| It doesn't have anything to do with the cell count, it has to do with how
| the voltage is controlled to your RX.
Well, yes, but the higher the cell count, the higher the voltage, and
the higher the heat released in the BEC ...
| Almost all ESCs are limited in the amount of current they can supply
| to your servos and RX, and pulling too much current will overheat
| the BEC portion of your ESC.
True, except for the `almost' part. :)
Here's a little math that might make things a bit more clear.
Suppose you're putting 12 volts (from a 3 cell LiPo pack, if you care)
into a BEC/ESC, and suppose that the BEC puts out five volts to your
RX and servos.
If your servos and RX are drawing 500 mA, that means that your BEC is
converting (12v - 5v) * 0.500A or 3.5 watts of electricity into heat.
That may not seem like very much, but it's probably all coming out of
one chip and it may not even have a heat sink. To compare, your
soldering iron may use 25 watts, but the tip is probably larger than
the chip on your BEC, and it's not insulated by anything, and consider
how hot it gets.
(Of course, to be fair, 500 mA is a bit on the high side for average
current draw on a small plane.)
The other source of heat in a ESC is the ESC itself. If we look at a
Sprite 25 brushed ESC
, it's rated at 25
amps/40 amps surge and the rated resistance is 0.0025 ohms. Heat
produced = I^2 * R, so for 25 amps that's 1.6 watts.
... which seems too low. I suspect that this internal resistance
figure is the lowest possible value, when the ESC is at 100% output.
As you lower the throttle, the effective internal resistance probably
increases signifigantly. Certainly, in most cases an ESC will get
hotter at 50% throttle than at 100% throttle.
In any event, if you wrap the ESC in foam, that heat will have a hard
time getting out, and it'll get hotter, and hotter and hotter --
probably until something fails.
Airflow is your friend. Your RX doesn't dissapate much power at all,
so cooling isn't needed. Your battery, motor and ESC are not so
lucky, and you'll definately want cooling for them, and may even want
to design your plane so some air flows over them. (Your servos
probably don't dissapate too much power so they probably don't get too
hot, but they do probably use a lot more power than your RX, so while
you probably won't have to go out of your way to make sure they get
good airflow in most cases, you probably don't want to wrap them in
Note that there are some BECs out there that do not use a simple
voltage regulator and instead have a full switching power supply.
While these still aren't 100% efficient, they're far more efficient
than the standard voltage regulator type BEC, and so the math I gave
above doesn't apply. But this sort of BEC is usually not included in
a ESC -- it's usually a seperate unit, and somewhat expensive at that
-- so unless you know otherwise, you should probably assume that your
BEC is of the more common voltage regulator variety.