Battery Isolator-which one!

[Dave Wortley]

What is the reason for using the earth battery terminal for the isolator switch? If the switch is very near to the battery why not use the positive terminal with a very short lead then it is more convenient to operate. Do you really want to open the bonnet every time you leave/return to the car?

[JonE]

I found that the cheaper ones had poor screw threads which were not going to last very long. I've used the rotary switch versions in the positive supply, about 6" from the battery. Either the repro black switched lucas type, or discounted Durite ones, high ampage "just in case".

[Austin in the Shed]

I'm no electrician but I think most of the isolator switches are made for the earth side of the battery,ie the contacts are not insulated from the mounting/ body.I found this out wanting to fit an old lucas one on the live side.I can't remember the numbers of these  (Lucas)but only one of the four types is insulated so spent a day or so converting it.

[Dave Wortley]

I have used the one from Auto Electrical Supplies with removable red plastic key for about £8. Plastic body,good threads about 5/16” BSW on terminals. Have 3 cars with this and never had any faults. Mounted so I can switch them from the driver’s seat.

[Paul Nott]

Thanks for all your replies, looks like my best option may be to connect via the battery earth lead and bolt isolator to the wooded floor.
This approach will keep the earth cable as short as possible, I have no plans to move the battery from its under floor position.
Isolators that connect direct to the battery such as the green knob terminal switch are of limited use as carpet, wooden floor and cover will need to be removed to access, in an emergency situation this may take to much time.

Thanks for all your replies, really appreciate the advice.
Green spark plug company website also looks interesting.

[jansens]

It depends a bit on what you want the switch to be used for. If to just isolate the battery when the car isn't used then close to the battery on either positive or negative will work. On my MGB I have a big red key kind on the positive lead mounted to the panel behind the drivers seat (the battery is behind that). Positive lead so I can have a thin, fused wire, to keep the clock and alarm powered. I never actually use the alarm though, instead I just take the big red key with me!

If it is more for safety as in to stop a running car will an ordinary cut off switch do it? Disconnecting the battery alone might not. It won't on an alternator car I think which is why in the US for racing they specify a battery cut off on the positive side but they use special switches with extra terminals that also cut off the alternator (or ignition?).

What happens on an A7 if you turn off the switch with the engine running? It would be like disconnecting a battery lead which I am sure they warn against doing. The generator will still generate voltage so won't the engine keep running? But now you don't have the battery there acting as a ballast so I imagine the voltage will shoot up and possibly damage things? I don't know how the cut off and regulator affect things?

I would always use a decent quality switch. When closed there is a lot of current passing through the contacts when starting using the starter motor and a cheaper one might not handle that reliably over time. Worst case would be the contacts weld together then when you turn off the switch it's not really off. I sometimes inadvertently check mine by forgetting to turn it on and wondering why the fuel pump/ignition aren't working.

Simon

[Roland Alcock]

I once had an "incident" where my passenger had somehow pushed hard with his feet and shorted out the live battery feed. When I went to isolate the battery, the green knob type of switch, had melted and welded itself together. Fortunately the right spanners were to hand to disconnect the other battery terminal. A bit of a scare that was.....

I now use. 




Roly

[Tony Press]

I once had an "incident" where my passenger had somehow pushed hard with his feet and shorted out the live battery feed. When I went to isolate the battery, the green knob type of switch, had melted and welded itself together. Fortunately the right spanners were to hand to disconnect the other battery terminal. A bit of a scare that was.....

I now use. 

Roly

Wouldn't the same thing happen with this switch ?

[Chris KC]

Must admit I hadn't really envisaged use of such a switch while the vehicle is running, more to immobilise the car when parked and minimise the risk of a garage fire.

Do the 'kill' switches fitted to racing cars isolate the battery completely or merely break the ignition circuit?

Bit like Roland I once had an incident where a battery cable fretted through its insulation through constant chafing against the edge of a floor panel. The resulting firework display beneath my leaking petrol tap was a little alarming. I dealt with the sharp edge as best I could but now double insulate the cable in areas of potential fretting by sleeving it with heavy rubber hose.

[Dave Mann]

I use the isolator switch as a safety & security issue, wouldn't want the car to set the garage and house on fire. I have occasionally operated the isolator before switching the engine off and the engine stops because at it's low idle speed the dynamo isn't generating sufficient to close the cut out. I was asked to help sort out a sports model where an auto electrician had replaced the Lucas PLC switch with the result that the engine wouldn't stop on operating the ignition switch because the dynamo was feeding the ignition. It took a while to sort out because all the wires were black.