I am building a hexacopter and will be using PixHawk as the flight controller and a Raspberry Pi for gimbal control as well as image processing. Now I see that I can use a 4s or 6s battery for powering my set up and a BEC can be used to convert the 14.8V/22.2V down to 5V for the electronics. Additionally, the ESC will use BEC to provide the correct voltage to obtain the required rpm of the props.

Now, from my research on BECs I understand that most use a resistor to reduce the voltage, thereby wasting energy in the form of heat. Based on this understanding, I feel that the 4s is a better option, considering total capacity is constant, for longer duration flight as in case of 4s the voltage drop required is less and hence less energy wasted. Is my understanding correct?

Second, is a resistance type BEC capable of handling 6S battery?

Further I understand switching BECs do not waste energy, so if I get a switching BEC which battery is better 4S or 6S, all other parameters remaining same and constant?

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There are linear BEC's that use a voltage regulator (and dissipate the energy in heat) for low current from a 3s battery and switching BEC's for higher voltage/current that are very efficient (90%+). 

Use a switching BEC sometimes called an SBEC or UBEC as they are the only practical way to get 5v from a 4s-6s battery. If you are feeding video equipment then an additional filter or a low-noise BEC may be necessary. 

Peter

V = IR (Voltage = Current * Resistance)

"or" I = V/R

It doesn't matter you use 4S battery or 6S battery or 8S battery. The current draw from a battery is always going to be dependent on the source which is drawing the current from the battery. Remember voltage stay the same regardless of the device unless it is also stepped down from a higher source.

That said the first thing your copter will use a voltage regulator, like the one that comes with Ardu and pixHawk. Why we use a voltage regulator to begin with, it is to provide a constant source of Voltage and current draw (that's why we go higher rating). So stepping down higher voltage and current gives you that consistency that your flight controller and ESC is looking for.

Secondly, all electronic devices (in 99%) cases have built in voltage regulators that even further reduce the +12V down to +5V or +3.3V depending upon if they are using TTL IC's or CMOS etc. on the circuit board.


So using an external UBEC (For Step down) is really not bad thing. Just make sure you check both voltage and current rating of the UBEC. No its not going to waste anything. Every electronic device dissipates heat, its normal.


There are BEC out there can reduce any voltage down to +12V or +5V or so on. Check Ebay, cost like $2

Here is an article on switch vs. Linear BEC Switch Vs. Linear BEC

Arunava,

the solution with a resistor would not provide a fixed voltage. You can hence assume that all BEC utilise a step-down converter. Their efficiency is in the 80 ~ 90 % range. I would look at their weight as most important factor. Mind you that the power-module for your Pixhawk does have a 5 V BEC on board. If you want to maximize on performance, check the hall-sensor version offered by craft & theory (MAUCH).

The cell-count depends on your setup. If in doubt, I reccomend going with 6S.

I have not understood what you meant. Could you please elaborate a little. Sorry, I am new to this thing.

Thomas Scherer said:

Arunava,

the solution with a resistor would not provide a fixed voltage. You can hence assume that all BEC utilise a step-down converter. Their efficiency is in the 80 ~ 90 % range. I would look at their weight as most important factor. Mind you that the power-module for your Pixhawk does have a 5 V BEC on board. If you want to maximize on performance, check the hall-sensor version offered by craft & theory (MAUCH).

The cell-count depends on your setup. If in doubt, I reccomend going with 6S.

Arunava,

for a start simply check whether the BEC you're interested in does support the required input-voltage (usually expressed in xS or for an 6S you want to see something like 26V). Two things to watch out for:

  • the standard power-module for the pixhawk does not support 6S batteries
  • do not use and not connect the ESC-BEC if they come with BEC. Else you end up with 6 power supplies which will lead to conflicts.

and again: if it is 5 Volts you're looking for - the power module provides for that already. You'll need a seperate BEC once anything requires 12 V (Gimbal etc.)

Additionally, the ESC will use BEC to provide the correct voltage to obtain the required rpm of the props.

Arunava, you are completely wrong.

The tension of the battery goes directly to the ESC and the required rpm to prop is not provided by the BEC but from the three fase drive of the ESC to the brushless motor.

So the tension you'll need to reduce from 4S or 6S  to 5 Volts it is just for servos, receiver or flight control board not for the ESC/motors supply.

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