I use the Mauch power units. IIRC some of the cheaper generic pixhawk power units only can handle 4s, had to look at the ratings to make sure they could handle 6s (what I have). I used the 6s generic one for a bit but I learned about the Mauch sensors and how they’re higher quality and made the switch. https://www.mauch-electronic.com/
Can I connect my Multi-meter to measure max current? I know this is mad idea, but that will measure accurate value.
Yes, you can use a multi-meter to measure max current.
- Current measurement on most meters is 10A so dont use that.
- Clamp On Current meters have much higher Ampere ratings just be sure it can measure DC in your range
You can use the voltage setting of your multimeter to measure the voltage drop across a known resistance value. This is how current measurement is done using current sense resistors instead of Hall Effect sensors.
I = V/R
You can use many things as your resistor as long as it is low resistance milli-Ohms as it will get hot (P = I^2*R). Products use metal rods ( very large currents), large AWG wire, power resistors, current sense resistors, PCB traces, etc.
Pass a known current through a “resistor” at a given voltage and you can determine its resistance. (R = V/ I) Just be sure that it isn’t heating much as resistance changes with temperature…
Yes, you can use a multimeter on its 10A current measurement setting. But don’t use drone electronics as a load. At idle, in sub-1A consumption, the blinking leds will create enough fuzz so you have inaccurate results.
I use a pair of car headlamp bulbs in series as load, on my 6S setups, upto 4S a single bulb is enough. Will get current somewhere between 5 and 6A, which is a nice value to calibrate at. Keep the FC powered by USB during the process.
@ThePara Nice Idea, let me try that to calibrate my pixhawk power module, and if it will not give correct reading, then I shall measure it by DM( HTC DM 98 6000 Counts Digital Multimeter). Once i get accurate current then i am good to go deciding lipo or li-ion (18650 cell pack)
This page and video at the bottom explain calibration of the current monitor.
Note: Of course you can use your ammeter setting for a load under 10A. But… aren’t you trying to calibrate for expected flight? Thus figure out your minimum expected load and maximum both are likely to be above 10A and calibrate at those points.
If he is after maximum current draw at 100% throttle you are way over 10A coming from the main battery pack. Even measuring a single motor’s current would pop the fuse (hopefully) from these specs. here https://alpha-rc-heli.com/shop/tarot-4108-380kv-6s-multicopter-brushless-motor-silver/
Motors:6x Tarot 4108 with Tarot 1355 props
50% something around 36A (6A/motor)
100% 75A (12.4A/motor)
The part about how to calibrate the low end of the current range is great though. Old bulbs are awesome as a load but be sure you remember that resistance changes with temperature of the resistor AKA filament. AKA you cannot simply do what I described above.
This works if you just hover for a while to keep the current roughly constant.
Josh as you mentioned
'Motors:6x Tarot 4108 with Tarot 1355 props
50% something around 36A (6A/motor)
100% 75A (12.4A/motor)"
My hexacopter need max 75A.
So if we build 6S from " LG HG2 18650 3000mAh 20A Battery - LGDBHG21865"
We will have capacity of 18000 (3000*6) and it can give us current as 119.988A (C Rating 6.66).
That’s a 6S6P LiIo. 36 cells, yeah !?
The long-term trouble with LiIo cells is reliability. You can’t check individual cells in a 6P configuration, and if one goes down, it will affect the whole 6S chain. On the other hand, the majority of the LiPo packs are 1P, and you can measure cell internal resistance - most modern chargers do at the push of a button - so you know the quality of the pack, if it’s brewing for trouble and whatnot.
I’m flying drones comercially, for the last six years or so, and there’s no incentive in the equation to make me go LiIo. I’m still using 7-8 years old LiPo packs and I’m monitoring cell voltages using FrSky’s MLVSS to avoid any unpleasant surprise.
A few incorrect assumptions here:
- Estimates from motor manufacturer specs are a great first step, but there is no substitute for your own min. and max. current & power draw testing on your platform.
- As TheParra said you would have a 6S1P battery pack.
Voltage adds in series
Current and Capacity add in parallel, thus your battery pack is only 3000 mAh
You should use LiPo batteries until you gain a better understanding of the basics. Making battery packs is not easy and there are more pitfalls building your own.
You can check individual cells in a custom 6S6P battery to do so just takes more thought in the electrical aspects. Of course you would have 37 wires to measure each cell. A 6P pack is just 6 1P packs in parallel all you need to do is have separate monitoring per pack.
Sorry for delay in reply, here is what I worked on
My Hex Configuration
- Frame: Tarot 680 PRO Folding Hexacopter Frame
- Motors:6x Tarot 4108 High Power Brushless Motor (380kv)
- Propeller:3x Tarot 1355 Carbon Propellers (Two Holes / CW and CCW)
- ESC (Electronic Speed Controls):6x Hobbywing XRotor 40A-OPTO ESC
- Flight Controller:PIXHAWK PX4 2.4.7
Frame weight :2142g
Battery weight: 1627.2g (6S4P pack of “Milocel 21700 P42a 45 amps”)
Total weight : 3769.2g
Thrust to each motor : 628.2g
Amp draw : 18Amp (As per tarot (3Amp/650g )*6))
Flight Time will be
Battery capacity : 16800mAh
Average amp draw :18Amp
Flight Time : 56mins ((16800/18000)*60)
Theoretically are these calculation correct?
I know practically it will take some more/less avg amp to hover.
Looks good to me - just be sure to integrate the fact that you should probably target 80-85% battery usage during flights. So 0.85*56 = 47.5 minutes.
yes, thats enough. now i am getting only 10mins
Flight time will vary for the price of the advice
This computation looks ok from a quick check at 22.2V. But there is one glaring, item remember your voltage decreases over time… so hover at full charge (maximum voltage) is less current then hover at minimum charge (minimum voltage).
The current and hover thrust from the motor table is given at 22V.
Here is when you need to use power to get the current at minimum voltage. Depending on what you select as your minimum voltage say 3.2V/cell will change how far off your assumption of a constant voltage source is 15% approx. Also remember you will get less thrust as your voltage decreases so that adds in another item. This is why some motor manufacturers will give test data at variety of voltages.
^^^ I did not check the above. But you are likely within the maximum discharge rate of your cells.
If you had fewer packs in parallel this would be important to check say for a tightly optimized solar aircraft.
Remember to check the maximum power and current draws as well. AKA 100% throttle. It looks like you are ok on that front as well. On hover from these numbers you are right at 1C discharge rate.
If you are going to make you own pack be careful and be sure connections are mechanically strong and stress relieved. Test on the ground first! There are some good youtubers who make packs.
Get battery monitors! You don’t want to over-discharge these batteries. They make great fireworks…
If you don’t need to charge quickly try to stick under 0.5C charge rate you will get more charge cycles and less capacity degradation. Of course this varies per cell type and I know chemistry has changed since I last looked at long term studies.
My guess is 33 minutes Hover in place to 85% capacity. Voltage sag will not be like a Lipo so you will have determine from test flights what works. I use a 2S Li-ion on a small quad and the voltage drop is precipitous as it’s nearing 85%.
And your pack will take forever to re-charge
Yes, recharging will take time, and not like lipo. thats a concern
It will. Josh is right about .5C charge. That’s what I use on a 3500mah li-ion pack I have.
Yes, you are right, its hard to maintain and risky to make 6S6p.
But we can make two packs of 6S3P and connect them parallel.
All, I highly appreciate your time for detailed reply.
Many many thanks.
I shall keep updating and disturbing you all