How to know if a BEC is necessary?

[_Ben]

I'm planning to either build a new rock racer/crawler or upgrade the electronics in my RTR Rock Rey, but I'm confused about how to read the statistics of the various electronic parts.

Here are the specifics: I've read that Savöx Servos tend to draw a relatively large amount of current, and that you generally need a BEC for them to work at full efficiency. So let's take this servo as an example. It draws 7.4 amps according to the technical details.

However, A 30c, 5000mah battery could handle an output of up to 150 amps, right?

All the servos I've looked at, from ProModeler to Hitec and Protek, seem to fall within +- 5 amps of that, which seems like a tiny variance when compared to 150 amps.

So, naturally, I next wanted to figure out how many amps the rest of a hypothetical set of parts would draw. If the rest of the system's peak draw + the Savox's draw at peak load was > 150 amps I'd need a BEC, right?

So I tried using this formula with this Castle Motor and ESC.

The formula says w = (V - R * I_0) / Ke. Most manufacturers don't seem to publish resistances for their motors, but some forum posts suggested 0.0005 or 0.0008 ohms as a good estimate.

However, when it comes time to use the formula, I often seem to get nonsensical numbers. For example, if I plug in the stated maximum speed of that motor (60,000 RPM) I get a negative number for I_0.

Also, intuitively I would expect that a higher resistance would lead to more current being required to achieve a particular RPM. However, it seems that the opposite is true of this formula.

Given how hard it's been to figure this out, I'm guessing that this isn't actually the right way to go about picking parts or determining if I need a BEC. How do people actually go about determining what sort of electronics they need in a crawler without buying them first?

 

[Topher Builds]

For the savox servo, the output on the esc internal bec is what’s going to matter, not the battery.

Unless you’re using a direct battery powered servo (holmes v3 for example) the servo gets its power through the receiver from the esc output.

Most internal bec’s on escs are around 3-5amps so sometimes a savox servo or similar amp hungry servo can cause brown outs. In these cases a bec helps. In addition to amps, the voltage that you want to run your servo at also matters. Not all escs can run more than 6v so in these cases a bec can help.

 

[_Ben]

Thanks for explaining. That makes a lot more sense.

So if the ESC's BEC can be adjusted to up to 8 amps max, and the transmitter amp draw + Servo peak amp draw is < 8 amps, then there's no need for an external BEC, right? Assuming that you're fine with the ESC getting hotter than normal under peak loads?

And the motor doesn't factor into this at all?

 

[Xx Losungen xX]

You don't need a Bec with that servo and ESC.

 

[Topher Builds]

You don't need a Bec with that servo and ESC.

Agreed.

Thanks for explaining. That makes a lot more sense.

So if the ESC's BEC can be adjusted to up to 8 amps max, and the transmitter amp draw + Servo peak amp draw is < 8 amps, then there's no need for an external BEC, right? Assuming that you're fine with the ESC getting hotter than normal under peak loads?

And the motor doesn't factor into this at all?

Motor does not factor into it. That gets its power from the esc itself not the internal bec of the esc. That servo won’t constantly draw that many amps just when it’s really working hard.

 

[Inspector86]

Thanks for explaining. That makes a lot more sense.

So if the ESC's BEC can be adjusted to up to 8 amps max, and the transmitter amp draw + Servo peak amp draw is < 8 amps, then there's no need for an external BEC, right? Assuming that you're fine with the ESC getting hotter than normal under peak loads?

And the motor doesn't factor into this at all?

Generally speaking you cannot adjust amps on an ESC, In some cases you can adjust voltage. But you have to keep in mind that the ESC also supplies power to the receiver as well as the servo. If you up the voltage to supply a powerful servo you must verify that your receiver is also able to handle the higher voltage. If the receiver cannot handle higher voltage then you need to install an external BEC to supply the servo and power the receiver with the ESC.

 

[QuesoDelDiablo]

I'm planning to either build a new rock racer/crawler or upgrade the electronics in my RTR Rock Rey, but I'm confused about how to read the statistics of the various electronic parts.

Here are the specifics: I've read that Savöx Servos tend to draw a relatively large amount of current, and that you generally need a BEC for them to work at full efficiency. So let's take this servo as an example. It draws 7.4 amps according to the technical details.

However, A 30c, 5000mah battery could handle an output of up to 150 amps, right?

All the servos I've looked at, from ProModeler to Hitec and Protek, seem to fall within +- 5 amps of that, which seems like a tiny variance when compared to 150 amps.

So, naturally, I next wanted to figure out how many amps the rest of a hypothetical set of parts would draw. If the rest of the system's peak draw + the Savox's draw at peak load was > 150 amps I'd need a BEC, right?

So I tried using this formula with this Castle Motor and ESC.

The formula says w = (V - R * I_0) / Ke. Most manufacturers don't seem to publish resistances for their motors, but some forum posts suggested 0.0005 or 0.0008 ohms as a good estimate.

However, when it comes time to use the formula, I often seem to get nonsensical numbers. For example, if I plug in the stated maximum speed of that motor (60,000 RPM) I get a negative number for I_0.

Also, intuitively I would expect that a higher resistance would lead to more current being required to achieve a particular RPM. However, it seems that the opposite is true of this formula.

Given how hard it's been to figure this out, I'm guessing that this isn't actually the right way to go about picking parts or determining if I need a BEC. How do people actually go about determining what sort of electronics they need in a crawler without buying them first?

I think you might be getting volts & amps mixed up.

Volts are pushed and can be adjusted, amps are pulled and cannot be adjusted.


Basic rule of thumb to determine if you need bec is that unless you have a Mamba X or something else with an 8Amp onboard bec, you need an external bec.

If you have a HW 1080 you can run most servos with no external bec but not Savox, they need more juice.

 

[Greatscott]

You are overthinking this, a lot.

As mental practice, it is fun to play with the numbers to actually understand what is drawing what given voltage and such. But in practicality, just run by a couple of rules of thumb and you are good. And, sometimes those rules fail and you have to change... happens.

Generally, your servo is your big consumer of amperage. But, the consumption is not constant, and it depends on the loading of the servo; if it is bound up on the rocks, it will draw more amperage to try to move to where you are telling it to move. In most cases, if your servo is < 300oz, a 3A BEC will work just fine. If your servo is > 300 oz, then you really want more amperage. How much? The higher tear ESCs come with 8 amp BECs, and that should be more than enough for most servos out there. So, give the ESC you are running, along with the servo will tell you if you need an external BEC.

Traxxas XL5HV- You need a BEC. The on board BEC is only 1A. The Traxxas external BEC kit is nice because it is plug-n-play, but it is expensive and only 3A, but if you are running a 300oz servo, that is plenty.

HW1080- It has a 3A BEC built it. This is plenty for most applications, but if you start running more powerful servos, you'll want to install an external BEC like the CC-10A.

Mamba-X- It has an 8A internal BEC. If you over tax this BEC, you are running seriously monstrous servo.

 

[_Ben]

Thanks again for all the explanations, everyone. This clears things up a lot.