External BEC needed?
 

The bec in my hobbywing stock spec ESC blew. I hooked up an external BEC and everything worked fine. So my question is: if the servo and the receiver and transponder can handle 7.4 volts, can I just run a wire from the battery straight to the receiver to run everything?

Best not to do that. The voltage from a fully charged 2S battery will be at 8.4V or higher.

Where do you think 7.4v as a battery specification comes from? Which battery is 7.4v specified? Right, it is a 2S LiPo/Li-Ion battery where the nominal voltage of a cell is determined at 3.7v. So yes, if electronics are specified for 7.4v then they wil work on a full charged 2S LiPo.

ESC BEC voltages tend to range between 6V and 7.4V. That's not just a nominal, that's a peak. Servos rate their operating voltage range, if they specify up to 7.4V they're not rated for 8.4V. The servos I tend to use, SRT 6012, have an operating voltage range between 6V and 7.4V. And not all servos are rated for 7.4V in the first place.

The 5v specification is very old fashion which comes from the 5v components of the receivers (74xx TTL logics) like the old Robbe Economic. Later receivers were using 40xx CMOS logic IC's which can handle with easy up to 7v.

The 6v specification was based on the 5 cell NiCad/NiMh receiver packs but those batteries in full fast charge can give up to 7.5v. Most servo's were using the ZN409 servo controler which was able to work up to 6.5v

The 7,2v specification for ESC's is based on 6 cell NiCad/NiMh batteries but in full fast charge they can have a voltage beyond 8v.

The 6.6v is the nominal voltage specification of LiFe but comes from the charger with 7.2v

And yes, the 7.4 specification is (again) the nominal voltage specification of a LiPo/Li-Ion battery.

There is a lot of confusion about 7.4v, if you do not agree my standpoint then you must explain where the 7.4v comes from. To set more strength to my standpoint is that 7.4v is also a HV specification and HV is also a name for 2S LiPo. And also explained LiPo 2S Battery 101: All About LiPo 2S Batteries (batteryequivalents.com)

Yup, I've done this myself just fine, in fact I've even used the balance port from a 4S pack to draw unregulated 8.4V however I found it necessary to only draw the positive lead from the balance port, it was necessary to use the negative lead from the BEC to prevent steering glitches as shown in the following thread:
https://www.rcgroups.com/forums/show...0#post40061288

https://c1.staticflickr.com/9/8797/1...b4f1c60e_c.jpg

Have you measured the voltage of a fully charged 2S lipo battery?

Well I tried it and it works fine. I would guess it's the same thing as 1/8 nitro because they use 7.4 receiver packs nowadays.

I know about nominal lipo voltages but servos etc aren't rated against a nominal, they have a minimum and a maximum defined.

Why would you measure that? On a stock charger that will be 8.4 up to 8.44v max because that is the charging voltage.

Then explain where the 7.4v comes from. On a daily base I do read many datasheets and no product is talking about something like 7.4v, if close then it is 7.5v.
So we do run 6v servo's (specs say 4.8-6v like you explain) on 2S LiFe -we even advise it- and even 5 cell NiMh that will start just as Life at 7+ volt.
You were talking that 7.4v is peak, no RC manufacturer does specify voltages with a peak moment. Absolute maximum? Yeah right.....
My Futaba receivers are also rated to 8.4v while the used voltage regulator can handle 10v up to 12v. No max voltage specification is an absolute maximum and still thats why I say the 7.4v stands for 2S so quit scaring people with your miss-information.

Like I said earlier, BEC outputs tend to top out at 7.4V maximum not nominal. They're regulated. Look at a servo data sheet, they tend to list the speeds at a few different voltages, and list the operating voltage range. Those are actual voltage inputs, not ones you translate after considering the max voltage of an equivalent nominal battery.

What do you think 8.4V volt actually means in this table if it's supposed to be scaled up from a nominal value?

https://cimg5.ibsrv.net/gimg/www.rct...78a6395b34.png
Advising people to run 6V servos direct from a 6.6V LiFe source is reckless as it's running out of spec and can shorten the lifespan.

Eh, even the tekin rx8 gen3 bec runs at 8.05-8.1ish volts despite being listed as 7.4

If your electronics can't handle straight lipo, put them in the garbage where they belong.

7.4 and 8.4 is the same thing. Every RC I have, has 8.4 to the servos. And has been since 2008.

as stated the lipo is 8.4v on a 2s at full charge some things can handle a full 2s lip some cannot. its also best to run servos off the BEC or External BEC in this case as it will supply a regulated voltage. if you power direct it will and can have a fade effect. can also leave you with no steering if the lipo voltage drops to much.

yes but it is unregulated. a regulated constant supply voltage is best.

also many servos are rated to only 6v not 7.4v or even 8.4v. 8.4V is actually not common

Cheaper and more reliable than a BEC is just using 2 diodes in series in the power cable, it will lower the voltage by 1.2~1,4 volts. Just find a diode that can handle 5A or more, otherwise a simple diode bridge rectifier is also a solution (if you look close it has 2 parallel lines of 2 diodes in series)

Yeah, well, the $20 regulators aren't exactly constant.

If you want to be that picky. Use a Hitec servo on straight lipo.....say 2000mah+ reciver pack. Low amp draw servos ain't going to make the supply bounce much.

It's just a complete non issue with quality gear.

All the rulesets I've seen ban running auxiliary batteries.

If you're running a 2s electric car, connect directly to the ESC's + solder post.

Cut the red wire in the ESC's servo wire, and move it to the solder post.

Been there done that. Many 1/8th E racers with obsolete 6v ESC's are doing this. Direct connecting to one of their 2s lipos (the one with the negative wire, for 8.4v, not the 16.8v side). So that they can properly power up a modern servo.

It depends on the applications. I have vehicles with multiple servos, multiple LEDs, winches, fans etc. BEC is the best option. Direct power is a good option as well but sometimes extra lipo pack is not easy to fit in the build

Depends on the car. But all of my underweight cars have ample room.

For 2wd buggy and such, no room needed. Just pull power from the main 2s pack. I've used an Rx pack for ballast weight. But it's kinda meh, now that Tekin ESC's have a 8.4 output. And lead weights are lower CG.

My E 1/8ths have room, but I only use a maxed out BEC built into the Esc, because they're over weight. Bad idea adding another component.

But a 50cc airplane, has a seperate lipo for the Rx, and often another for servos. Sometimes 2 batteries for servos. Some surfaces critical. Some aren't. Failed retract servos have killed many power systems.

Some guys use multiple BEC's, or complex power supplies. Some just use multiple little lipos.

None of the above is redundant. We just dont want 1 failed servo, browning out the whole aircraft.

We used to run tiny 800mah lipos in nitrobuggy. But since theyre lighter now, a 1500+ makes more sense.