Crash Investigation, Why did it crash?

Just as in full scale aviation, crashes are often because of a sequence of things going wrong. I received a letter recently asking my opinion on a set of circumstances. At the time of this writing there is no 100% sure answer, not enough info is provided, however there is enough info to draw boil it down to a few likely reasons. It’s easier to take a cucumber cool Sherlock Holmes look at the facts when it’s not your own model. However, you should always investigate crashes and failures with cool logic looking for every possibility, particularly the human element. Read through John’s letter and see what idea’s you have based on the info provided

Hi Dave:

I have an Orbit Charger–up graded by you to accept A123 cells.–Number: 0650-05601OK

Until recently it has worked flawlessly–but that came to a “Crashing” end several weeks ago when I took of and immediately ran out of power, airspeed and ideas, all about the same time.

If you will bear with me I will run through the sequence of events:

*Prior to going to the Field I threw my Plane on Charge (6 x A123)–The Charger quickly indicated “Full and I assumed I had charged before leaving the field the previous day.

* Commenced T.O. and promptly BECed at about 15 ft–gliding smoothly into a vertical Metal Post just off the side of our runway—cleverly ripping the wing in two and tearing it off the Fuse.

*Checking the battery back at the Pits, with my Buddy’s similar Orbit Charger–the pack indicated full and would not accept any charge.

OK, that is what happened–here is the rest of the story:

On arrival at home–(I did not take my charger to the field as we have similar chargers and packs and intended to use my buddy’s) I put my Pack on MY charger–and was horrified to see, when it was turned on–“LiPo 4.2” instead of the usual 3.6!

So all was clear–somehow my Charger reverted to Lipo 4.2V/Cell, overcharged (very quickly I might add) and ruined my 6 cell pack. This without any abnormal heat or any indication (other than the initial charge settings–which I have never paid as much attention to, as obviously I should have– as it’s been “Bullet Proof” for years).

Battery Pack ruined–Airplane wing history, ego damaged–etc!

But what I would like to know–is WHY???–did my charger revert to 4.2V/Cell as I have never used Lipo Batteries with this charger since you upgraded it– and I must say it has preformed flawlessly–until now!!

I hope this ramble was understandable to you,

Regards
John, Canada

John, Sorry to hear of losing a favorite model. You say it ruined your pack, however, you don’t provide any cycle data or voltages to back that up. So, I’m going to make a guess or two to follow that presumes your pack is not ruined from over voltage charging.

You may not know, but when A123 first came out there were no chargers really. We all charged them with Lipo mode chargers. As long as they are perfectly in balance this can be done. The high voltage fuse does not open up and fail the cell until just a little bit over 4.2. So, if a fuse opened up (your pack would be reading zero) it would have to be out of balance to begin with. You don’t say your pack is reading zero, just throwing out there some information you may not be aware of. Obviously it is less risky to the HV fusing if charged to 3.6 per cell and as chargers (Orbit was one of the first) became available to do it correctly we started only charging them in A123 modes.

There is no capacity (or not much) above 3.6v so there is no advantage to charging them higher than 3.6v, just risk. They are not lipo which will puff or risk of fire if you charge them even a tiny bit above their max voltage of 4.2. A123 is a little more forgiving of it’s 3.6v cap. Not suggesting you take up this practice, just history to chew on which bears on your question.

No idea why it may have reverted. Have you run a test to satisfy yourself it is true? If it is, you could tell it that it was charging a Lipo pack of 1 cell less and it would stop at 4.2v x cell count. In this case, set to 5 cells it would stop at 21v on your 6S A123. This would prove for certain your correct.

Also, bear in mind you had some period of time of testing it and it worked perfectly. It’s possible something happened here that you may have not expected yet was exactly what was supposed to happen given the circumstances. Bear with me while I roll through the some possibilities. Anything is possible some some things are more likely than others.

