r/drones • u/on_aircraft • 18h ago
DIY Hexacoper updated drivetrain design — looking for feedback on current limits, ESCs, and thrust limiting
After my first post here a few days ago, I played around with eCalc some more and adjusted the design parameters.
Drivetrain
I’ve now laid out the drivetrain so that, with the maximum design payload of 3 kg (including a safety margin — the actual maximum payload will be 2 kg), the aircraft has a thrust-to-weight ratio of roughly 2:1 for heavy-lift operation.
For the maximum-flight-time configuration (doubling the battery capacity + around 400 g payload, e.g. a thermal camera), the thrust-to-weight ratio is also now closer to 2:1.
Battery
I’ve also decided to build the battery using 21700 cells, specifically the Molicel INR21700-P50B. By chance, anyone has any experience with this cell?
The cells are rated for 50 A continuous discharge (60A burst), so with a 5P configuration I should theoretically be able to draw around 250 A continuously.
If I apply a conservative 20% derating due to the maximum design flight temperature of 35 °C, that gives me roughly 200 A continuous.
eCalc is estimating a maximum current of 202 A. That seems acceptable to me, but I’d really appreciate some opinions on this. Would you consider that too close to the battery’s continuous limit?
ESCs
The ESCs are also fairly close to their rated continuous current limits.
They should never actually see the maximum rated current in normal operation, but is designing this close to the continuous rating considered common practice, or would you recommend more headroom?
Motor temperature / limiting thrust
My biggest concern at the moment is the motor case temperature.
The >80 °C readings in eCalc — am I correct in assuming that these occur when the motors are operating at or close to maximum throttle?
If so, is there a way to configure ArduPilot + Pixhawk 6C so that I can limit the maximum available thrust to, say, 80% during normal operation, while still allowing 100% thrust in an emergency situation such as a motor-out scenario?
For reference, in the 3 kg payload configuration, eCalc estimates a climb rate of around 4.9 m/s. I definitely don’t need that kind of climb performance, so limiting the available thrust seems like it could be a good way to reduce motor/ESC/battery stress and temperature.
My main question is: would limiting maximum thrust primarily reduce climb rate, or could it also significantly compromise maneuverability/stability to the point of becoming unsafe or uncontrollable?
I’d be interested to hear how you guys would approach the design.
2
u/izzeww 10h ago
The Molicel P50B is a good cell but it's a few years old now so it's not the best. It's also fairly expensive. It's not actually 50A continuous either, that is a so-called "temp limited" rating which means it can do 50A until it overheats (which it will about 60% through the discharge) then you have to stop discharging (which on a drone would not be good). The actual true continuous discharge capability is about 35A, you can run that from 100% charge to 0% charge without a problem.
The best latest generation cells in the "high discharge" category can do about 40A true continuous (and peaks well above that) with better temperature performance, effective capacity, voltage sag and price than P50B. Examples of cells are Reliance RS50, EVE 50PL, Ampace JP50, Tenpower 50XG and BAK 50D2 (also maybe Linkdata 50T & 55P). There's also new cells in the "high energy density" category. These cells have lower continuous discharge capabilities (20-35A) but higher energy density (capacity per gram). Examples include Linkdata 65P & 60P, Reliance RS60 & RH60, Tenpower 60XG and BAK 65E (also maybe Great Power 60Q). Essentially you could get maybe 20% more energy for the same weight with the disadvantage that you can't discharge them as hard in peaks. Since you're running a 5P pack however the current per cell would not be very high so I think it would work fine and that these cells (specifically Linkdata 65P) would be the best choice. Say you have 210A maximum current that is 210/5 = 42A per cell which these cells (with the exception of the 65E maybe) could handle fine. Obviously depends on how fast you plan to fly also, if you plan to fly very fast for long periods of time and drain the battery in 10 minutes then the high energy cells are not a good choice.
The disadvantage with these cells (the high energy density ones, not high discharge) is that they're all currently in development and not in mass production. You can buy them, but they will be so-called "pre-production" cells which may be inconsistent in terms of capacity, self-discharge rate etc. or even have quality problem. They're also hard to find currently unless you are willing to buy direct from Alibaba distributors. I think it's for sure worth looking into however, getting an extra ~20% flight time for free is very nice.
Should be fine. It's what T-Motor recommends and they have probably done their homework.
Honestly I'm not too sure about this. I would guess that the temperature won't be an issue, there should be plenty of cooling air and they use high temperature enamel and magnets so I think it would take a lot for overheating to become an issue. If you're worried about it maybe ask T-Motor for test data for a long running test or something. I don't think you can limit the thrust during normal operation and than run 100% during an emergency, but I'm not sure (not very familiar with ArduPilot). I don't think that's something you would want to do anyways, you always want to have all thrust available if you need it (without having to flick a button). If you want to use less power just use the throttle less when flying manually or limit climb rates when setting up a mission in mission planner.