RX Batteries/HV Servos/Voltage Cutoff/BEC/Electricity?

[WilliamM]

Been enjoying my 1/8th scale Kyosho Inferno Neo Race Spec RTR nitro buggy for almost a year now and have decided to upgrade my servos. I want to go with two HV servos, probably savox for the steering and throttle. This desire has led to a whole host of questions for which I need your help (my understanding of electricity is pretty limited):

1. What is the amp draw of a typical HV servo? (and for that matter what is an amp?)
2. I am having difficulty finding an RX battery that will fit in my battery box. One that I think will work is the Venom 5C 2S 2100mah 7.4V LiPO. From what I have read this battery will put out a consistent 10.5 amps, will this be enough to power two HV servos?
3. Is a BEC the same as a voltage regulator and do I need one?
4. My LHS recommended the Castle Creations BEC, is has a maximum of 10amps, will this provide enough power for the two HV servos?
5. The Castle Creations BEC does not have a voltage cutoff, how do I protect my RX LiPO battery from over discharge?
6. Does anyone know of a BEC/voltage cutoff combo unit, or is there a separate voltage cutoff unit and would I install it between the RX battery and the BEC?
7. I just purchased a Kyosho starter box, and was planning to power it with LiPO batteries. I see in this forum people are recommending a 4S LiPO in the starter box, so same series of questions. Do I need a BEC and voltage cutoff? And does anyone have a recommendation?

I am going to replace the stock RX/TX with my Spectrum DX3, so the RX will be able to handle the 7.4V power.
Thanks, Bill

 

[hamz9561]

I don't know the current draw off the top of my head, but the HV stands for High Voltage, so your servos shouldn't need a voltage regulator running a 2s LiPo. A BEC is normally used to bypass an ESC in an electric vehicle.

 

[Greywolf74]

1. What is the amp draw of a typical HV servo? (and for that matter what is an amp?)

I can't tell you exactly and it would vary from model to model. I can tell you that a 228oz/in servo at the standard 6V draws roughly 1.1Amps. At least the ones I use. Even if you doubled that for HV servos your still well below 10As. As for what is an Amp... Well think of electricity like this Voltage X Amperage = Watts. The Amperage is like the opening on a Garden hose or how much water is coming out of it. Voltage is like how much pressure the water is being pushed through the hose. Watts is the measurement of total power output.

2. I am having difficulty finding an RX battery that will fit in my battery box. One that I think will work is the Venom 5C 2S 2100mah 7.4V LiPO. From what I have read this battery will put out a consistent 10.5 amps, will this be enough to power two HV servos?

Yes it should be fine for 2 servos and your RX if you wanted to but most RXs dont like more than 6Vs. If you want to run your RX from the lipo pack also I would just order standard 6V servos it will make wiring everything up much more simple. You can get 6V servos all the way up into the 400+ oz/in range.

3. Is a BEC the same as a voltage regulator and do I need one?

A BEC takes a certain amount of power say if you attached it to a 14.8V 4S Lipo and outputs a specific Voltage usually between 5 and 6 volts so you can power your RX and servos without melting them with the full voltage of the lipo. In the case of the CCBEC its settable via an optional PC "link" card to anything from 5V to 9.5V if memory serves. Do you need one? Well that depends. Are you going to only power the HV Servos or are you going to power your RX as well? If your only gonna run your HV servos off of it and they can take as much as 8.4V then you dont need one. If you want to power your RX as well then your going to need one for the RX and the Servos can run from stright off the lipo. This starts getting a little tricky to wire but sounds harder than it really is.

4. My LHS recommended the Castle Creations BEC, is has a maximum of 10amps, will this provide enough power for the two HV servos?

The CCBEC is a nice one but a tad pricey IMO and you'll have to have the "Castle link" in order to change the voltage settings on it. The Castle Link I believe is about 20 bucks and the BEC itself is $25 again going from memory.

5. The Castle Creations BEC does not have a voltage cutoff, how do I protect my RX LiPO battery from over discharge?

You can do an ebay search for "low voltage alarm lipo" and pull up all kinds of small alarms that plug into the balance port of your lipo and will start going off when the voltage is getting low. becareful though, they can be had cheap and theres nothing wrong with the cheap ones but the really cheap ones come from china so becareful where you order from if you arent willing to wait 3 or 4 weeks for it to arrive. Make sure you find a US seller if you want it quickly. You LHS most likely has them too, probably for an inflated price.

6. Does anyone know of a BEC/voltage cutoff combo unit, or is there a separate voltage cutoff unit and would I install it between the RX battery and the BEC?

