Isolator switch for engine battery

[JimC]

Is this necessary? my boat came fitted with one but surely the alternator diodes effectively isolate the engine battery, preventing any outward current flow. Doing away with the isolator would obviate the danger of damaging the alternator through accidental opening of the switch and would permit a shorter and more direct engine/battery connection - maximising cranking amps. This is how road vehicles are wired after all.

 

[Jaramaz]

Necessary?

Is this necessary? my boat came fitted with one but surely the alternator diodes effectively isolate the engine battery, preventing any outward current flow. Doing away with the isolator would obviate the danger of damaging the alternator through accidental opening of the switch and would permit a shorter and more direct engine/battery connection - maximising cranking amps. This is how road vehicles are wired after all.

Firstly, a boat is not a road vehicle. It is very good to be able to switch off a battery if anything happens, like a small fire or something. Boats for leasure are not used in the same way as cars, and do not work under the same conditions.

Secondly, it use to be a requirement from the insurance company.

It could be noted that some insurance companies require a similar switch on older cars.

/J

 

[maby]

On modern Jeanneaus, the engine battery isolator switch is an essential security feature - there is no ignition key so if you do not isolate the engine battery anyone could motor the boat away without keys.

 

[William_H]

Engine battery isolator switch

Yes I agree with OP although all the other posters are correct. No we don't see problems with cars not having an isolation switch for the starter. However all small aircraft have a similar electrical system to car or boat but invariably have an isolation switch in the form of a relay which carries the starter current. Why? I dunno. except they have to meet a design standard which may demand it. On the other hand outboard powered power boats where virtually all the electrical system is in the engine do not have isolation for engine battery. So no answer at all olewill

 

[cimo]

I was asked to fit one to an old JCB once - parked behind his house, its owner noticed the hazard lights flashing one evening - turns out, the wireloom decided to have a meltdown, (perhaps it was tired).

I've also been asked to fit isolators as anti-theft measures.

A single battery?, enclosed next to the inboard?, no electrics other than a basic engine instrument panel and all well maintained?, I honestly can't see a good enough argument that would insist on one either. By the same logic, if there was one, I couldn't make the argument to remove it. So, its looking like a non answer from me too. .

On a serious note though - If ever concerned about a fire risk, it would be prudent to disconnect the battery lead when unattended. These switches are just a convenient way of doing that, that's all. The removable key versions provide an additional security feature.

 

[little_roundtop]

On modern Jeanneaus, the engine battery isolator switch is an essential security feature - there is no ignition key so if you do not isolate the engine battery anyone could motor the boat away without keys.

That's not what the isolator switch is for, it's there to disconnect the batteries in case of an electrically caused fire.

Not only do I not isolate my batteries when leaving the boat (and I have an older Jonno), I leave the (standard Yanmar) key in place. The reason is that over the years we've saved three badly dragging yachts (one which was nearly on the rocks) only because we were able to start their engines and motor them back to a safe anchorage.

BTW. My insurance company know this and have agreed that as along as I lock the boat up it's ok with them.

 

[prv]

I don't have an engine isolator. The battery sits in a compartment next to the engine bay, a couple of feet of wire run through the bulkhead between battery and engine. Can't see much to go wrong with it.

Pete

 

[halcyon]

Is this necessary? my boat came fitted with one but surely the alternator diodes effectively isolate the engine battery, preventing any outward current flow. Doing away with the isolator would obviate the danger of damaging the alternator through accidental opening of the switch and would permit a shorter and more direct engine/battery connection - maximising cranking amps. This is how road vehicles are wired after all.

Don't you have a ignition key with your car ? plus alarm, plus engine imobiliser, and if it is a very new one explosive main power fuse.

The key switch is to isolate engine when you leave boat, safety and anti theft, isolates power to engine in case of engine fire, also the alternator regulator has a permanent supply thus permanent drain on battery.

Just think of it as a kill switch.

Brian

 

[RupertW]

That's not what the isolator switch is for, it's there to disconnect the batteries in case of an electrically caused fire.

