When a BEC is required

[gfmucci]

The theme of this post is to solicit recommendations for when a SEPARATE BEC should be installed.

The beginning example I'll use is a Castle Creations Sidewinder 3 ESC and a reasonably high torque Savox steering servo providing 222.2 of torque at 7.4 volts.

Interestingly to me, the graphic wiring diagram provided by Castle for the Sidewinder shows no separate BEC and wires the steering servo directly from the reciver.

Yet in many applications a separate BEC is said to be required, especially for servo torque in the 300 to 400 range. What is the threshold? If you could give specific component examples as a reference point, that would be helpful.

[EeePee]

Yes you need an external bec

[helhedded]

allthetime.gif

[waka789]

You always need an external bec it's just cheap insurance for $20 you have the peace of mind of that it won't go out taking out your speed controller.

[helhedded]

My first crawler 7LB SHAFTY used a Hitec645mg Torque oz./in. (4.8V/6.0V): 107 / 133

still needed a damn BEC

[helhedded]

My 2lb MRC comp crawler running a JR8711 347 oz-in @ 4.8V; 403 oz-in @ 6V

still needed a dang ole BEC

[gfmucci]

Thanks for all your answers that "yes", you need an external BEC all the time. But I am aware that:

• Most models of ESC don't have a "built-in" BEC for no reason, or because an external is ALWAYS required,
• Many systems satisfy the operators needs well without an external BEC.
• Not all operators run the snot out of their systems

So my initial post deals mainly with the question of what are the electrical thresholds where it becomes required to have an external BEC versus it being merely "insurance."

For example, in a 2S system with a 3300K motor when a servo draws more than X amps and the ESC has a 70 amp limit. Recognizing there are a number of variables in a system, what aggregate numbers within those variables scream, "hold on, this is beyond my capacity, you need a dog gone external BEC!"

ESCs with built in BECs don't typically include the BEC's output capacity in their specs, do they. From other sources I understand that most integrated ESC/BECs provide around 3 amps to run servos. Should this be my limiting guideline on when an external BEC is required, i.e. when I expect more than 2.8 to 3 amps to be drawn?

So then this brings up the question: How many amps does a moderate (200) to high (400) torque servo draw?

And yes, I reviewed the other threads and learned this:

For competion: one or more external BEC's are essential
For the newb: not so much
For those wanting to know when a BEC is most beneficial: It is good to know the electrical thresholds when circuits will be tripped under certain conditions without an external BEC. That's me.

[EeePee]

I ran my Losi with a Holmes controller and HiTec 7950 and some other mini digital (dig) servo with no BEC for a long time. Then when I needed it most, at a good sized comp, it went pop.



And then

[EeePee]

Originally Posted by gfmucci

ESCs with built in BECs don't typically include the BEC's output capacity in their specs, do they. From other sources I understand that most integrated ESC/BECs provide around 3 amps to run servos. Should this be my limiting guideline on when an external BEC is required, i.e. when I expect more than 2.8 to 3 amps to be drawn?

So then this brings up the question: How many amps does a moderate (200) to high (400) torque servo draw?

Yeah they do and it's typically about 2-3 amps (and can be 5-6 volts) at 7.4 volts with the typical 2S lipo battery. Volts go up, amps go down, thus the problem. 3S batteries are the norm for the most part. When crawlers stall their servo there's the huge amp spike and then the typical brown out. Or worse.

Spend the stupid $20 and buy a BEC. It's not that hard.

[gfmucci]

When a maximum amp draw of a particular servo is specified, what is a typical "spike" when stalled relative to its amp rating? Might it be its amp rating plus 10 or 20%, or might it be 200 or 300%?

Or behind curtain number 3, it varies all over the board and we don't really know.

[EeePee]

I'm going with curtain number 3.

For one, I never cared enough to get an amp meter and find out. Two, people that said they would never did. Three, there's no point. A $20 Castle BEC is cheaper than an amp meter and actually fixed the problem instead of just analyzing it.

[gfmucci]

Here are the amp draw specs of the Savox steering servo I am interested in:

Running current (at no load): 300mA @ 4.8V
Running current (at no load): 350mA @ 6.0V
Stall current (at locked): 2600 mA @ 4.8V
Stall current (at locked): 3200 mA @ 6.0V
Idle current (at stopped): 5 mA @ 4.8V
Idle current (at stopped): 6 mA @ 6.0V

Stall amps is roughly 6.5 times the running current (no load).

So, in combination with the shifting servo I need, I would be at the point of a BEC being required. That was easy.

[snapon boy]

On my sons dingo with the ae5 esc and the tatic servo that comes stock he killed 2 esc and 3 servos. Now he has a bec on the dingo and all is well till Murphy finds us again! So in short I thought if it was stock electronics he didn't need one. Now all 7 of my rcs have a bec

[Grizzly4x4]

Internal BECs are useful for go fast stuff where the tires are hardly ever on the ground.
Even then, an external BEC is better. My HPI mini trophy truck had really weak, slow steering. many of the other rally racers replaced their servos. I wired a Castle BEC to the stock servo and it steered much faster and better. I still use the stock servo today and have won races with it.

My touring car based rally car has an external BEC just so I know the steering is as good as it can be.

Rock crawlers always need an external BEC. It takes a lot of load off the ESC and will help RTR ESCs live a longer happier life.

The only time you shouldn't run an external BEC is if you feel $20 is too much money make your rig work better. Go ahead and justify your choice with amp ratings if you want, but your setup will work better wth an external BEC.

[EeePee]

Originally Posted by Grizzly4x4

Go ahead and justify your choice with amp ratings if you want

He's trying!

[Grizzly4x4]

Originally Posted by EeePee

He's trying!

His prerogative I guess.

[gdb85]

My 2 cents if I may,... a bec is a must in any crawler (comp or scale) the stress you put on an internal bec is overwhelming and it will be frustrating or impossible to enjoy.

You can get away without one in a go fast rig (EXO, SC, Yeti) if it's got the correct motor/ESC combo. You need very little torque or speed to turn the wheels since the front end is extremely light most of the time.

That being said, you will need a bec with a Savox servo, regardless. They are power hungry, voltage & amp robbing, evil little bastards!


Typing as Grizz posted, sorry, hate to be repetitive.

[EeePee]

Originally Posted by Grizzly4x4

His prerogative I guess.

Probably into helis.

[losikid]

Even if your esc built in bec can handle your servo, BUY A BEC.

If you constantly abuse the bec in your esc it will burn up and make your esc useless (your esc also needs it). Would you rather spend $20 on a nice bec or take the chance and ruin your esc?

Buy a BEC, if you're trying to save money, eat ramen for a couple days and spend that extra $20 on a BEC.

[Reddog95]

Question: When do I need a BEC?

Answer: As soon as you buy a crawler.