Yeah, I’m planning on a full micro on there anyways - monitoring current and voltages - so totally easy to disable the pre-charge relay on a timer once the rail is charged.
I haven’t seen many cheap/light automotive contactors that run at 12V and can do 300A in a small package - would love a reference if you’ve got it!
Leaning towards something like this to pass the bus bar through:
Which is $20 and has a ratiometric output and runs at 5V-ish which is great for the micro’s ADC.
PITA is driving the ~15W needed to drive the coil on the big relay - just means needing cascaded relays
The links I posted above is what I was referring to. They are in the $100 range and 500A rated. Weigh just under 1lb but have low current draw. That weight may be an unfortunately reality.
Hall effect current measurement would be really cool. Thinking out loud; you could add some logic such that the main contactors cannot open unless the current is less than some amount to avoid any risk of arcing. Maybe add some other switch that is an ‘E Stop’ that opens them no matter what in an emergency.
Yeah, I think the weight is what it is… I found one that I think is closer to .8 pounds, but it’s unfortunately thicker than the 35mm we have between the CF plates of the main body, which means it’s a clunky thing on the outside, instead of something nicely integrated.
We might be better to find some other mechanical failsafe and manual procedure for power up. Less than ideal, but adding a pound and an ugly mess on the outside won’t for fly for many folks I’d guess. Current measurement can fit inside cleanly, so that’s good.
Exploding bolts for emergency disconnects? haha I’ve always wanted to do something with exploding bolts, but perhaps this isn’t the project…
Perhaps worth considering replacing the contactors with some fat FETs in series, and put the whole thing on the controller board? You then kill many birds with one stone. FET RdsOn will be 2mOhms, so still some power loss, but 4 of them will give the start of the redundancy that is possible. Can design combined controller and power board if it is of interest.
Yeah, I’m starting to think that’s the way we’ll have to go to get anything reasonable in terms of size/price. Would allow a built in soft start as well. Getting this kind of current across a PCB will be interesting. But I think it’s the only way we’ll get things compact at all
Getting that current across a PCB will be okay, provided that it is split per ESC. You can get FETs for 2-3Euros that will handle the problem with a sub 2mOhm Rds.
Another thing that occured to me: If the reason that the mechanical switch is failing is solely due to arcing from inrush current, then what if only a small precharge circuit was added while keeping the mechanical switch?
Suppose only a small relay and power resistor were added with a switch. Cheap and light. You would throw that one first, and then close the marine switch.
Might make the difference in these switches surviving or not.
Yeah, separate pre-charge switch is what I’m adding to my system right now - step 1 It won’t be the nice long term solution and it won’t be fool-proof (i.e. people could forget to use it). It also doesn’t get us the safety that we might want in case of a short in the system somewhere and the ability to break the circuit in flight for safety.
Some fine suggestions in here.
I’m personally interested in killing throttle PWM signal to stop the props in case of a fault in the throttle (stage 1 emergency).
But also realize that isolating the power is important. It needs to be done easily, in-flight .
I’m thinking of adding additional link connectors on the main wiring harness. This additional link is attached to, say, a reserve handle behind your head. A solid yank on it will disconnect power (stage 2 emergency).
Bit low-tech but simple to implement and troubleshoot. Combined with a manual (press button) pre-charge resistor - Pilot has complete control over all systems.
I got a fresh switch and did some more testing. After a few actuations the pitting from arcing is already starting to show on the contacts. I also did thermal camera imaging of the components
Test Conditions:
25C ambient - all components (incl batteries soaked to 25C)
running from full batteries (which is lower current than at the bottom end of the batteries at full throttle)
open strap down test (so airflow to wiring is likely better than in the flight configuration)
ran for 3 minutes at full throttle
In this picture we see the two bolts coming out of the back of the switch, connected to my bus bar setup.
Observations:
the bolts were too hot to touch continuously (so over 50C)
you can see the battery side of the bus bar heating up and sinking heat from the switch
you can also see the body (plastic) of the switch heating up.
Conclusion: I think we’ve already mostly been suspecting this, but we really should find a new switch
Other conclusions:
10 AWG for distribution to the motor controllers seems fine and well within thermal limits
running the 4 Bonka config - no significant heat, even with the 10AWG wires coming from the batteries (vs the 8AWG we expected)
I would think that your aluminium bus bars would act as a heat sink and keep those switch bolts a bit cooler and that the current should flow good through the aluminum. However, I haven’t had any problems with my switch (yet ) which leads me to ask what is different. I used the same yellow crimp connectors that you did except I soldered them on and totally saturated each joint with solder. I bent them in an L shape and bolted them like flower petals to the switch. Perhaps all those 8 AWG wires attached to my switch are making a better heat sink. Hard to see in the picture because I covered it in electrical tape but here it is:
Also, I don’t think the 2 feet of wire that came with the kit was enough to work with so I bought some stiffer copper wire with fewer strands but still stranded for the inner workings (that’s why all the interior wires are black).
So, hopefully the same problems don’t develop with my switch.
Nice setup @GliderPilot. After my first switch failure I’ve been trying to beat it up - I don’t want another in-air failure, hence all this strap down and thermal testing. Ultimately I think we’re pushing it to its limits and it’s quite susceptible to wear.
You can see from the thermal imaging that the bus bar is sinking heat. The bus bars are way less valuable than I expected, so I’ll likely ditch them and do closer what you did.
Did you drill out those yellow connectors to fit on the power switch bolts? How did you bring in the power from the batteries? Some 8AWG crimps?
The yellow connectors that I bought were big enough on the bolt end to fit the switch bolts. The two red wires going into the switch are attached the same way and those connectors are also soldered on. The red ones are easier to see in the picture because there isn’t tape covering it.
I was looking at perhaps an added safety measure to the circuit in case something funky was going to happen. A device easily accessible to shut down everything. The curent switch is a nightmare to access while sitting in the harness.
haha I’ve always wanted to do something with exploding bolts, but perhaps this isn’t the project…
) which leads me to ask what is different. I used the same yellow crimp connectors that you did except I soldered them on and totally saturated each joint with solder. I bent them in an L shape and bolted them like flower petals to the switch. Perhaps all those 8 AWG wires attached to my switch are making a better heat sink. Hard to see in the picture because I covered it in electrical tape but here it is: