# Where is the bottleneck for current in my system?

I have built a quadplane, and it works fine. However, after attaching a 1kg load to it, it is no longer able to take off. eCalc tells me that with my current configuration, I should be able to fly with a 1kg load. Here is the eCalc link: eCalc - xcopterCalc - the most reliable Multicopter Calculator on the Web

I’m trying to figure out where the bottleneck might be. eCalc says I should be able to draw up to 95A at max motor speed, but at full throttle I’m only pulling around 45A.

• Is my battery’s c rating too low?
• Is the BMS on the battery limiting the max current being drawn? The battery pack says its max continuous current is only 25A, which is concerning, but the BMS has never kicked in to cut off the power when the current gets too high.
• Is my wire gauge too small? I’m using 18 gauge for the motors and 16 gauge for the battery.
• Is my throttle being limited somehow in Ardupilot or on my transmitter?

Battery Pack:

Cells inside battery pack:

One bottleneck would be of course your battery.
If I look to your last picture you can be happy thar it only smeled some isolation and not burn completly. If the blank cans get contact you will get a shortcircuit which really can start open fire.
The Battery label shows a maximum continious current of only 25A, so why you believe you can use it upto near 100A. If you look to the datasheets of such single cells they also show a continous max current of only 20A-25A. For peak current (less than 1 sec) 100A is possible but not continous.

1 Like

That last picture is not the battery we use in the quadplane, its a duplicate that we took apart for testing.

The obvious answer is that the battery isn’t capable of supplying enough current, but why does eCalc not point that out? And what specifically in the battery is the bottleneck? Is it the BMS, the wires coming off the battery, or the cell chemistry itself? I’m just trying to determine if there’s a way to fix this without needing to buy a completely different battery.

Probably because you selected the wrong cells. On the cells of the opened pack it sure says 2500mAh and you selected a cell with 2500mAh, but on the unopened pack are 3500mAh per cell and 21500mAh total indicated. The cell you selected is rated for 8C/12C, your pack however is rated at 25A/21.5Ah = 1.16C.

If you choose a battery cell that is closer to your actual battery, like the “Panasonic NCR18650B - 1C/2C” (although it still hasn’t enough capacity), the new calculation is closer to your experience.

Mainly cell chemistry, although the BMS could be a little involved too. With lithium cells there’s always a tradeoff between energy density (high capacity per weight) and power density (high current delivery capability per weight). 18650 cells with 3500mAh are very much on the high capacity side, meaning they are terrible at delivering high current (as you have seen). If you want more power delivery (at the same weight), the capacity will suffer. Or if you want high capacity and high power delivery, you have to use a heavier battery (like a LiPo).

1 Like

Alright, sounds like we just need a higher discharge battery. We just ordered a 6s 10000mAh LiPo.