wasted watts equals I² x R.
Because losses increase with the square of current, doubling the amps
(as you’re doing if you run a 6-cell pack instead of a 12S pack) carries a
hefty penalty in lost efficiency, and this penalty gets larger still if your
motor has relatively high resistance or if you’re pushing the capacity of
your wire or connectors.
Table 1 shows some realistic
figures for power losses
for 360- and 600-class
helicopters. The 360-class
heli is shown with 3S and 6S
battery packs, and the 600 is
shown with 6S and 12S packs.
This table gives a lot of detail, but the important numbers are in the far
right column. The point of the table is this: Even allowing for thicker,
heavier wire and the typically lower internal resistance of high Kv motors
and large LiPo packs, the high-current setups generate 60 to 70 percent
more waste heat than the high-voltage alternatives. Furthermore, this
is actually optimistic for two reasons: First, this table doesn’t capture
the increase in resistance as things heat up during flight. Second, the
Scorpion motors I used as examples are some of the best on the market;
a cheaper, less efficient motor would do much worse.
So what can you take from all this? Voltage is your friend. Given the
choice between high amps or high volts, the latter wins almost every
Battery Pack Motor Setup Parameters Resistance (Ohms) Waste Heat*
Battery Wt (g) Motor Kv Amps Duration Wire Gauge Rm Battery ESC Wire Ω/Ft. Total Res (Joules) ( Watts)
3S2200mAh40C 204 HK-2221-10 3000 40 3: 18 12 0.031 0.012 0.003 0.0019 0.0493 16,578 79
6S1300mAh35C 234 HK-2221-1630 1630 20 3: 54 14 0.079 0.030 0.003 0.0030 0.1175 9,873 47
6S5000mAh40C 836 HK-4025-1100 1100 80 3: 45 10 0.008 0.012 0.001 0.0012 0.0234 31,396 150
12S2650mAh40C 936 HK-4025-630 630 40 3: 59 12 0.024 0.030 0.001 0.0019 0.0589 19,804 94
Joule’s Law and High-Power Helicopters
Even if you’re running high voltage, you should make every effort to keep power leads as short as possible and to
use high-quality connectors. Note how I configured the Castle Edge HV 120 speed control on my Gaui R5 with an
integral-series wiring harness rather than following the common practice of making a separate harness. I’ve also
used high-quality Castle connectors to keep resistance to a minimum.
Wire Gauge Resistance (Ω/ft.)
This table shows side-by-side comparisons of equivalent power systems for 360- and 600-class helis. The low-voltage setups (3S for 360 and 6S for 600) have
resistance losses 60 to 70 percent higher than the high-voltage alternatives.
This table shows the significant
reduction in resistance with heavier-gauge wire. The problem is that heavier
wire is just that: heavier. Remember
that resistance is a function of gauge
and length, so you should keep battery
leads as short as practical, particularly
for high-current systems.