Important! Wire sizing for current draw


#1

This started in a build thread, but I think it should be made its own as this is an important issue.

I am concerned with the wire sizing being used with the smallest battery combo (12S1P - two Bonka batteries). It seems that many of these batteries shipped with 10AWG wire instead of the expected 8AWG.

If the current draw of this machine is really 300A peak at full throttle, then it is dangerous to be drawing that much for any length of time from a single 10AWG wire.

Take a look at this ampacity chart. The FUSING current for 10 seconds is 333 amps. That means the wire melts! We should design to be far far away from this number! The insulation will likely fail at a lower value.

The results could be catastrophic. I really think that the smallest battery combo (12S1P) is too small for this application. Adding another set of batteries in parallel will likely resolve concerns and halve the current (so long as the harness is sized right that connects them). It is reasonable for a customer to want to fly full throttle and climb for an extended period - a use case that is in the danger zone here.

I suppose the first step is confirming the true current draw of the system. Anyone have a real measurement?


#2

In Paul’s latest flight video, he was running two 22000 Bonkas, started at 100% and ended at 20%, on a flight that lasted about 10 minutes, but there were only a few times he climbed with full throttle.

Here’s the raw video: https://www.youtube.com/watch?v=ttfOoZFWsUU


#3

Right, but his batteries had 8AWG wire…

And he did not do extended full climb out. What if someone goes full throttle until they deplete the entire pack (might be only a few minutes!) on a 10AWG wire? Totally different scenario.


#4

Agreed with @jhair
In some other threads, there were discussions about prop types. The original parts were 22x10. For guys wanting more thrust, there was also a 22x14 option.
With the 22x14 props, the ESCs will pull over 110A each according to Paul’s measurements, so 440A total. With the 22x10 props, a figure of 330A was given. In the UK, there are not so many sites where it is possible to Buzz around at 10-20feet, so a climb to 1000ft is almost certainly a requirement, which is 120seconds at high/full throttle.
Paul has done the hard work in getting the project to where it is now, and I am sure that more users having first hand experience of the units will help to come up with a best solution for the high current supply and wiring.
Short suggestion is to test your anticipated flight thrust levels and durations, with the paramotor solidly mounted on the ground/rack, before taking off.


#5

Yeah, I’m also worried about this. I’ll be doing some thermal measurements, but this is something we likely want a much higher safety factor on. This is part of my rationale for moving to bus bars… but I fear the 10AWG on the batteries will be the weak link. This is a really complicated “swap” so I’m not sure how we best deal with this with Bonka. Field modifying the batteries is terrifying :slight_smile:


#6

Dave, are you running a single set of batteries (12S1P - two Bonka’s)?

If you go to 4, with a second set in parallel, the numbers get a lot safer.

Changing wires in packs themselves is sketchy, i would not attempt or encourage anyone to do lipo surgery.


#7

I’m running 4 Bonka’s. (12S2P). I still don’t like the wire sizing based on AWG specs tho!


#8

No answers yet, but we’ve got our best people on it :slight_smile: I’m now set up to do some of these temp measurements and look for hot spots. Tomorrow I’ll set up a test where I can run full thrust for a full battery and record temperatures.

I’ve disassembled the frame and just running the body/batteries at the moment:
FLIR00024

You can see the heat in the motor and motor controllers
FLIR00023

Running for now without props to make sure I can make a good setup
FLIR00019

More tomorrow… :slight_smile:


#9

Thanks Dave. Wow! These thermal images will be extremely useful. I am particularly interested in the junction area between the 10AWG wire to the Bonka cell(s). Are you able to image that area in your setup?


#10

Absolutely will be able to image the batteries and I plan to. I’m trying to sort out my strap down setup that can take the forces. I’m running without props at the moment, so these aren’t representative loads yet. Plan to look at:

  1. batteries
  2. wires to batteries
  3. my bus bar Distribution setup
  4. motor controllers
  5. all connectors and power switch
  6. and really all wires.

I’ll be sure to record ambient, so we can work out actual temperature rises, and I’ll document my setup and wire gauges.


