Thursday, February 08, 2018

Aviogard first try

The MGL Aviogard came today. I hoked it up on the kitchen table together with all the other stuff (stretching the patience of my wife to the max). The Aviogard has two purposes:
  1. Isolate the electronics from spikes and stuff from the engine and supply a steady 13.5 "ish" V to the EFIS and other delicate instruments.
  2. Hook up a backup battery on the isolated side for an automatic battery backup. The battery is charged normally with 13-13-8 V, and if the main power should fail, it will supply power.
At least that's the theory. I had a fully charged "main" battery, and I had used the backup battery, so the charge was around 11V. I hooked everything up, and what happened was not what I expected. The Aviogard started to heat up. When reaching 120 deg C, it will automatically shut off, which it did. The reason is that charging the backup battery took took a lot of current, probably much more than the 6-10A max rating. When the Aviogard switches off, all the current is drawn from the backup battery, which already was low on power. This kind of defeat all the purpose of point 2. I had a fresh newly charged main battery, but the low charge on the backup battery caused the whole system to shut down. Even with the battery charger on the "main" battery, the backup battery wouldn't charge This failure mode is just insane. A partly charged backup battery should not cause the whole system to fail !!!

I could have this wrong. When going from backup battery mode to main battery mode, it seems a delay of about a minute or two is needed. Else, the Aviogard will just switch into backup battery mode automatically. This is also strange though, and it would e nice if the manual could explain these things.

Anyway. The reason for that failure mode is of course the simplicity of the design. That simplicity is also a good point. Even if the main supply and even if the Aviogard itself breaks completely down, the backup battery will supply power as long as it lasts. The downside is to check the voltage of the backup battery before every flight, and remember to charge it regularly. The installation manual also define a separate panel mounted switch for the backup battery. In flight, if the EFIS shows lower than 11.5-ish voltage, turn off the backup battery and see if the voltage increases. If it does, the backup battery is either not charged enough or faulty. The system works, but fully automatic - not so much.

The backup battery is charging now, and I will see how the thing behaves with a fully charged battery.

Now the backup battery is fully charged. It's rather obvious it is (almost) a pure backup battery for emergency only, and it's far from fully automatic. Once it takes over, in the event of main power failure, it will be the only supplier of power, even if the main power comes back on. This means the manual switch must be used actively. It must be the last switch to turn on during start, and preferably also the first to be turned off during stop. Rather strange actually. In "normal" mode, when "on", it's just a normal battery in the circuit and will help supply a steady current even when there are lots of intermittent activity going on.

But, how to monitor the supply of power? I got the main battery and alternator with it's voltage. Then I got the voltage out of the Aviogard, the voltage out of the ECB, and the voltage of the backup battery (when off).

The EFIS monitors the V out of the ECB. The ECB also monitors the current going through itself, and is sent to the EFIS. The RDAC has a current meter, but no voltage. I should probably monitor the voltage of the backup and main battery. I hope there is an easy way without too many additional gadgets.

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