Sparks Question


Active member
31 Oct 2001
I've just purchased a battery master switch. Now the terminals are marked for the battery cableing and are straight forward, the 2 other terminals are marked for the Alternator "field" terminals. I suspect to protect the diodes when switching batteries. Now my question is "does this just break the circuit or does it keep the diodes "In Cuitcuit". and do i have to use this or just leave them out. I cant see myself swithcing over batteries while the engine is running, its mainly so i can run the limited domestics off of one batt and use the other for engine starting, ie out on batt 1 return on batt 2 to keep them both charged. I'm open to your wisdom.



Well-known member
30 May 2001
North from the Nab about 10 miles
Disconnecting a running alternator causes a voltage spike which will destroy the diodes. The 'field' connections of the switch are designed to prevent this during switching.

However certain you are that you will 'never'switch over on load, just bear in mind that a certain law demands that sooner or later you may need to switch over in a hurry - and you might just forget.

Anyway a 'helpful' visitor might also decide to do it for you one fine day....

In view of the cost of alternators, is it worth the risk of NOT wiring it right?

To find out exactly what each terminal does, use a meter or continuity checker across the terminals before you install it. There is no risk of damaging anything.


New member
5 Jul 2001
Winchester, UK
If it is a standard 1-both-2-off switch, there should be no problems at all when switching between the 3 "battery on" settings. The only problem is if you actually switch all batteries off while the engine is running. When going between the other three switch positions, there is at least one battery connected at all times, and I would not worry about changing while live. If you use the switch as designed, you can start the engine on battery 1 and then charge both once the engine is running.

I suspect that the "field" contacts remove power from the regulator and/or field windings when in the "off" (no battery connected) position, to avoid damage to the alternator. However, if you have a simple internal regulator, it may well be that you cannot disable/disconnect it anyway, as it is permanently connected internally to the alternator output. This is why you often have to modify the alternator internal wiring slightly when fitting an external regulator.

Should be easy enough to check what the "field" contacts on your switch do - my guess is that they are closed in the 1-both-2 positions, and open in the "off" position.