Of those who have this setup what size battery is being used as the
primary unit and what size unit is used for redundant or essential bus
use
I have two independent electrical circuits... the only common part being a 2AWG
ground wire joining the two ground distribution blocks. For diagnostic purposes
this link can be removed.
The primary power supply is charged by the engine alternator and has a 24Ah RG
battery located under the baggage bay. This is fed by 16 gauge welding wire
which is the same diameter as 4AWG wire but is much (MUCH) more flexible. A
master breaker located on the baggage bay bulkhead acts as the primary
protection mecahnism for this feed.
Using my fluke voltmeter down to 3 decimal digits I cannot measure any voltage
loss between the battery and the firewall distribution point. I tested the
cranking ability using the same length of wire and emulated junction points on
the 912 in ZK-UBD and it turned over even with the battery partly charged. This
primary circuit runs all the primary functions including the Airmaster prop and
the primary pump.
The secondary power supply is charged by a B&C Specialities generator mounted
on the vacuum boss and has an 11Ah battery located on the footwell. I have also
run 10AWG wires aft and, if necessary can relocate the battery anywhere in the
rear of the fuselage if I need to adjust the C of G. If I have to use lead
weight to adjust the C of G it may as well be doing something useful! If I
don't need to relocate the battery then these two wires will be cut and
terminated.
The secondary circuit runs the backup fuel pump and a small selection of
electrical items. This supply also feeds the secondary power input into the
MicroAir radio and transponder. This removed the requirement to implement a
cross feed bus as these are the only two instruments I felt that were essential
if I lost the primary feed.
The primary pump is turned on by a second set of poles on the primary master
switch but can also be dropped off independently by its breaker/switch. The
secondary pump is turned on by an independent switch on the right hand panel as
are all services used regularly in flight.
In NZ you are very rarely more than 20 minutes from an airfield so my primary
design requirement was to keep the engine pumps working and, if necessary, tell
people where I was.
Tony
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