Battery isolator

[zoidberg]

I'm considering fitting one of these isolators on the Negative terminal of my start battery so I have a convenient means of taking it out of circuit when I'm likely to be away for some time. There's insufficient space for a physically larger unit, such as the BEP isolating switches I have on t'other batteries elsewhere.

There's an emphatic suggestion elsewhere that it should rather be fitted onto the Positive side, but no reason offered. This YouTube video - one of several - prefers the Negative.

Perhaps some kindly knowledgeable soul would clarify pros and contras....?

 

[sarabande]

If you put the isolator on the positive terminal (different size terminal BTW) it prevents the amp things going from the positive terminal round the circuit towards the negative. On the neg terminal, current could flow from the pos round a circuit into a device which had a short or was using current and connnected to a neg busbar.

An isolator on the pos is "upstream".


Or you could put a standard super cheap red 'switch and remove' isolator where you have space, and run some battery cable from the pos to the red switch, and onwards.

You can also buy isolators with fuses on them.

 

[Baggywrinkle]

Putting the isolator on the negative lead is a convention from vehicles where they are "negative earthed" ... that is, the body of the vehicle itself is connected to the negative terminal of the battery.

This means that any failure of insulation on positive wiring, or contact with metal tools to the body when working on positive wiring can easily cause a short - The isolator on negative ensures the body is isolated first to allow safe working on the vehicle positive circuit.

Obviously a plastic boat can't be "earthed" in the same way, but often negative bus-bars or connections are left un-insulated - as is the engine - so the same convention is used.

As long as there is nothing by-passing the isolator (e.g. connected directly to the battery clamp) then IMO, on a boat it doesn't really matter if the isolator is on positive or negative. Look at your battery installation and if it is at all possible to contact an opposite polarity connector or bus-bar with tools then I'd put the isolator on the side that isolates the potential hazard - like the car body example.

I'd be very interested to know any other arguments for +ve or -ve isolators.

 

[LadyInBed]

If you put the isolator on the positive terminal (different size terminal BTW) it prevents the amp things going from the positive terminal round the circuit towards the negative.

[PEDANT] 'amp things' (electrons) flow from negative to positive around an electrical circuit [/PEDANT]

 

[pvb]

If you put the isolator on the positive terminal (different size terminal BTW) it prevents the amp things going from the positive terminal round the circuit towards the negative. On the neg terminal, current could flow from the pos round a circuit into a device which had a short or was using current and connnected to a neg busbar.

An isolator on the pos is "upstream".

If the negative is isolated at the battery terminal, the negative busbar you mention isn't connected to the battery, and no current can flow.

 

[sarabande]

There is some debate on the direction of flow. I understand that the modern view is from pos to neg.

https://www.google.com/search?q=cur...rome..69i57.5775j0j8&sourceid=chrome&ie=UTF-8


In Zoidberg's case, with multiple batteries the use of a single isolator at the terminal of one could leave parts of a circuit still 'live' if permanent connections (e.g. VHF, bilge pump) are made at the terminals of other batteries. Which is why a picture is worth a thousand words.

 

[zoidberg]

I say again, for the hard of comprehension,

the Negative terminal of my start battery

The other end of the Negative lead is connected directly to the'N -' point on the engine....as designated by Beta Marine.
The other end of the Positive lead is connected directly to the 'P +' point on the engine....as designated by Beta Marine.

VHF, GPS, induction furnace, crew's hairdryer, BOSE soundbar, Fracino Classico Espresso Coffee Machine are supplied with 'lecky on other circuits.

 

[VicS]

I'm considering fitting one of these isolators on the Negative terminal of my start battery

As far as I know this type of isolator is only available to fit the negative battery post............ someone will now post a link to one for the positive (I can hear the keyboards being pounded to find one just to prove me wrong.)

They are also available with a fuse, which bypasses the isolator, and therefore allows low power devices to operate even with the green knob removed but which will blow if a high power device , eg engine starter, is operated

 

[jiris]

As long as the size fits, a negative insulator will work equally well on a positive terminal ;-). Possibility of mixing up the polarity because of color - a few drops of red paint will prevent this.

 

[VicS]

As long as the size fits, a negative insulator will work equally well on a positive terminal ;-). Possibility of mixing up the polarity because of color - a few drops of red paint will prevent this.

Positive and negative posts are different sizes. The positive one is a larger diameter.

 

[Boo2]

Does anyone know what the macimum current carrying capacity of these isolators is ?

Boo2

 

[VicS]

Does anyone know what the macimum current carrying capacity of these isolators is ?

Boo2

They seem to vary but often no current rating is given. If you are thinking of buying one check the spec of what you are looking at.