Also, havn’t used one lately in A123 mode but I am wondering if the screen reads differently? It’s late or I would check it, time to go home.

****For the benefit of readers, the Orbit charger starts the charge procedure as soon as you plug in the pack. Plugging in the pack is like hitting the “start” button on many chargers. It looks at the pack for a few seconds then displays on the screen the number of cells it “thinks” it’s connected to. The user then adjusts by hitting the up or down arrow and once you hit “start” and approve it, it starts applying current and charges the battery.****

Scenario’s I’d put forth:

1) If it were in fact in Lipo Mode, an A123 Discharged to 2.7v per cell would be only only be 16.2V, the charger would have guess it was only 4 cells and charged it to 16.8v. You’d be not fully charged and your flight would end very early. Your charger would “peak” out quickly. It would be very reasonable for you to fly the pack down to that level, so this scenario could hold some water. If your BEC voltage is hard set to something reasonable like 2.3v per cell or so, you can see how quickly putting a load on the partially charged pack would cause you to hit BEC cut out.

2) If it were a tad higher than 2.7v, lets say 2.9v, it might still only guess 4 cells. Same scenario as above, you’d be not fully charged and your flight would end very early. Your charger would “peak” out quickly.

3) If you had a short flight and the cells were at 3v+ then the total would have been 18+v. The Orbit tends to guess a tad low (to be fire/damage safer on your packs) at higher cell counts, so I’d expect it to count 5 cells in Lipo mode at 18v and only charge your pack to 21v. A full A123 would be 21.6v, it would have flown fine and you’d not have detected the slight short charge.

4) The charger could have been in A123 mode, the pack could have been discharged lower than normal. Did the last flight on the prior day end in a BEC cut out? Was it a great day, perfect conditions, where you reallying enjoying yourself extra well? Maybe you put a couple more minutes on it. Maybe it was discharged a little further than normal habits and counted 5 cells. Out of habit you approved it without looking and you had a short charge at 18v to start the flight.

My guess without having anything here to measure or double check is that scenario 4 happened It is even possible you could have had the pack so low (depending on BEC settings and nature of model) that the orbit only counted 4 cells and only charged to 14.4v. This kind of oversight when your distracted or in a hurry is very easy to make. And with a floaty model with very low cruise power requirements, it’s even easier to fly a pack very deep by mistake.

I would wager (unless there is mechanical damage) that your pack is just fine, just undercharged due to approving the charge at the wrong cell count and not hitting the up arrow to correct it.

If in fact it really and truly is in Lipo mode, I have no idea how that could have happened and it’s likely that scenario 1 describes the chain of events.

Some positive action to take:

When anything happens that is unexpected, it’s worth a closer look. You had the little red flag early on that the pack filled quicker than usual.

Really looking at that screen before you approve the charge is the first step in not having a problem. The second is looking at the finish voltage when you take the pack off. Your packs should be reading close to 3.6×6 or 21.6v. Seeing this end voltage is your second charging check and really your first step in pre-flight inspection of the model. The third check is listening to the ESC and make sure it’s counting 6 beeps. If it only beeped 4x or 5x when you plugged it up, it’s a big red flag something is amiss. If you pay careful attention to all three of these steps, it would be very difficult for something like this to slip by you again.

(You might actually be listening for 5 beeps as most ESC’s presume they are counting LIPO’s to set BEC Cut Off voltage. Being aware of the regular beep count that represents a full pack is the point, not the precise number in this case since your flying A123 cells and I dont’ know which ESC your using or it’s settings. So, correct could be 5 beeps, wrong could be 4.)

Hope those ideas will let you craw back through memory and your equipment and find a satisfactory explanation. Obviously, anything that goes unexplained can easily happen again so you want to figure out as much about it as you can. This should allow you to be alerted before flight if ever there should be a next time that circumstances stack up in a similar way.

If you did mechanically damage the battery, we can usually repair them. We keep cells on hand here.

Dave

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