Why is the alternator routed through 25A breakers?!?

[Chris B]

Recently I was stuck in the run-up with six other aircraft trying to get out of RHV IFR. RHV shares the approach corridor with SJC (and NUQ & PAO), and NorCal gives priority to the big aluminum cans. For added interest, when things are backed-up NorCal usually gives the RHV tower less than 60 sec to release an aircraft.
So you wait... wait... wait..., and then suddenly are cleared "with no delay."

After 50(!) minutes in run-up, we're finally cleared for takeoff. Immediately after punching through the marine layer, the Main Tie breaker pops. No VFR airports within at least 50 miles. Awesome...

Per a recent DAN thread (link) this behavior is a feature, not a bug. The battery is connected to the Essential Bus with a 70A breaker, the alternator is connected to the Main Bus with a 70A breaker, but the Main Bus and Essential Bus are tied via two (series) 25A breakers. I am trying to understand why Diamond would design the DA40 electrical system so that when the alternator runs anywhere near rated capacity, either the Main Tie or Essential Tie breakers (or both) will pop. For an aircraft totally dependent on electrical power, this is quite confusing.

A related design decision is providing no indication that the electrical system is being drained. The G1000 ammeter only shows the alternator output, not the net load on the electrical system. As long as the alternator is pumping ~>8A everything is "fine." Even if the system draw is 20A, and the battery is seriously depleted...

Does anyone have a clue what Diamond was (is?) thinking?

FWIW, this incident has changed my run-up process. Once we announce ready for release (/departure), the engine stays at fast idle. The extra gas is definitely worth avoiding any excitement on climb-out.

Chris

[Colin]

The suspense is killing me. Did you make it back down?

[Chris B]

The suspense is killing me. Did you make it back down?

LOL...

Thankfully I'd previously experienced this "feature" in CAVU conditions, after playing with the G1000 too long on the ground. So I had an excellent idea what was happening.

In this case, shedding every possible load (principally exterior lights) and a few judicious resets - between long pauses - did the trick. But I almost declared an emergency, and would have if the low-voltage warning sounded.

I just can't figure out why on an otherwise extremely well designed aircraft there is such a seemingly bone-headed alternator/battery interconnect. But maybe this prevents other issues?

Chris

[CFIDave]

My guess is that the wiring is only capable of safely handling 25A. So rather than wondering why Diamond installed such a small breaker, perhaps the better question is why Diamond didn't install heavier gauge wiring -- perhaps due to weight/space considerations?

BTW, a somewhat similar thing happened with our former DA40: my pilot wife discharged the battery through multiple attempts to start a hot engine (i.e., lots of starter cranking). After she finally got the engine started and taxied to the run-up area, increasing the RPM (and thus alternator output) during the run-up caused the 25A breaker to pop. She not-too-surprisingly decided this was a failure and cancelled the flight, stranding her away from home in another state. I then persuaded a pilot friend to fly me to her remote airport location so we could "rescue" her and the malfunctioning plane. We ended up putting the battery on a charger overnight and of course the breaker never gave us any more trouble.

I later learned the "secret" to keeping the 25A breaker from popping after the battery has been heavily discharged is to operate the aircraft on the ground for a few minutes with a fast idle (e.g., maybe around 1500 RPM) -- fast enough to charge the battery, but not so fast that the alternator output exceeds 25 amps. You can do this by carefully advancing the throttle while monitoring the ammeter. Operating like this with a fast idle for a few minutes will charge the battery sufficiently that full power (or 2000 RPM during a run-up) won't later cause the alternator to exceed 25 amps and pop the breaker. Obviously this procedure won't work very well once you're already in the air...

[rwtucker]

Dave,

Your limited charge sounds like the best solution and something we should all watch for if we are in a situation where the battery is discharged before takeoff. However, I'm not so sure that the wire gauge is the problem. For example, three feet (each way) of 8 ga will drop only 0.4% at 25 amps; 10 ga only 0.65%. Even 12 ga will only drop a percent, still delivering well above the minimum charge differential from the alternator's 28+ volt output. Because the distances between the battery to breaker to alternator are so short, these figures, even if twice that, are within acceptable drops.

My guess as to why we see a 25 amp breaker in the line goes to the nature of the Recombinant Gas AGM cell. These cells do not hold up to high inrush charging currents. When I ran the numbers for a friend who had killed his battery with the infamous map light problem, I came up with a suggested 20-22 amp charging limit. I suggested that if the initial charge was higher than 22 amps (he was using an outboard charger) he should first trickle charge or intermittently charge (30 seconds on, 30 seconds off) until the initial continuous charge was 22 amps or below. Forty amps delivered to our Concorde battery could easily fry or, worse yet, rupture a cell. In theory this initial charge might be self-limiting but inserting a limiting breaker is not a bad precaution, although it would have been nice had Diamond warned us of this issue. It is a potentially serious problem in IFR.

[ihfanjv]

Is an alternative - if this were to happen in cruise - to lower RPM, shed load, reset the breaker, then manage alternator output via RPM? I'm not sure how low you would have to go with RPM because this has never happened to me.

[Chris B]

Is an alternative - if this were to happen in cruise - to lower RPM, shed load, reset the breaker, then manage alternator output via RPM?

AFAIK, only if you can effectively glide until the battery recovers. Which may be a viable strategy in some circumstances, although climb-out is not one of them.

Per this chart from Plane-Power (link), anything over ~1500 RPM is generating >50A. Normally this is considered a great feature. But in this particular situation, not so much...

Even at fast idle the Plane-Power alternator is pumping near the rated breaker limits. OTOH, I now know from personal experience that the "25A" breakers actually pop at >40A.

Chris

[ihfanjv]

This is one of the craziest design flaws I have ever heard of in an airplane.

Quick, someone start working on a STC for a 40 amp breaker in lieu of the 25 amp breaker!

[rwtucker]

One implication of Chris' chart is that the Plane Power, even though it is an overall superior alternator, is slightly more likely to cause this kind of problem. But, again, the prevention is not taking off with an undercharged battery. (Maybe a checklist item?) Again, I don't think it is a design flaw. I think it is designed to protect the AGM cells that cannot safely take a 70 amp charge rate.

[Chris B]

...recently stuck in the run-up with six other aircraft trying to get out of RHV IFR. <snip>
After 50(!) minutes in run-up, we're finally cleared for takeoff.

BTW, this must have been supremely frustrating to the Meridian behind us. All he wanted was VFR on top, which would have taken <60 seconds to punch through from throttle-up.
But he probably burned ~25 gallons of Jet-A sitting in run-up.

Chris