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MG MGB Technical - battery isolation switch

I'm considering fitting one of these at the same time as fitting a single 12V battery. I have seen the cheap sub £10 versions on ebay as well as the FIA approved ones (more expensive). Which type do users recommend?
Steve Church

I fitted one of these to my car, I got it from an eBay seller. I checked his feedback to ensure people werenít complaining about them and its working fine (so far). A PO had changed to a single battery and put it in the passenger side (left hand side for our colonial cousins), they joined the battery inter-connection lead to the main lead going to the starter. It was therefore an easy job to put a hole in the heal board just behind the drivers seat, mount the switch, break the connection in the 2 cables and connect them to the switch. I soldered large lugs to the end of the cables and they fitted straight onto the switch.

It makes it much easier when working on the car to be able to disconnect the battery with the switch. It also acts as a simple anti-theft device and I can disconnect the battery when the cars in the garage (I have a dread of an electrical fire)

Being behind the driverís seat itís also easy to switch on/off and remove the key when exiting the car.
B Davis

Cheapie's fine! FWIW the accepted wisdom is to put it in the -'ve for a -'ve earth car. . (Similar to always disconnecting the ground lead last) But plenty put it in the +'ve.
Michael Beswick

If using the cheap version and you have an alternator, don't use the isolator switch to stop the engine running!
Dave O'Neill 2

Iíd agree with Michael the ĖĎve (earth) lead is definitely the place to put the switch from a safety point of view. The only reason I put the switch where I did was that the PO had already put the 12V battery in the passenger side and this was a quick fix. Over winter I plan to move the battery to the driverís side at which time Iíll move the switch to the earth lead.

The reason I didnít move the battery was because the one as fitted is a bit large and is a struggle to get in and out. Iíve read various articles about suitable batteries, does anyone have suggestions regarding type and supplier.

B Davis

FIA switch not required, that has additional contacts for the alternator and ignition to ensure the engine stops. Note that with the simple switch turning it off will NOT stop the engine. Some say that this blows the alternator, lights and goodness knows what else, due to over-voltage from the alternator. All I can say is that having done it several times in the V8 there has been no harm done (yet ...).

I've put mine in the 12v lead right where that lead runs down the heelboard behind the drivers seat. Not that different to put it in the earth lead, just an additional length of cable rather than cutting the 12v cable. If you only needed to remove the 12v connector from the battery then having it in the earth lead saves you removing the earth connector first, but apart from that there is no benefit either way, I think. I was pre-conditioned to put it in the 'live' from many years custom and practice elsewhere.

PaulH Solihull

If you plan on using a battery conditioner over the winter through the cig lighter socket, it pays to have an accessible 3 or 5Amp fuse across the isolation switch contacts. Also keeps the radio presets and works the interior light. Blows when the fuel pump etc kicks in, if you forget the switch....
And yes, I do have the fuse in the -'ve!! (Mounted under the carpet on top of the battery compartment lid)
Michael Beswick

You can always use a 5amp Polyswitch resettable fuse which does exactly what it says, but automatically, once the overload is removed. It can then go out of sight in a safe place. You can buy them from Rapid Electronics.
Richard Coombs

Don't forget that if the reason for fitting a cut-off switch is to save the battery when not used regularly (as mine on the V8 was) then don't fit or use one with a bypass fuse! My radio is a fully removable unit so doesn't need a permanent supply to keep the presets. However I got fed-up with keep resetting the clock, so ran a wire from the battery connector via an in-line directly to that (which I probably couldn't have done if the switch had been in the earth leg). It subsequently occurred to me that I could have run that wire just to the hatch light and removed the purple fuse instead of running it all the way to the clock, but at least now I can check I have turned the switch off just by opening the door!
PaulH Solihull

I just wandered over to the MGA board and found an interesting thread there about a battery cut-off switch!
Dave O'Neill 2

I've heard this theory of current getting 'used up' before it gets back to the battery before :o)
PaulH Solihull

This thread was discussed between 22/09/2010 and 09/10/2010

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