BECs burning, wiring advice?

[thisrealhuman]

First, the obligatory thank you to everyone in this forum. I've been lurking and learning for probably three years and I wouldn't have made it this far in my build without you, so thanks!

Build Specs:
2x 5s Lipo Zippy Compact 5800mah (used in series, charged in parallel)
120A K-Force Opto ESC
245KV Eflite Power 160
ADS UST Mount 12T/36T
77mm Speed Vent (115mm Tom Shanahan Inc. Powerboard wheels shown at bottom)
GT2B with Badwolf v2 (will be posted when complete)
Castle Creations 20A BEC PRO (dead)
CNC Skateboard LED Riser pads (http://cncskateboards.com/led_skateboard_lights.htm They're spiffy! I'll get some video and full review once my power is back.)
Pelican Micro Cases 1040 and 1060 (http://www.pelican.com/us/en/product/watertight-protector-hard-cases/micro-case/standard/1060/)

Now here's my problem; My xt90 loop key rattles loose and falls out cutting my battery connection, but when moving at speed the motor creates energy that feeds back through my ESC and only has one place to go; into my BEC. I've killed two BECs this month and I'm looking for a better wiring solution. Relocating the loop key will happen too, but that will be a side effect of better wiring.

(I don't understand why some images are showing and others are links)
Here's my wiring diagram:



And the actual wiring (sorry for potato quality):



When I had a non-OPTO ESC (Turnigy AE-100, VERY BAD), the disconnected battery would leave nowhere for the power to go in the event of a disconnect, and the power would loop back into the motor and cause it to lock. With an OPTO, the same is true, but having my BEC connected means the power has a place to go, so the motor won't lock until after I blow the input cap on my (one week old $45) BEC. (mumble, mumble, false economy...)

After my loop fell out, I tried restarting the system and noticed my lights came on and wouldn't shut off using their individual power buttons, and while I was trying to turn them off, my antispark resistor burned up. I assume this was because the input cap on the BEC had already popped and was sending more power than it should have been. My lights run on 4.5v (3 AAA's), so I'm using the CC BEC Pro default power of 5.1V (9A) to power them and my receiver (GT2B). My loop key wasn't in and the board was not moving when all this happened.

I went with the Turnigy K-force because I feel more secure with a DRAG brake. Future builds will use VESCs, but I'm not ready to open that can of worms until the product is a little more polished.

I'm going to get some fuses to put inline near the batteries and just before the BEC. I'm not sure if I trust a simple button and relay, so the loop key will remain king. New enclosures will be fiberglass or carbon fiber once I have all my components and wires figured out. Those wires are sealed to the case with two layers of shrink tube and Shoe Goo. I can use a razor to remove all of it and have already done so three times, everything is temporarily permanent.

So here are my actual questions:
Is there a specific BEC for 10s lipo that I should look into?
Other than adding fuses, is there a way to wire this with an automatic disconnect when the loop is out?

Extra shots:


View attachment 2

 

[Grizzl-E]

Just a thought here but your bec may simply be too low of voltage for the application. If the cap is popping it's likely just not rated for the voltage being produced by the back emf. It fits if your lights were getting higher voltage as well, the bec is a switching power supply. So one of the fet's is likely locked on, one of the common possibilities when they are destroyed. Easiest thing is just to make the loop key never come out. Or use a higher voltage vreg for a bec. Or switch to a controller that limits the regen output. Or put a separate switch for the low voltage circuit and wire it upstream of the loop key. Or a couple other things, but they are all to fix the bec's low input capacity.

Edit: actual help - vreg good to 60v input http://r.ebay.com/pz8N2I

Btw that board looks sick! I love the detail you put into it.

 

[thisrealhuman]

I'm using 10s, the BEC pro is good for 12s. It came with a set of wiring diagrams, one of which is to connect to only ONE of the lipo packs, but I didn't want to run extra wires and also I was worried it would throw my packs off balance.

The other BEC I burned was a Dr. Mad Thrust Dual output (11s). http://www.hobbyking.com/hobbyking/store/__34852__Dr_Mad_Thrust_series_5A_HV_BEC_with_Inbuilt_Aux_Controlled_On_Off_Switch_for_Accs.html

I was only using 5v for my receiver at that time and since they both burned when my key fell out i figured that was the cause. initially, I thought it was just over heated since it's in an airtight case and there's really no heatsink. That's why I went with the Castle, it has a heatsink.

Would it be feasible to use a set of diodes with a loop back that charges a secondary cap bank? So that current flows into the Esc through the diode on one wire, but when braking or just rolling while disconnected the flow goes into the cap bank and is regulated back into the battery? The BEC would be on the input wire before the diode. This is dependent on where the current flows during regeneration; is it on the positive or negative wire?
My first ESC caught fire while drag braking down a small hill, but my timing was set high and my PWM freqency was set to 12khz. I'm always worried that regen braking will kill my setup.

 

[thisrealhuman]

...
Edit: actual help - vreg good to 60v input http://r.ebay.com/pz8N2I

Btw that board looks sick! I love the detail you put into it.

Thanks, that will work out nice for my extra lights as well, and much cheaper than my current ideas. And thanks again, it makes me happy. the green on top is flourescent, I have blacklight leds that i'm putting in my badwolf controller so the top glows at night. I ride in the bike lane, 24mph is my current top speed.

EDIT: That converter is only 3A. I'm sure the leds are using less than 1A, but most BECs have a 5A output, there must be a reason? will my GT2B receiver pull enough amps to kill it?

 

[Grizzl-E]

I have a couple of those receivers, they pull a fraction of an amp. The cap idea is neat but the size you'd need wouldn't fit on your board, think this size:



Caps hold a fraction of the capacity of a battery. You could put a graphene lipo in as a cap, they have very high c ratings (and the graphene is just a substrate for the silicon anode but they are calling them graphene). But again your kinda fixing the wrong problem.

There are a ton of options for v regs, check aliexpress too if you want a little higher amperage or voltage. Just search dc to dc step down converter. The bec's need to power servos so they have larger capacities.

Edit: btw don't your feet go numb at those speeds? Or is the pavement where I live just that bad. I've disassembled my board cause long distances were intolerable.

 

[thisrealhuman]

My toes do go numb sometimes. On a smooth road ill be numb after a mile of 15mph or more, on a rough road i tend to shift around more. My big wheels will be much nicer, they are the reason i went 245kv and 1/3 drive ratio. Im going to a wide belt before i get to those.