Not only do I not isolate my batteries when leaving the boat (and I have an older Jonno), I leave the (standard Yanmar) key in place. The reason is that over the years we've saved three badly dragging yachts (one which was nearly on the rocks) only because we were able to start their engines and motor them back to a safe anchorage.

BTW. My insurance company know this and have agreed that as along as I lock the boat up it's ok with them.

Hmmm...not the only reason. I isolate the engine battery whenever the engine is off because the thing is wired so that the oil pressure alarm go off if I don't, but the main use for isolators for engine and domestic is to stop the batteries getting flat when you leave the boat for extended periods, due to some small voltage leak, or a switch left on or damp causing a short or whatever. Fire is there as a risk but a long way down the list.

Much easier (if lazier) to isolate everything than switch off the various light circuits, water pressure circuits, Dec plugs, stereo etc etc.

 

[boatmike]

Is this necessary? my boat came fitted with one but surely the alternator diodes effectively isolate the engine battery, preventing any outward current flow. Doing away with the isolator would obviate the danger of damaging the alternator through accidental opening of the switch and would permit a shorter and more direct engine/battery connection - maximising cranking amps. This is how road vehicles are wired after all.

Having read other posts I think one point has been missed here. The batteries are not just connected to the alternator but to services generally. Even with a single battery and no services you have a live wite to the control panel and instruments. Switching off isolates everything and ensures that there are no current leakages or fire risk anywhere. This protects both the battery and all wiring when not in use.

 

[prv]

Having read other posts I think one point has been missed here. The batteries are not just connected to the alternator but to services generally. Even with a single battery and no services you have a live wite to the control panel and instruments.

You're right about the loom to the engine control panel (which in my case is mounted essentially on the outside of the engine compartment, so it's not exactly a long vulnerable run). But no other services - we're talking about the engine battery, and on my boat that only supplies the engine.

Pete

 

[cimo]

yes. the thread title is

"Isolator switch for engine battery".

 

[fastjedi]

On modern Jeanneaus, the engine battery isolator switch is an essential security feature - there is no ignition key so if you do not isolate the engine battery anyone could motor the boat away without keys.

I fitted one after a new D1-20 was fitted because

1. No ignition switch ....Added security required!

2. No ignition switch ....The Volvo Dealer had wired the fuel solinoid and fuel gauge (was on the switched side of the ignition switch) to 'permanent' 12v. It was drawing 0.6A as soon as the 12v was switched on for domestics.

There is Volvo optional extra consisting of a small relay, cable and connector to overcome (2) but it was beyond my budget!!!

 

[JimC]

Thanks to all posters for opinions expressed. I was prompted to start the thread by a conversation with a marine electrical specialist who told me how he was often called upon to help boaters with flat starter batteries who had blown their alternator diodes through inadvertently open-circuiting the alternator output. Apparently some makes of 1/2/both switch are frequent culprits.

Having studied all the posts I think my own opinion is tending this way: the high current wire from the battery +ve terminal to the engine should go there by the shortest and most direct route with no intermediate connections or detours, the best place for an isolator being directly on the battery terminal, conventionally the negative one.

My reasons:
Fire safety - the biggest danger is a short on the high current wire from the battery which is usually un-fused, so keep this as short & direct as possible. The distance to a remote isolator switch will often be greater than that to the engine and may pass through more structure. Also arcing at connections is less likely with only 2 connections.
Cranking power - with a failing battery a least-resistance path to the starter motor may just make the difference and save the day.
Security - the 1/2/both switch offers no security. The various key switches with removable red keys are so commonplace that any self-respecting boat thief would surely equip himself with an appropriate key. In any case shorting out a switch wouldn't be difficult. A terminal isolator with its removable wheel would be harder to circumvent.

Here's an example of the type of isolator I'm favouring for my boat http://zoomerroo.co.uk/index.php?target=products&product_id=82

 

[cimo]

pretty sound approach imo Jim.

why is it so awful hard to win in this life though......

.............. any self-respecting boat thief would surely equip himself with an appropriate key.............

while the ZoomerRoo blurb says...........

"For added security the wheel can be completely removed. Spare wheels are available in our shop"