodes drop voltage limiting battery charging, it may be compensated for, it may not, once you have this knowledge a sensable decision can be reached.

Brian

 

[JimC]

Thanks, I've done this: the voltage is just under 14 volts which is about what one would expect allowing for the forward voltage drop in the splitter diodes. My query was more about whether my incompletely isolated batteries can be considered safe. When one switches off the isolator(s) prior to leaving the boat one likes to think that the batteries are truly isolated - in my case they're not, and I suppose the possibility of electrical fires is what's in the back of my mind.

 

[VicS]

The system as you describe it is fine except for the volts drop across the diode splitter that Halcyon mentions.

The diodes in the splitter effectively isolate the batteries from the alternator in as much that current cannot flow backwards through them to the alternator. More effectively isolated than in some of the schemes promoted on here recently.

The volts drop across the diodes may be the problem... as Halcyon indicates.

If the system has been properly designed the alternator may be "battery sensed". In other words instead of controlling its output as measured at its output terminal it controls its output to deliver the correct voltage at the battery, although, as always, there is more than one way of skinning a cat.

You will find simple descriptions of diode splitting and battery sensing in the notes from TB TRAINING

 

[JimC]

Thanks Vic, I intend to replace the diode splitter with a Victron split charge relay to get round the volts-drop problem. When this is done the only thing stopping the engine battery from discharging through the alternator will be the alternator diodes themselves but I should have thought this would still be OK? after all isn't this the same as on cars and their batteries don't go flat when left ?

 

[VicS]

I intend to replace the diode splitter with a Victron split charge relay

I have looked at the website but not sure what item you intend using

 

[JimC]

I have looked at the website but not sure what item you intend using

This one: http://shop.solar-wind.co.uk/acatalog/Victron_Cyrix_Battery_Splitter_100_brochure.pdf

 

[VicS]

Ah the Cyrix battery combiner an electronically controlled relay. A VSR in effect and a 2 way one at that

Much favoured by Sailorman of these forums

Snag there, if it is snag, is as you notice there is no isolation between the battery and the engine. You could separate the alternator output from the feed to the starter and isolate the stater and all the rest of the engine electrics as you do at present, just leaving the alternator connected.

There is an argument for making the house battery the priority one but that would not be for me,

 

[halcyon]

Thanks, I've done this: the voltage is just under 14 volts which is about what one would expect allowing for the forward voltage drop in the splitter diodes. My query was more about whether my incompletely isolated batteries can be considered safe. When one switches off the isolator(s) prior to leaving the boat one likes to think that the batteries are truly isolated - in my case they're not, and I suppose the possibility of electrical fires is what's in the back of my mind.

If the alternator output is 14 volt, and you get 14 volt at the battery, then you can just move the two diode splitter outputs to the switched side of the isolators. Job done, batteries fully isolated.

If you have a volt drop, then a VSR will solve it. Alternator output to starter motor feed from battery,use existing large cable to charge battery. Connect VSR between the output terminals on the isolators, VSR negative sense wire to battery neg, this should have fuse / circuit breaker to protect VSR if not integral.


Brian

 

[PeterR]

Most new yachts with the smaller Volvo engines also have this wiring system. Volvo supply a high output alternator (115 amps) with a battery sensing connection to get round the charging problem caused by diode voltage drop. I havent heard of anyone having problems caused by the isolation system.

 

[William_H]

Battery isolation

i think I would be inclined to take the output of the two diodes of the diode isolator to the load side of the 2 isolator switches. This would isolate totally the batteries when turned off. The chances of a short to negative and a fire are pretty remote but it will be more in line with most standards.
If the isolation diodes are working ok and batteries are getting enough voltage then leave it as is. Obviously either the alternator is battery voltage sensed or the output has been elevated to suit the diode volt drop.
good luck with the new boat olewill

 

[boatmike]

I don't disagree with other posts but IMHO they miss the point of your question. It is quite permissible to wire the charging circuit direct to the batteries but it does mean that there is a small chance of current leakage which should be at least minimised by having a diode splitter or relay in place. Its also safer if there is a short in the charging circuit to wire on the other side of an isolating switch. The advantage of wiring direct is that if you switch off your batteries, (or in fact there is a short circuit or other fault that does it for you) while the engine is running you won't blow your alternator controller. There is no right or wrong way. Depends on what you feel is most important.

 

[JimC]

Thanks everyone for dealing so comprehensively with my query. What I've decided is that I'll leave the system as it is apart from substituting a vcr for the splitter diodes, so the engine battery will still be permanently connected to the alternator (the domestic battery will be isolated by the vcr when the engine isn't running). I think that if somehow a short did develop in the charging circuit it would simply blow the alternator diodes. The only fire danger would be if the wire from the engine battery to the alternator somehow shorted to earth, but this would be just as likely to happen to the wire from the engine battery to the isolator switch in the alternative setup.

I suppose the only way to have bomb-proof battery isolation is to have one of those isolators that's part of the battery terminal, or to take the terminal off the post when you leave the boat. I guess I'm getting a bit paranoid in my old age.

 

[JimC]

sorry meant vsr not vcr !

 

[BabaYaga]

fuse

If the risk of a short/fire is your main concern, I'd suggest that you install a fuse in the positive wire, as close to the battery as possible.
I have a set up similar to yours, in that one of the batteries (or rather the house battery bank) is permanently wired to the alternator, without any isolator switch.
I was advised to install a 80 A fuse. Hopefully it will blow if for instance the wire came off the alternator terminal and made contact with the engine block.
(I have a fuse also in the engine battery wire, this is 200 or 250 A, as it must be able to take the full cranking current).

 

[halcyon]

If the risk of a short/fire is your main concern,

Also the VSR is perminently live, so needs protection, as well as a perminent drain on the batteries.

Brian

 

[cimo]

If the risk of a short/fire is your main concern, I'd suggest that you install a fuse in the positive wire, as close to the battery as possible.

straight answer to a straight question!
A big 2nd from me.
Install fuses/breakers to prevent electrical fires.
rgds
c