On Sun, Jan 13, 2019 at 9:00 AM Stephen Izett stephen.izett@gmail.com <flyrotary@lancaironline.net> wrote:

Hi there people

We are in the process of getting the Certificate of Airworthiness for our Glasair powered by Renesis 4 port.

Our AP is questioning the electrical system and wondering if we should have redundant supplies in case of electrical system failure.
Can any of you make comments on risk or preferred systems.

Our setup has one automotive 40amp alternator and two HC20 (PC680 equivalent) paralleled (two batteries are used due to physical location) and mounted in tail for W&B and to keep cool.

My planning was:
1. Keep battery cool
2. Replace every two years
3. Battery Failure modes:
        Open circuit cell - Alternator and second battery do the job (No indication that one battery has failed)
        Short Circuit cell - Battery voltage drops and overcharges ??
4. Alternator failure modes:
        Open Circuit - Low current and Low voltage alarm with 30min electrical reserve capacity
        Short Circuit - Charge circuit fuse blows then same as open circuit

I understood that a shorted cell was very low risk in AGM’s if not overheated.
I thought creating redundancy may introduce other failure modes.

Appreciate any feedback.
Steve

Hi Steve,

Are you signed up for the Matronics Aeroelectric List, and do you have a copy of the Aeroelectric Connection book? If not, then stop, do not pass go, and do both. Those issues are pretty well hashed out there.

Now to specifics. Have you done a load analysis for your plane? If not, you might be a bit surprised at how much current can get sucked up by modern homebuilts, especially running automotive style injection. FWIW, 55-60 amp alternators are available for very little money (at least here in the USA) and likely will weigh no more than your 40A model. The higher output models will be running a lot farther from their max output (think heat).

Failure modes: I'd agree that properly maintained, the battery is one of the least likely things to have a catastrophic failure. Alternators, not so much, though modern ones are pretty tough. You left out one failure mode: overvoltage due to regulator failure. That is the one that presents the biggest risk, because it can take out everything electrical in the a/c. There are simple protections for it, which result in the same plan B as your other failure modes: get on the ground within 30 minutes. Some have tested battery life in VFR a/c using Tracy's engine controller & minimal airframe electrical loads, & saw around 35-40 minutes life with a PC-680 size SLA battery.

More later; time for breakfast.

Charlie

Ok, refueled.

The 'simple protections' mentioned earlier are an overvoltage detection module that monitors bus voltage and if it gets too high, has the power to shut down the alternator. B&C Specialty used to sell a little 'crowbar' module that did this job. It's fairly simple to fabricate your own, or you can buy something 'off the shelf' that can work in a similar manner. ex:
https://www.tomtop.com/p-e1021.html

Here in the USA, inspectors have accepted single alt, single battery, but most don't even question it; it's 'experimental'. But if you accept the premise that odds of multiple failures in a single flight *extremely* unlikely, and battery failures are almost always 'graceful' (meaning gradual and detectable), then the battery becomes the backup for the alternator. It's our job to decide how long we need that backup to last in operation, and to do our due diligence in maintaining the battery in good condition. That does mean doing capacity testing, because battery power and battery capacity are not the same thing. A very unhealthy battery can start an engine, but might have very reduced capacity to keep the electronics alive for the expected time. In my mind,  the biggest risk is the battery's variable capacity as it ages.

I chose to add a 2nd identical alternator (~10 lb weight penalty), and my wiring architecture has my engine functions on one bus and the rest of the plane on a separate bus. Either bus can feed the other if there's a power feed issue to a bus (bad switch or contactor), and the engine will continue to run if the master must be turned off ('smoke in the cockpit' scenario).

Charlie