Circuit breaker popping when battery is low

[rbickford]

I ran my battery down low trying to restart the engine. In the no good deed goes unpunished category, I shut the engine down, after a cold start, as a courtesy, when several parachutists touched down and were walking across the apron to get back to where they were hanging out. After they cleared the area, and after several attempts to get re-started, and with the engine hardly turning over from the depleted battery, I finally re-started. When I did my run up at 2,000 RPM, a breaker popped on the Main Buss. I could re-set it, but it kept popping any time the engine was run up over about 1,800 RPM. Come to find out, if the battery is as low as it was, the charging system puts a large amperage demand on the system once the RPMs are up high, and that pops the breaker. I've since found that if this happens, I can re-charge the low battery by running at 1,800 RPM or less, for a period of time, to get the battery sufficiently charged so that the system will not pop the breaker at high RPMS. Seems odd that this issue is designed into the charging system. Is this normal for a 2004 DA 40, or could there be an issue in the charging system failing to regulate the amperage demand. Thanks for any thoughts on this. Apologies if this is an old topic.

[Rich]

A common problem. The alternator output is on the main bus and the battery is on the essential bus. When the battery is heavily discharged it'll take up any spare amps around. When the alternator reaches a point where it can deliver enough more than the various non-essential bus equipment needs, the battery (plus the essential bus equipment) sucks up the excess, which exceeds the 25 amp breaker capacity. In my experience with my plane, this tends to happen around 40 amps of alternator output. Typical vehicle charging systems don't have current limiter circuits that are not circuit breakers. I can conceive of a possible approach to fix this, but haven't really gone through the possible negative side effects of limiting current draw by the battery/essential bus subsystem.

[Sandy]

I had discussed this issue in another thread. See, viewtopic.php?f=8&t=5091&p=69788&hilit=Sandy#p69788

As I stated there, the Main CB and the Essential CB are each rated at 25 amps, and they are connected in series, unless the ESS switch is flipped. That means that when you have a low battery, the alternator will try to pump current that may very well exceed 25 amps, so whichever of the two CBs actually flips at the lower current will pop. As you noted, you can deal with this issue on the ground by limiting your RPMs to limit the current that the alternator can supply. However, if it happens when airborne, you may not be able to throttle back enough to limit the current, so I do not know why Diamond does not include a current limiting circuit in series with those two CBs. FWIW, I discussed this with my mechanic, and he told me that Bonanzas do have current limiters, although I never confirmed that.

Sandy

[rbickford]

Hi Again Sandy. I found the other thread and appreciate the input here and from the other post. Thank you. Bob

[Rich]

Did some digging through the wiring diagrams (no small task) and testing. Some data:

-The breaker for the alternator to the main bus (and the rating of the alternator) is 70 A.
-The breaker for the battery to the essential bus is also 70 A.
-Those two breakers Sandy speaks of are both strictly used for this bus tie subsystem.
-The "Low Voltage" annunciation on my plane looks to be only relevant to the main bus. But sometime around 2005 such an annunciation was added for the essential bus.
-The Davtron unit (if you have one) displays its input voltage, which is from the main bus.
-The VM1000 is supposed to flash the voltage display when the voltage it detects is "low". This is actually the essential bus voltage, contrary to what the manuals say. But it does not appear to be adjustable. I don't know at what level this happens, but it is not the same as the main bus low voltage annunciation.

I feel the best solution would be twofold:

1. Increase the capacity of the bus tie subsystem. This would require changing out the breakers, the relay, the wiring to the two bus bars and the diode. It's tempting to say just bump it up to 70 A, but 50 might be sufficient. This isn't entirely trivial, as these current components are all behind the instrument panel. Might be a tricky squeeze.
2. Add in some prominent annunciation for the state of the battery in these situations. I think it would be good to have current detection to show the rate of charge/discharge of the battery. This was actually common in older planes (though implemented in a crude way compared to what we could do now) but we don't that information today. It would be useful enough if it just read current flow of the circuit between the battery and the essential bus, ignoring the other minor draws. Why do we need this? So you know if you're taking off with a fully charged battery or one still sucking 10 amps because you started out with a low battery condition and it's not really quite ready to deliver full capacity.

Some other thoughts:

Current limiter? Where, exactly? And at what level? Using just this approach has the effect of perhaps unnecessarily extending the battery recovery time.

What's the max charging amps the battery should be subject to? I've seen just over 40 amps coming out of the alternator right before the CB popped. Maybe this or even some higher number is fine. But this could be a more appropriate use of a current limiter, if necessary, in addition to what I suggest.

[rbickford]

Hi Rich, I think Diamond in Neustadt should hire you (an if you have not been there you should go! You will for sure get to fly a factory plane over Austria)

I am not an engineer, but it seemed odd that this system flaw lives on, or lives at all. After 'uncovering' the issue for myself, I now see this is an understood by the Diamond community. I tell every Diamond owner I meet, and mechanics also, about the issue just to get the word out.

This was happening the other day to the mechanic that was tracing down a spark plug issue on my plane. Lots of starts in a short period of time drained the battery. Brand new Gill 7243-16 (pure lead) battery so no inherent issues there. He noted that he saw 43+ amps when breaker popped on him.

Would be interesting to spitball this with some of the Diamond engineers (after they hire Rich) Thank you for all the information on this.

[Rich]

Other than the low battery condition, it works fine as is. Once you wade through the multiple versions of the wiring diagrams the weak point becomes pretty obvious. I had to print out all the candidate pages to get my head around it. Couldn't really deal with the clumsiness of these in the pdf.

[Steve]

This may not be a flaw at all, but a design "feature". I'm not enough of a battery chemist to know for sure, but even though AGM batteries are fairly tolerant of high charge rates, the DA40 was originally certified with a flooded battery. Charge these too fast and you get hydrogen gas fizzing, heating, and maybe...

...boom!

Steve

[Rich]

This may not be a flaw at all, but a design "feature". I'm not enough of a battery chemist to know for sure, but even though AGM batteries are fairly tolerant of high charge rates, the DA40 was originally certified with a flooded battery. Charge these too fast and you get hydrogen gas fizzing, heating, and maybe...

...boom!

Steve

This is potentially true, but the first question is what the max charge rate should be. Then, if necessary, a one-way current-limiter is a better solution and it may very well be higher than the 25A allowed by the current system. And keep in mind the 25A itself is not the current available to the battery, as it also includes whatever equipment on the essential bus happens to be turned on while the battery is being charged.

I have done some digging but have been unsuccessful finding what the max charge current rate should be. Too high a charging VOLTAGE is a definite no-no, but that's not in play here.

[rbickford]

The battery danger it is an interesting point which the circuit breaker certainly mediates. Still seems like something to design around in terms of limiting amps somewhere in the circuitry. On the battery danger topic, if you ever need a question answered about battery design and characteristics call Tom Jones at Gill, or stop by the Gill booth this week at Oshkosh, and ask him. I doubt he could be stumped on any topic I can say that for the Gill 7243-16 I have the spec sheet recommends the regulator sweet spot is 28.6 V, and then + or - .3 V. I did not see any warning data in the battery spec sheet about too high voltage when charging, but it may be in there.