I'm not aware of one but that doesn't mean it doesn't exist. the much simpler and probably cheaper alternative is the above mentioned CCBEC and a low voltage alarm.

7. I just purchased a Kyosho starter box, and was planning to power it with LiPO batteries. I see in this forum people are recommending a 4S LiPO in the starter box, so same series of questions. Do I need a BEC and voltage cutoff? And does anyone have a recommendation?

No you do not need a BEC. 4S is the right size for the job. You can also use two 6 cell NiXX stick packs ran in series also. This is assuming your bump box uses a 12V lead acid battery to begin with. some of the newer boxes use other size packs so youd have to know what the boxes native voltage is. Again a low voltage alarm can be used for this or you can periodically just check the voltage with a volt checker since a bump box isnt something that sees alot of continuous use.

Hope this helps

 

[SchrodeMode]

2. I am having difficulty finding an RX battery that will fit in my battery box. One that I think will work is the Venom 5C 2S 2100mah 7.4V LiPO. From what I have read this battery will put out a consistent 10.5 amps, will this be enough to power two HV servos?

Yes it should be fine for 2 servos and your RX if you wanted to but most RXs dont like more than 6Vs. If you want to run your RX from the lipo pack also I would just order standard 6V servos it will make wiring everything up much more simple. You can get 6V servos all the way up into the 400+ oz/in range.

3. Is a BEC the same as a voltage regulator and do I need one?

A BEC takes a certain amount of power say if you attached it to a 14.8V 4S Lipo and outputs a specific Voltage usually between 5 and 6 volts so you can power your RX and servos without melting them with the full voltage of the lipo. In the case of the CCBEC its settable via an optional PC "link" card to anything from 5V to 9.5V if memory serves. Do you need one? Well that depends. Are you going to only power the HV Servos or are you going to power your RX as well? If your only gonna run your HV servos off of it and they can take as much as 8.4V then you dont need one. If you want to power your RX as well then your going to need one for the RX and the Servos can run from stright off the lipo. This starts getting a little tricky to wire but sounds harder than it really is.

GW, the SR300 is the rx that comes with the DX3. It's rated up to 9.6v, are people having problems with it on the 7.4v? I'm not trying to start anything, just trying to figure it out as well, I'm about to order a new rx battery and was planning on going lipo.

 

[Greywolf74]

I'm not familiar with that one bro. I dont use one. If its rated for 9.6V it should be fine. I've only used a couple of TX/RXs. Thats why I said most. But really its technically unfair of me to say that because I dont have personal experience running HV on most RXs. That is technically an assumption on my part based on my limited experiences and stuff that I've read.

 

[WilliamM]

Hey greywolf74, awesome response. Thanks for all the help. I'm off to get some new servos and see if I can put this plan into action.

 

[SchrodeMode]

I'm not familiar with that one bro. I dont use one. If its rated for 9.6V it should be fine. I've only used a couple of TX/RXs. Thats why I said most. But really its technically unfair of me to say that because I dont have personal experience running HV on most RXs. That is technically an assumption on my part based on my limited experiences and stuff that I've read.

I was just making sure, most of the Spektrum rxs (that I've seen) are rated for 9.6v. Like I said though, I just wanted to make sure you hadn't heard of them failing, even though they're rated to handle it.

 

[Greywolf74]

Thanks man. I've never used the spektrums yet. good info to know

 

[hamz9561]

I have four of those RXs and have never had a problem with them.

 

[SchrodeMode]

I have four of those RXs and have never had a problem with them.

Are you running a lipo rx pack?

 

[hamz9561]

Just on my electrics through the ESC. The gas and nitros run NiMH for now but will probably be switched for LiFe in the near future.

 

[WilliamM]

Hamz9561 or others, is there an advantage to using a LiFe rx batter in lieu of a LiPO rx battery?

 

[Greywolf74]

LiFe packs have a lower nominal rating than lipos. For example a single Lipo cell has a nominal voltage rating of 3.7V and a maximum voltage of 4.2V (or in this case of a 2S pack it would be 7.4V and 8.4 volts respectivly. A 2S LiFe pack will have a nominal voltage of 6.6V and a max voltage of 7.2V. So if you have a RX like the spektrum that can handle up to 9.6V and an RX that handles up to 8.4V then it doesn't matter which one you used. However if your RX and or servo have maxium ratings of lets say 7.4V then you could use a LiFe pack instead of a lipo and not need a BEC. If your servos and RX can only handle 6V max then you'll have to use a BEC regardless of what kind of pack you use.

Life is also a little more stable than lipo but is subject to the same risk factors that lipos are and LiFe is generally a little more expensive I believe as well.

Hope this helps.

 

[WilliamM]

Yes, very helpful. Thanks again.