18 replies to this topic

[Yoda74]

A short question, does a battery cut-off switch have to be on the positive or negative side of the circuit? And why?

 

 

[IronmanG]

On a negative earth car it goes on the positive

[PoolGuy]

It's a bit of a contentious issue. But putting it on the gnd/neg cable is often easier for a road car, reason being there'll often be extra wires running from the pos battery terminal. Also it's good practice to remove the gnd/neg terminal first when disconnecting a battery.

 

However, if it's going on a race car and intended to stop the engine, you'll need an FIA style switch that cuts the alternator wiring too, these go in the pos cable.

 

IMO it makes sense to put it in the easiest place unless you have to comply with some regulations, motorsport or otherwise.

[nicklouse]

It depends on why you are fitting it.

 

and then which one you are fitting. 

[Yoda74]

I already fitted one some time ago for two reasons.

One is to keep the battery from discharging when parked in my garage for a longer time and no short circuit will occur when not around.

Reason two is for security so I will be able to cut electric circuit in case of electrical problems or fire.

I always thought I could shut off the engine when turning the breaker, but last week I tried it and my engine kept running….

As you probably already guessed, it’s on the negative side.

I always use it when parking the car for longer than 30 minutes.

I put it in my dash so I can reach it easily.

Its one with the red detachable key.

Edited by Yoda74, 19 May 2022 - 04:49 PM.

[nicklouse]

I already fitted one a few years ago for two reasons.

One is to keep the battery from discharging when parked in my garage for a longer time and no short circuit will occur when not around.

Reason two is for security so I will be able to cut electric circuit in case of electrical problems or fire.

I always thought I could shut off the engine when turning the breaker, but last week I tried it and my engine kept running….

As you probably already guessed, it’s on the negative side.

I always use it when parking the car for longer than 30 minutes.

I put it in my dash so I can reach it easily.

Its one with the red detachable key.

The position in that case would make no difference. You have the wrong switch.

 

you need a motor sports one that has 4 terminals on the bottom. This when wired correctly stops the alternator keeping the car running. 
 

[IronmanG]

Mine is fitted for the same 2 reasons. Except on the positive side

[PoolGuy]

 

I always thought I could shut off the engine when turning the breaker, but last week I tried it and my engine kept running….

 

I'd discourage anyone from trying that unless it's an emergency, it can make alternators get a bit grumpy.

[Yoda74]

If I change it to the positive side, I still won’t be able to shut the engine off with this breaker?

Will the radio keep its memory when the breaker is on the positive side or is that a funny thought?

I thought I once heard someone say that.

[IronmanG]

Whatever is between the battery and the isolator will retain power. Only after the isolator will the feed be cut

[stuart bowes]

some race cars run without batteries anyway, my boss for example has a number of 'F' cars (a few 4's, 2 3's) amongst other things and they all use a battery jump pack in the pit and once you're running that's it, unplug the connector and don't stall the car or you're done.  in fairness most of the time if you come to a stop you're chassis-deep in gravel and going no-where anyway.. batteries are heavy and 10ths of a second are the difference between podium and not

 

the cut off switch then presumably is just to cut off the alternator supply

 

weirdly (and wandering slightly off topic) I once saw an open-wheel car of some junior variety which had the alternator driven by belt from one of the drive shafts? you can't even stop and keep the clutch down on that one..

 

anyway.. so yes the car will keep running if you just cut the battery out and as already pointed out you need to 'break' other wiring as well

 

you could run a separate feed to your radio easily enough, through a much smaller lower rated switch that you only turn off if it's unused for more extended periods of time

Edited by stuart bowes, 19 May 2022 - 06:54 PM.

[nicklouse]

If I change it to the positive side, I still won’t be able to shut the engine off with this breaker?

Will the radio keep its memory when the breaker is on the positive side or is that a funny thought?

I thought I once heard someone say that.

Will  make no difference. Like I said you have the wrong cut off. To do what you want you need one of these https://www.merlinmo...ff-switch-coswd

[Gaz66]

Go for a genuine autolek FIA switch. This will do what you want.
Dont buy an autolek FIA "type" switch from ebay and the like, even though it looks the same, you will need muscles in your p**s to turn it off!

[stuart bowes]

why would you need to anyway, just get in the habit of switching off with the key like a normal person lol then isolate when you need to for any reason

 

better still work out the cause of the drain, fix that and/or add an isolate switch that just turns that thing off

 

the only real reason for having a safety cut off is as prescribed for racing safety purposes, is that part of your plans for the car

Edited by stuart bowes, 19 May 2022 - 07:14 PM.

[Yoda74]

@Stuart: There is no real drain, but a battery will always discharge a bit when not used for a while. It's also put on a trickle charger mostly.

 

I don't really mind that the radio loses its presets when using the breaker. I alwys listen to mp3 on a usb-drive, takes about 10 seconds to put the sound-settings right.

 

Question still is, do I keep the breaker on the negative side or do I change it to the postive side.