I had my first testflight yesterday, and a second one right after! Yay!
TL;DR
The good thing: I was up in the air and the BMS works as designed: keeping the Battery Pack secure!
I love the OpenPPG, it’s just so light, no stinky gas engine running idle when getting ready for launch, no torque, instant power on the press of a button, motors stop and don’t run idle when I let go off the throttle up in the air, no vibration!
The bad thing: I have to re-think a few things. The BMS shut off the battery three times. once on first launch (because of too high amps), and twice up in the air, because of battery temperature.
I am not mad, because I think I know how I can fix it, and these three shutdowns confirm that the BMS was the right choice, it works as designed, keeping the battery (and me, the pilot) safe.
Here’s the long story:
When launching with a paramotor on my back, I am used to go full throttle, even with my strong gas paramotor. I usually very quickly gain altitude.
Against my expectations from my post above…
… I am now thinking there could be a little more thrust, but it will work as well as is.
As mentioned above I had three shutdowns: once due to high amps being drawn out of the battery, and twice because of temperature. Here’s the details:
Problem 1)
After pushing gently, and then stronger, during launch, I did not feel any lift.
So I went full throttle for probably a couple of seconds (the BMS settings has a factory default of 5 seconds shutoff delay on exceeding the set Amperage maximum of 300A).
So it happend, the BMS shut down and I had to abort. The bluetooth App on my phone confirmed the shutdown due to too much Amps draw.
Possible solutions:
- add a second battery of the same size, so any power needed is devided between the two
- loose some weight (yeah, that would be awesome)
- reprogram the throttle controller, so that all the way down is equal to max 95%, or allow 100% for maximum 4.5 seconds, then reduce to 95% automatically
- not sure if it’s possible or advisable, but change the settings in the BMS from 5 second delay to 10 second delay before shutting off.
- a mix of the above
Problem 2)
The first flight was around 9 minutes, I did not gain much altitude (approx. 190 meters. Divide that by 3.3 for feet 
), because I did not want to go full throttle due to the experience above (problem 1). Flying was great. I really enjoyed it, until all over the sudden the motors went out.
The battery temperature reached 60° Celsius, and the safety measure kicked in.
I landed safely and checked the App again. The temperature of the BMS was fine, the four temp sensors that I placed evenly spaced in the middle of the pack however were showing 60°, 60°, 59° and 58°. I took off my gloves to feel the battery, but I felt… nothing.
I touched below, on top, on the sides, no heat at all.
And that’s the key takeaway here! I did insulate the battery too much, not only to protect all the terminals (which are all over the place on four sides) but apparently also thermally.
I wrapped the battery, mainly to protect it from shorting, with foamed rubber. I secured the foamed rubber with stretch film which I tightly wrapped around the whole battery. It did give a nice pack in the end, but apparently it was too much!
So even though the battery was at 60°C, I did not feel a thing. I then reached with my fingers underneath the wrapping (I left vent holes on the bottom and top), and there was the heat.
That was the moment were I knew that I had insulated the battery too much, and that I need to rethink securing the battery against shortage.
For the fun of it, I waited like ten minutes until temperature was low enough again (the ESCs at some point just start to beep again, which tells you that power is back and you can arm your ESCs again).
This time I was about 6 minutes in the air before another shutdown. The temperature had reached it’s maximum of 60°C again!
Possible solutions:
- replace the foamed rubber and shrinkwrap with a (plastic) case that has plenty of vent holes on all 6 (!) sides
- possibly add active cooling with fans
- add a second battery so that the load is evenly split between the two batteries
- don’t place the battery directly onto the carbon fiber backplate, leave at least 1cm space for the air to circulate
I’d like to add that changing the BMS’ settings for max temp of more than 60°C is NOT an option, because that’s the recommended maximum discharge temperature by the manufacturer.
My personal opinion: the thermal insulation was so good, that it’s no wonder that the pack overheated.
leaving 1cm of space for between the pack and the backplate, as well as leaving away the thermal insulation and replacing it with laser-cut acrylic sheets with plenty of vent holes on all sides should be enough.
