Main power plugs/connectors have a finite cycle life (how many times they can be plugged & unplugged) before they start wearing out, pitting & carbon burning from ESC capacitor charging arcs, oxidation, dirt, or the spring tension of the contact points starts getting weak. 

All conditions or a combination will give you an increasingly poor connection with increased resistance over time. This generally starts showing up as power drop-offs or worse, cut-outs & restarts under high current loading meaning it's time to replace your connectors. Yep, I've been there and have the sad remains of the heli as a reminder! 

Why did this happen and what is the RC connector lesson to take away from this crash? As it says above, a hidden problem with the male sprung bullets in the EC5 connector. 

Because spring bullets have a sleeve that wraps around the main body of the pin, there actually is not that much contact area in relation to the length of the pin. Only the small top and bottom area of the sleeve are in direct contact with the inner pin.

In addition to a rather small "contact patch", this area is completely hidden from view. In fact, this is the very last connector I pulled out to check because on the outside, it looked practically new, and was still very stiff to plug/unplug. Only when I decided to pull the sprung sleeve off did I notice the significant carbon pitting on both the top and bottom of both pins where the sleeves make contact.

I still get sick to my stomach thinking about how a $2 connector destroyed a rare 700 Blackshark F3C fuselage and why I didn't catch it before hand considering my almost obsessive maintenance regiment. Of course this had to happen when I was way out at one end of the field over a large stand of pine trees as I climbed to do a half cuban 8.

The ESC powered down as I was pulling almost full collective, and I didn't have the altitude or height to auto back over the the safety of the field. If something bad is going to happen, it will usually do so at the most inopportune time. 

I later confirmed several voltage drops and then the big one that powered down the ESC by looking at the Castle Link flight log data (a wonderful tool). Thus the search & checks of all battery wiring and the connectors, only to find the very last one on the ESC was the issue even though it looked perfectly fine. I have since talked to three other people who have been experiencing power drops and coincidentally, all were using sprung sleeved bullets. Upon inspection, the same hidden carbon pitting was verified.

This is why I'm now converting all my EC5's over to XC90's. They are a "split pin" type so there is no hidden part of the pins and they have a larger contact area. I'm not saying the EC5 or sprung bullets are bad connectors, I have had nothing but good success with them up till now, and this is only my personal experience and a few others - totally anecdotal evidence.

If you use sprung sleeved bullets however, it would not hurt at all to pull the sprung sleeves off the male pins to inspect for hidden pitting underneath every now and then; especially if you are getting the odd power drop while pulling higher current loads during a flight/drive.

Only problem with that is once you remove the sleeve, it will generally be a loose fit afterward over the pin. Compression pressure when it's slid into the female end should then hold it tight & make good contact; but I would be somewhat worried about a subsequent increase in resistance.

So, think of all these larger RC LiPo battery connectors as "wear & tear" items and keep a watchful eye on them.

I say primarily the larger ones because they handle more current, are stressed more, and often arc when plugged in. You don't see the same wear & tear with a smaller RC LiPo connector for example in most cases. Some of my smaller Servo & JST RCY connectors are over 15 years old and working like champs still.

Regardless, replacing RC connectors when they start looking worn or are getting really "easy" (loose) to plug and unplug is cheap insurance considering what can happen if they go ignored!

Two other things I want to stress here:

  • Maximum current connector ratings are when the connectors are brand new; all RC connectors (especially the larger high current ones) will generate increasing resistance which decreases their current ratings as they are cycled over time.
  • The RC LiPo Battery Connector is almost always the weakest link between the battery and the ESC or the ESC & the motor. In fact, I actually prefer to solder my ESC to motor connections if at all possible. No question, I would like to do that between the battery and ESC as well, but that of course is not practical. The more connection points, the more weak links you introduce into your power system. This is why I strongly recommend against using converter connectors, as they introduce two or more extra connection contacts.

Learn to solder RC LiPo battery connector / s in other words so you don't have to waste money on "weak links".