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Battery isolator switch


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I expect many of you have fitted battery isolator switches to your vehicles.

A simple and effective item for safety as well as security, but my question is this -

 

Which battery terminal are you isolating? Positive or Negative?

 

There seems to be an abundance of conflicts on various forums and groups and I'm unsure which one to do?

My mind says Positive, but there seems many who chose Negative?

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I'd choose the negative (assuming it's negative earth). Then any wire can do anything it likes to touch the chassis but nothing will happen as there is no earth reference. That's why when changing a battery the negative cable should always be the first to be removed and last to be replaced.

 

Andy

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Close to the battery is important of course but also within easy reach of the driver. Fractions of a second count when your loom is starting to overheat.

 

Although either positive or negative switching will obviously kill the supply. I favour in a negative earth vehicle, switching the negative because then you can use a spanner to tighten or undo the positive connector with no fear of a meltdown if the spanner touches the bodywork.

 

In a positive switched system you would need to undo the negative terminal first, in order to put a spanner on the positive terminal safely. Not a big deal but you have always remember to undo the negative one before the other.

 

The risk is increased in a 24v system inevitably having two 12v batteries. Switching the negative off allows safe spanner access to three otherwise live terminals.

 

If you switch the positive you really should undo the negative earth terminal before spannering the other three terminals.

 

PS Agree with Andy was composing this whilst he posted.

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Dear All,

 

Most battery isolator switches for commercial vehicles are double pole so both neg and pos are disconnected so no risk even when disconnecting batteries. Some isolators can be switched off remotely, eg the one fitted to the Bedford TM.

 

Having thought about it carefully, I think with a single pole isolator it is best to fit it to the negative line.

 

Always disconnect the earth (usually negative) terminal first. That way, when you undo the pos term it cannot be live to the chassis.

 

I have had a lot of problems at Bordon with leaving the master switch "on" for a few weeks ands so ruining the low maint batteries. On the Conqueror ARV I have fitted a timed cut out so that a Contactor opens after 30 minutes or so. No more problems. Maybe not such a good idea for a road vehicle!

 

Finally, think very carefully about fitting wires for accessories such as a fridge or radio straight to battery, esp with an isolator. If isolator is "off" and you try to operate the starter, all the current could go through the radio supply cable! For that reason, if you draw straight off battery, both neg and pos cables feeding need to be fused with an appropriate rated fuse.

 

John

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Surely it makes no difference where the isolator is placed - whichever way, the battery still sits as a potential energy source, but without a circuit until the terminals are joined.

 

You're right. The only time it would make a difference is if there are multiple grounds.

 

From a single vehicle point of view, if you've disconnected either side you've broken the circuit, whether it's ground or supply.

 

The only thing you are interested in is controlling the battery, which cant be switched off. You want the switch as close to that as possible, or to make the cable between battery and switch as safe as possible if you need to route it somewhere else.

Edited by Lauren Child
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...Having thought about it carefully, I think with a single pole isolator it is best to fit it to the negative line….

 

Having thought about it carefully, I just got even more confused! :-D

 

The other thing about isolators is to get one with an adequate capacity. Any arcing or dirt build-up on the contacts will create a poor connection and could, especially with 6V , give slow starter issues. Only had problems with one - on a GPW. Reckon there were other 'lead connection' issues which became aggravated by the additional albeit small resistance of the isolator switch.

 

These are the two I use (but several makes to choose from) - the red plastic key for GMC and smaller (6, 12V), anything bigger (12, and all 24V) the metal key. Prices are just a guide - should find a little cheaper but gives an idea of quality and capacity:

 

Screen Shot 2015-02-08 at 09.23.55.jpg

Screen Shot 2015-02-08 at 09.24.43.jpg

Edited by N.O.S.
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Surely it makes no difference where the isolator is placed - whichever way, the battery still sits as a potential energy source, but without a circuit until the terminals are joined.

Strictly speaking that's correct - Lauren's charge can't go anywhere unless there is a path between both battery terminals. However from a practical point of view it makes sense to put it in the earthy side for reasons already explained.

 

Andy

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  • 3 weeks later...

This week I fitted an isolator switch to my 110 Wolf using the Land Rover / MOD modification kit.

 

This kit switches the negative side of the system and also creates an auxiliary earth post within the battery box. This earth post then takes any extra negative cables that were fitted to the negative terminal of the battery i.e. the earth from the interstart socket.

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Hi Charlie

 

I got it from Blanchard's via eBay. It's just the isolator kit and doesn't include the Auxiliary Earth Terminal Kit referred to in the Mod Inst. The latter is just a 60cm long 25mm² earth lead with a 10mm terminal one end and an 8mm terminal the other so easy to make. A few pictures attached.

 

Nick

 

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Nick,

 

is the slave socket set up part of the kit and if not what is its part number and where do you get them from?

 

I have not seen that slave socket for sale anywhere, it is what I am looking for on a different project

 

any leads would be great, thanks in advance

 

Robin

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Hi Robin,

 

not sure what vehicle you are doing this on, but have a look at an old 5/4 ton chevy, on the drivers side of the engine bay, you will see a batt. master there. If you are up here next week, Ed's salvage yarrd on Dwyer Hill rd. has a couple of these that are being parted out.

 

cheers

 

Nick

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Thanks for that Chris, is that really a fair market price?

 

Robin

 

Hi Robin

The others beat me to the reply! As for price, retail from Land Rover is £246 +vat so the one from Thexmod is a reasonable price, but there are at least 3 on eBay including the one David has linked. The other two are new and listed at £74 each.

 

http://www.ebay.co.uk/itm/L-K-LAND-ROVER-POWER-SOCKET-BRAND-NEW-GENUINE-PART-RRC8692-/400483854816

 

http://www.ebay.co.uk/itm/military-landrover-wolf-vehicle-interstart-plug-RRC8692-/360539701364

 

Nick

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