Important! Wire sizing for current draw

@GliderPilot, have you felt any of the connectors or specifically the switch bolts after a flight? Based on touch temperature, I’d say mine get to 50-60C or so after 1-2 mins at full throttle. Which means things inside are hotter.

Investors redone my config to look more like yours. Removing the bus bar as another variable.

From the balancing port to the other side of the switch (you’ll see a red/grey scope probe in the photo) I still see 300mV of drop at full throttle

I will do current measurements next. However, if this up near the 300A we expect it to be, it’s still like 90W going into heat, which will definitely melt some stuff. Any chance you (or someone else??) can do some voltage drop measurements?

Edit: so I measured the current using a LEM HASS-50. At full throttle its 75A per channel. So confirmed, we’re at 300A when at full throttle, so we’re definitely at the limit of the switch, and this confirms the voltage drop I see represents ~90W of loss

After a 20 minute flight yesterday I felt the switch and the wires coming from the switch and I could barely feel any warmth at all on the wires and no heat from the switch housing. I wasn’t able to reach the switch bolts though but I tried.

Are you using 10 AGW from the switch to the speed controllers? I’m using 8 AWG to the XT 90’s but then the speed controller wires came with 10 AWG.

Are you seeing less voltage drop without the bus bars?

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I do see less drop yes. About 10-20% less, but I’m also measuring at a different point. I’m using 10AWG to the XT90’s, but this is measuring before that. I don’t have any discernible heat on those wires. This voltage drop is just from the batteries to the other side of the switch.

I couldn’t feel the heat on the switch, just on the bolts.

Are those yellow connectors crimped or soldered?

On mine just crimped. @GliderPilot both crimped and soldered.

Hey guys. Did a bit of a wire resistance/power analysis. Trying to figure out if my measurements all check out and if the thermal situation I’m seeing matches the math.

I modeled the overall configuration as follows:

I’ve taken numerous observations - under both full load (4 motors @ 75A each), as well as with 1 motor spinning (just safer to take measurements :slight_smile: and the batteries last longer). I’ve used measurements from the battery balancing ports, as they should be good ohmic points for voltage measurements. I’ve also measured voltage drop across various components to estimate resistances. Also used wire resistance estimation based on cross sectional area and AWG charts for copper wire resistances (mOhm/ft).

I also confirmed the current through the system in the 1 motor max throttle config using a LEM HASS-50. It’s great at measuring the full transients, but I’m really just looking at DC RMS values here.

My tests have focused on the the power distribution from batteries to the main rail, and not yet out to the motors. My big concern was the switch. Results so far are:

Label Resistance Name Type Length (ft) mOhm/ft Current (A) Voltage Drop (mV) Value (mOhm) Power Loss (W)
1 Bonka ESR Battery 150 0.00 0.00
1.5 #10 WIRE Wire 0.5 0.9989 150 74.92 0.499 11.24
2 XT150 Connector 150 86.17 0.574 12.92
2.5 #10 WIRE Wire 0.5 0.9989 150 74.92 0.499 11.24
3 Bonka ESR Battery TBD 150 0.00 0.00
4 #10 WIRE Wire 0.5 0.9989 150 74.92 0.499 11.24
5 XT150 Connector 150 86.17 0.574 12.92
6 #8 WIRE Wire 1 0.6282 150 94.23 0.628 14.13
7 Switch Connector 300 128.00 0.427 38.40
8 Ring Terminals Connector 75 0.00
9 Wire + XT90 Connector 75 0.00
10 Wire + XT90 Connector 75 0.00
11 Ring Terminals Connector 75 0.00
12 #8 WIRE Wire 1 0.6282 150 94.23 0.628 14.13
13 XT150 Connector 150 86.17 0.574 12.92
14 #10 WIRE Wire 0.5 0.9989 150 74.92 0.499 11.24
TOTAL 874.63 5.404 150.39

Punchline: At peak current (300A), we’re putting 150W into wires/connectors. This doesn’t get include the ESR of the cells or the XT90s to the ESCs. Current is I2R, so at 1/2 throttle, this is more like 37W. At peak current, the switch loss is close to 40W! My switch is a bit warn, so a virgin one is likely better. Continues to point out to me that a pre-charge switch to extend switch life is going to be crucially important.

Extra punchline: the quality of your joints is SUPER important with this much current going through them!

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Here is a Flite Test video by Josh and David that I found several years ago when I started building quadcopters: Flite Test - Solder Thick Gauge Wire - FAST TIP - YouTube If you want to eliminate the connectors and solder the ESCs in, this method makes for a very strong solder joint and an excellent electrical connection.

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An interesting article on switch lubrication - to me it seems to contridict itselft at the end a little:
https://www.machinedesign.com/archive/lubricating-electrical-switches

The system draws over 300A steady when at full throttle. The switch being used is one of the few out there that can handle that current in any reasonable form factor. If you pay very close attention to the quality of your joints and minimize your wire lengths you should be ok with the current switch. We are continuing to search for alternatives.

Im still liking this one – think I’ll get this one and try it out for myself if I am seeing problems with the stock
image

Cheers

That weighs nearly a pound! Let us know how you manage to get it fit in

Yep - big sucker - I was thinking it would have to be mounted partially through the back plate with a bit of a stand off spacer.

Any thoughts on this switch?

https://www.amazon.com/dp/B07DDGLRBD/ref=sspa_dk_detail_4?psc=1&pd_rd_i=B07DDGLRBD&pd_rd_wg=NiNy9&pd_rd_r=V325SB3Y27ZEE8PMK0VR&pd_rd_w=PHU4e

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I looked at that one when I was looking for alternates. It’s unfortunately rather large.

I’m rigging up a pre-charge switch right now, which I hope will eliminate wear and tear on the main contactor. I’ve also tinned the copper contacts within the current switch with solder and filed them flat. I’ve got about 45 minutes of flying on that configuration right now, and when I hit 60 minutes I plan to do a tear down and inspect the switch. See how things look.

My other plan is to put 2 switches in parallel. So 1 pre-charge switch/resistor and 2 main switches. We’ll see how things look after a tear down.

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So you would possible go with 2 switches and precharge circuit rather than one larger switch! Sounds like over complication IMHO
Cheers

If you can find me a small switch that can get a safety factor of 1.5 of 2x on the 300A max draw I’ll take it :slight_smile: At the moment, the switch we have is the smallest and highest rated I’ve been able to find, but we’re using it at it’s rated limit and seeing issues. Right now using 2 in parallel is smaller and lighter than any alternative I’ve been able to find.

Hi Guys

I have been reading the thread and thought i would butt in.

The switch may be rated to 300A but this is more than likely at 12vDC. If you look at the data sheet for this switch from LittleFuse, you can see it can provide constant 400A @12vDC however by the time you hit 36vDC it has dropped to 200A.

The OpenPPG is running 48vDC if i’m correct. If we ahve a proper datasheet for the switch you also have the values for the peak running through the switch.

Good theory, but it’s rated for 60V according the various websites selling it. Haven’t found a “proper” spec sheet

So I just got the second set of Bonkas today… Lo and behold they have 8AWG wires on them…


They also came in some nice shipping caddies:

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After about 100 flights my switch finally melted like yours. I was 3 minutes into a flight at full throttle and 850 ft high when it quit. So I was plenty high for a safe landing.

So now I’m looking at more reliable options… possibly a precharge button and two switches. Is that what you ended up doing? Can you share details? Also curious about the tinned contacts after 60 min.