Battery Switch Question

[Loki9]

My P30 has two 12V batteries and the usual off->1->all->2 big red battery switch. Printed right on the switch it says "do not change when engine is running". Why is that, what's the harm? It seems to be the make-before-break type of switch, so that if I have the cabin lights on and slowly move the switch from 1 to "all" to 2 the lights never flicker.

Is it safe to switch when the engine is idling and the alternator is not working?

I'd like to use one battery for starting the engine only and the other battery for everything else. However, can I start the A4 using "1" and then switch to "2" to charge the other battery only? I assume this will charge the battery that needs it faster than using the "all" position.

[hanleyclifford]

Although many switches will be said to be "make before break" I have never really trusted that claim. If you like to move battery switches with engine running it is best to use switches with field disconnect circuits. If an alternator is allowed to run even for a few seconds with no battery connected it is possible to fry the alternator.

[smosher]

Some Alternators are more forgiving than others and the Motorola is not forgiving at all.

I have a P30 and I believe it came with break before make as the PO
had the diodes replaced regularly, I have all of his repair data. When I
was redoing the wiring I noticed that there's a cutout for the batt switch
and it was replaced. I have checked it and it is a make before break. I think he
thought either the switch or bad or a break before make. When I had the motorola alt I switched while the engine was running, didn't have any issues.

I have now replaced the alt with the 55A and I have also switched while running with no issues.

I don't switch them often, I keep it on one battery and use the other as a emergency in case the one drains down. I did have to switch it last year
when the new walmart battery died at the end of season

[KeysCatalina]

perko makes a off-1-all-2 switch with a alternator field disconnect its about $6 more than the same switch without it. There are 2 extra poles on the back of the switch to connect directly to the alternator or diobe (regulator/rectifier). I believe certain alternators are more susceptible to being damaged if you turn the switch "OFF" while engine is running because you are taking the load off the alternator/regulator/rectifier/diodes... and that dc current has no where to go when the battery is disconnected from the circuit on the fly.

seems like the above posters have some good info i can contribute the attached diagram i got from perko as well.