Ive seen continuous ratings from 100 to 200 amps and intermittent ratings up to 1000 amps.

 

[superheat6k]

I fitted one to a third battery as it was a different size than the battery in the main bank. However the contact area was tiny, and there was no 'feel' to it as to whether it was making contact or not. If you want to include a negative isolator then try to find space for a decent brand of isolator rated to at least 1,000 amps.

I won't have the cheap red key type on my boat again, and wouldn't bother with these things again either.

 

[sarabande]

T

You now have me worried

What's up with the cheap red key ones please ? I know there are many suppliers and hence probably many differing load specs, but they do seem to be pretty much fail-safe (unless one drops the key in the bilges.....)

Those fiendishly clever rally car people seem to use a similar 'key' type but specced to 100A continuous and 500A for 5 secs.. I had to look up the current used by the starter for the engine (Yanmar 3cyl 10hp) and it says 1400 W ! Please tell me that must be the initial surge to get the flywheel turning, after which the energy required is less...

That example has a resistor which claims to protect the alternator. Does that overcome the dread I have of an inadvertent switch off causing the alternator to eat its diodes with back EMF , and much £££ for a alternator repair ?

 

[VicS]

T

You now have me worried

What's up with the cheap red key ones please ? I know there are many suppliers and hence probably many differing load specs, but they do seem to be pretty much fail-safe (unless one drops the key in the bilges.....)

Those fiendishly clever rally car people seem to use a similar 'key' type but specced to 100A continuous and 500A for 5 secs.. I had to look up the current used by the starter for the engine (Yanmar 3cyl 10hp) and it says 1400 W ! Please tell me that must be the initial surge to get the flywheel turning, after which the energy required is less...

That example has a resistor which claims to protect the alternator. Does that overcome the dread I have of an inadvertent switch off causing the alternator to eat its diodes with back EMF , and much £££ for a alternator repair ?

The power rating of a starter motor is usually the maximum mechanical output power.. The full spec should include the current when not loaded, the normal cranking current and the current when stalled , but they are details you seldom see.

For an 800 watt ( 1.1 hp ) Bosch motor on a 2.1 litre petrol engine I have the following

30 -50 A (at 11.5 volts) and 5800 - 7800 rpm, unloaded
185 - 220 A (at 9 volts) and 1060 - 1350 rpm, loaded
400 - 490 A ( at 7 volts) , stalled

As you can see the electrical power consumption when cranking is considerably more than the 800 watt power output figure.

 

[sarabande]

Thanks for finding those figures, Vic.

I expect that my engine, being diesel, requires more cranking power from the starter than a petrol one.

Not sure about interpreting the reduction in voltage, though. Is that always what happens during a starter motor cycle ? The load at 7V is enormous , but presumably momentary as the starter engages and before the engine starts turning ?

 

[VicS]

I'm considering fitting one of these isolators on the Negative terminal of my start battery so I have a convenient means of taking it out of circuit when I'm likely to be away for some time. There's insufficient space for a physically larger unit, such as the BEP isolating switches I have on t'other batteries elsewhere.
Perhaps some kindly knowledgeable soul would clarify pros and contras....?

As an alternative consider these "Quick release" terminal clamps, available as a pair , or individually for positive and negative posts

....... https://www.durite.co.uk/s/c/wiring...d-accessories/quick-release-battery-terminals

 

[pvb]

As an alternative consider these "Quick release" terminal clamps, available as a pair , or individually for positive and negative posts

....... https://www.durite.co.uk/s/c/wiring...d-accessories/quick-release-battery-terminals

I had some of those on my old garden tractor, they're poor quality and have a simple screwed clamp to connect the battery cable. OK for a tiny load, but I wouldn't dream of using them on a boat.

 

[zoidberg]

I had some of those on my old garden tractor, they're poor quality and have a simple screwed clamp to connect the battery cable. OK for a tiny load, but I wouldn't dream of using them on a boat.

Yes. I bought some, encased in tough packaging. And I, too, decided they weren't up to the job on my wee boat. I also have half a dozen 'Red Key' isolators. Various ratings are given on suppliers' sites, by not AFAIK on the products themselves. I decided on BEP isolators from 12Volt Planet.... both reliable outfits, providing reliable product and guidance.... following that from PR of this parish..... and cube fuses, for the 2 groups of 'services' batteries to P and S. These are positioned to be readily accessible.

The Start battery is unfused. Access is more difficult and I don't have the space for the BEP isolator product.

 

[VicS]

I had some of those on my old garden tractor, they're poor quality and have a simple screwed clamp to connect the battery cable. OK for a tiny load, but I wouldn't dream of using them on a boat.

Some do look pretty flimsy but the Durite ones are rated at 1000amps, but made in Italy :ambivalence: