Twin Batteries- Not all Redundancy is Created Equal, part 2

[Hartstoc]

Originally Posted by bret

Trying to digest your main goal here, one of your statements about having 800 CCA for starting with two batteries, if you are using the Earth X 900, you will not need two of them, one of them will crank your prop fast enough to taxi with the starter alone Also these new batteries keep their charge for a long time with very little discharge, these can sit for months with no trickle charger and start just fine, even in cold weather. I tried the schottki diode with heat sink for charging the backup battery but it did not do well, so I just tied in a manual switch to top off charge it during runup and taxi, then off. Ready to go for backup at the flip of a switch. Also, keeping the secondary battery isolated protects it if a runaway alternator goes crazy, or the crowbar fails and takes the main battery off line. With my set up if this happens, I just shut off the master, turn on the back up battery and everything comes alive......for about 90 min.

Bret- You are correct- but it is good to have the option. There is really no such thing as too much battery- the more available, the cooler and faster the starter operates. I don’t expect to use both for routine starts, in part because having one of the lightspeeds running off the “backup” allows it to have full voltage during cranking. There will be times, however, like after the engine has been sitting for a long time, when using both for an extended spin with the spark plugs removed will allow pre-oiling the engine.

As for the diodes, it is important to note that their use is actually optional in my design! They are high quality Schottky’s that are rated at 25Amps mounted on heatsinks, but their current will flow through a pullable 20Amp breaker at the feed points to each Essential Loads Bus(not shown on the schematic but I will add them to a corrected version). My reasoning is that with 45Amps absolute max output from my BC462-H spline-drive alternator, no more than 20 Amps will ever go to the backup considering that the main battery was used for starting and the avionics and lighting will be demanding at least 10 Amps off the top. Most of the time the charge current through the diode will be just a few amps, as SOP will be to have a single Lightspeed Plasma III as the only active load on the battery serving as”backup”. An option will always be to pull the diode breakers and manually manage charging of the “backup” by simply switching the battery’s roles every hour or so, putting the slightly depleted battery right on the main bus for a full-voltage top off. Duration of one of these batteries with a single lightspeed as load is in the range of 10hours!!

Before anyone reacts with shock that I’m using a spline-drive alternator as primary on an electron-dependent airplane, see this before responding:

http://www.vansairforce.com/communit...hlight=Bc462-h

This alternator may be the world’s happiest on any airplane, and I’m convinced that it is FAR more reliable than any belt-driven setup. -Otis

[MCA]

Hi Otis, You show dual power inputs to each fuel pump and ignition, neither of which (normally) natively have dual diode-isolated power inputs. How do you plan to connect these? Inquiring minds want to know.

Normally Bus A goes to Device 1, and Bus B goes to Device 2 if you have a dual bus architecture.

[ChattanoogaRV]

Originally Posted by bret

My backup battery is simple and no relays, just the 30A rated switch to tie into the three seprate fuse block busses, pump 1, ECM 1, Coil 1 on one fuse block, pump 2, ECM 2 Coil 2 on the other fuse block, and the avionics are on the third fuse block buss if I need to load sheed to keep the engine running longer, hand held for BU com. the Dynons monitor both battery volts and only switch on the BU battery during run up and taxi to top off, then switch off. To test the back up battery, BU battery on, alt field off, master off. fan still turning? then master on, alt field on, BU battery off.

Bret, Could I get more info on your system? Do you have a drawing?
Thanks in advance
Steve Michael