#11

Awesome. Look forward to your work!


#12

Setup just about ready for testing… did a few quick spinups and blowing everything in my garage around :slight_smile:

Need to rewire a because my defective power switch is getting hot and seems to be the weak link at the moment.


#13

Dave: Nice setup and thanks for all your sacrifice (blowing everything in the garage) getting us the data.

I got my kit 10 days ago. All mechanical work is done and I am ready to begin the electrical assembly so in about two weeks I’ll join you guys in collecting performance numbers.

Its interesting you mention the switch. Even though its rated for 300A, in some instances we will exceed that limit.

I was pondering the possibility of adding another switch to the other side of the center plate since the design allows placement of this switch right or left – Two switches instead of one!.
One switch would handle power to the “right” bank and the other switch handles the “Left”. The Flight Controller (whatever it ends up being) would monitor the state of the two busses and only allow ESC activation if both are active. The proposal would still require 4 Bonkas (with inherent 10AWG concern) but would eliminate the switch limitation worry.

This is only an idea to maul on…


#14

Vic, do you have the specs for said switch? Sometimes there is a continuous rating and a peak rating. Maybe the peak 10 second value is high enough - assuming you don’t go full throttle for a long time?

There are actually a decent selection of switches made for marine applications that have 300A and above ratings.

It is worth noting that the XT150 connectors are only rated for 250 amps peak, too. This is another weak point. It could be addressed with 2 parallel strings as well…


#15

Jon:

No, I don’t have detailed specs. Wish we did though (I’ll continue searching). I am basing my comments of the embossed number in the switch itself (says 300A!). I am pretty sure the peak will be higher… but again, no details on Max Value or Transient Time yet.

XT150…You are correct… This is another issue to consider…

Thanks!


#16

So I did some more strap down testing today. Didn’t make it through a whole battery - probably ran for 3-5 minutes - before I ran into a different problem. Was running ~full throttle for the whole test. The motor got so hot it melted the nylon in the lock nut on one of the motors!

FLIR00043

Battery wires looked and felt fine:
Now it’s a hot day - ambient of 29C, so wires were like 12C over ambient.
FLIR00034

As did the distribution wires, bus bar and motor controllers:
FLIR00035

So I’m feeling OK w.r.t. to the distribution and battery wiring so far, but I no longer trust the nylock nuts - i’ll be adding a metal lock washer in there! Not sure what caused it - perhaps a prop started to slip and started spinning on the shaft? (I thought I had pretty high torque on that nut, but it was on one of the CCW spinning props, where the natural prop drag torque acts in the loosening direction of the nut) Perhaps this caused the motor to go to insane RPMs and heat up? I’m not sure. It was running at full throttle at the time. I heard a change and shut down the test.


#17

Nut coming loose would only reduce the load on the motor and reduce the current the windings see. I think it just got pretty hot :slight_smile:

Some of my drones use CCW threads instead of locknuts, maybe a better option for the future.

Maybe the openppg is not well suited for full throttle super long climbs (5 min full throttle is a lot of power!), but it looks like your setup is pretty robust. Great job on the IR stuff.

Did you happen to get any readings on the cell face?


#18

Nut coming loose causes prop to slip/stop spinning and airflow drops. Temp then rockets quickly. At least in my experience.

The real trick with prop mounts is not to use a single shaft and friction, but rather a multi-pin prop mount that can’t slip :slight_smile:

Cell face temperatures were barely above ambient.


#19

Dave:

I believe you are correct in thinking the prop may be slipping. The temp on the washer just above the prop appears hotter that even the area near the drive magnets (which is where I’d expect heat to originate from). This strongly suggest prop slip. But it also points to the need to have tightening direction of the prop nuts be such that self-loosening is prevented. The kit comes with four prop mounts but I did not check to see if two of them tighten in opposite directions.


#20

I can 100% confirm that there was prop slip. The prop mount “ground down” the back side of the prop and the prop washer made sawdust :). All nuts are std CW threads. So that’s an inherent weakness.