Not really. But it poses a question in a 2 battery + 2 alternator electrical system. What do you do? There is only really one action plausible, which is “Both Masters OFF”.
When I did the Avionics, I figured I didn’t need a backup battery, because I had something better – a complete second battery/alternator system. That’s like a backup battery, but better … it can basically last forever. As an upside, it removes the weight and wiring associated with the backup battery. It was only when I went to do the complete wiring schematics that I asked myself the above question, and in so doing I realized that I did, in fact, need a backup battery. How else can I run a set of minimal Avionics after a “smoke in the cabin” event requires me to turn both electrical systems off. I couldn’t write an emergency procedure that said something like “turn one system off, wait a while to see if the smoke clears, turn the other off if it makes no change”, that would be silly.
So, I have to include the backup battery, to run a minimal set of Avionics in an emergency. However, there are many conditions where I would prefer to use the right electrical system as the backup, since it is far more capable. If the left electrical system went down for any reason, the right electrical system should be used as the backup. I didn’t want to start adding switches to manually select these choices, because:
- In any emergency situation, a pilot will tend to suffer a large drop in IQ. Adding switches and actions to deal with the emergency situation is just asking for trouble.
- Any sort of manual changeover could bring about a situation where the Avionics power went off, then back on, requiring the EFIS’s to reboot. This takes time – the last thing you need when an emergency occurs.
Therefore, what I need is some kind of automatic changeover. If the right electrical supply is present, then it should be selected as the backup source. If the right electrical supply is absent, then the battery should be selected as the backup source. Power diodes? No thanks, don’t like them. A non-latching relay would work, but then I’d need a switch or something in order to test the battery on startup. It’s starting to get a bit complex. There’s also circuit protection, and the need for a charging path to charge the battery. What I really want is a system that does the automatic changeover, with no energy loss when operating off the battery (no diodes, no relay coil current), and that yields good visibility to the condition of the battery, charging state etc. With this in mind, I designed a small board to manage the backup battery requirements.
The schematic for the board can be viewed here.
Over on the right, a latching relay is used to switch “Backup Power” between the right battery/alternator system and the backup battery. The FAN3240 and associated components drive this relay. In the center left, a small Arduino board controls everything. This needs a tiny amount of 5 volt power, whenever either the battery or right system is present. Up on the top left, protected by polyfuses, these two power sources are diode OR’d to provide 12V for relay power and a small linear regulator provides 5V for the Arduino board. On the bottom left, a level translator provides an RS232 serial port for remote monitoring. Finally, the “headset power” for Bose etc. headsets is tapped off a convenient point that has power if either the battery or right system is alive. In an emergency, the (trivial) power for ANL headsets will still be provided by the TCW battery – this isn’t a bad idea, in an emergency with minimal avionics, we would be better off hearing any VHF traffic clearly, right?
The Arduino controller has scaled analog inputs to sense battery voltage and right supply voltage, as well as the sense and “low voltage” outputs of the TCW IBBS. Two outputs from the controller are used to set or reset the latching relay. The code is trivial and simply selects the right system in preference to the battery, if it is present. A remote communications protocol allows a host system to determine the status at any given time, including measured voltages and the direct IBBS outputs, and to force the system one way or another for testing.
A modified backup battery harness is required to use this system with the AFS ACM. Page 155 of the ACM manual (v6.5) shows the standard harness from the TCW IBBS backup battery to the AFS control module. Here is a link to the actual harness I made to accommodate the new battery backup board.
I mounted the TCW IBBS on the back of the sub-panel, behind the AFS ACM. That makes it a bit hard to get to, unfortunately, but I didn’t have much choice by this point because all of the front sub-panel space was used up. The battery backup board is mounted near the ACM, I didn’t bother putting it in a case, there’s no reason to do so. Here’s a not-very-good picture of the first, hand assembled, prototype sitting in position on the sub-panel, ready for test. There’s one panel switch – the standard low-power one required by the TCW IBBS to enable the backup battery. Apart from this, everything else including test is handled remotely and can be viewed/controlled if need be from the small TFT display I’m using on the front panel for annunciation and monitoring of the EFI redundant power system.