Don't disconnect the battery!
 

I cannot imagine a scenario where you'd want to completely disconnect the battery from a bus (being fed by an alternator) that feed electronics of any kind.

Even a weak battery will still act as a capacitor to smooth out spikes, surges and over voltage from alternator (briefly) -- whether you have one or two alternators. This is essential to protecting electronics. So at least have a big electrolytic capacitor or small 12V battery across any bus feeding electronics.

For those of you with alternators that cannot be shut down by killing the field voltage (alt switch/fuse/breaker), the minimum, easiest and lowest cost solution seems to be the contactor (relay) in the line from the B+ terminal on the alternator. Alternator over voltage events may be rare, but very expensive when they happen. (Of course they could be the excuse to get the completely new panel you've been wanting.)

Oh, and if you don't know if you can kill the alternator output, please turn off all electronics you don't want to replace before testing.

But, really, really, reading of at least the sections on alternators and over-voltage protection in the AeroElectricConnection book should be required for anyone building or flying a HB plane.

I can't imagine spending $1,000s or #10,000s for avionics without automatic over-voltage protection (crowbar on field wire to alternator or a B+ series contactor).

Finn

i think the take away from this thread is that you need a way to disconnect the b-lead from the alternator to the bus. hopefully activated by a ''crowbar''.
disconnecting the field wire from the alternator MAY NOT stop the alternators output. there are failure modes in the alternator that will allow it to produce runaway voltage without a field wire.

If a battery cell shorts out, you have a 10v battery on a 12v system. Keeping it online with working alternator(s) can destroy the battery (a meltdown is possible).

So I can see why you want to disconnect a battery.

I agree with the BIG capacitors for voltage spikes...........

Paul, the master disconnects the battery. The Plane Power's field is typically connected to the main bus via a 5 amp breaker. Disconnecting the battery (master OFF) will not shut down a Plane Power alternator. If you wish to shut down the Plane Power in flight, you must have an alternator field switch or switchable breaker to disconnect the field from the main bus.