ORIGINAL: mattnew

How are you calculating how much battery you need when you put together a receiver system? Used to be easy, 95% of the time it was the battery that came with your receiver ( I've been away from the hobby for a few ( ten ) years... )

( keep in mind I don't want to get into a NiCd/NiMH/LiPo/LiFe debate... I've read plenty on the pros and cons on each, I'm more interested now in doing out the math so I know I have enough of.. whatever... )

I see current drain stats readily available for receivers: for example
Futaba R617FS
Power Requirement: 4.8 - 6V Current Drain: 80mA (at no signal), 74 mA(w/ signal no servos on 4.8)

However I don't see any current ratings on there servos either on Futaba's site or servo wiki's...
( just grabbing a fairly common 3004 technotes here... )
http://www.gpdealera.com/cgi-bin/wga...pgm?I=FUTM0004
http://www.servodatabase.com/servo/futaba/s3004

my assumption is you need to add up the MAX current drain under load of each servo, + the receiver draw + anything else connected up + fudge factor to get what you need in terms of current draw from your batteries.


so my questions are twofold
1. does that math seem right?
2. where are people finding the current ratings for different servos to do the above math?

Thanks for the help....

ORIGINAL: mattnew

I appreciate the comments, I'm not sure they are getting me to where I need to be though in order to buy/build my battery system.

So for any of you... if you had a futaba R617FS and 8 3004 servos.... what would you use for a battery and why.. how did you decide that that met your needs?

Again, I'm picking Futaba as just an easy/common example...

ORIGINAL: LesUyeda

I have never used anything over the Futaba receiver packs, and packs that I have assembled from purchased cells. I believe the max capacity I have used is about 900 mAh, BUT, I to run a "Voltwatch"; excersize the servos before flight, and if the Voltwatch drops into the red, I do not fly with that pack.

Les

ORIGINAL: modeltronics

I don't want to take this off subject and don't want to start something but.....

Be very careful in how much trust you put in a voltwatch type device.

If while you move your servos around is it fast enough to catch a 50mSec voltage dip? And even if it is, can your eyes catch it that fast.?

ORIGINAL: modeltronics

I don't want to take this off subject and don't want to start something but.....

Be very careful in how much trust you put in a voltwatch type device.

If while you move your servos around is it fast enough to catch a 50mSec voltage dip? And even if it is, can your eyes catch it that fast.?

ORIGINAL: airraptor

I would run a single LiFe at 1800-2000 Mah range for all day flying. as i dont know your set up and flying style.

I always run smaller batteries than most would but I monitor them all the time.
Measuring what your servos pull on the ground is meaningless same as how much thrust your engine makes on the ground. Its in the air that mattters.

Here are some sample planes and draw on them per flight

WM T-34 set up for the racing class. it has 5 Hitec HV digital standard servos. I run a 2cell 1300 Mah LiPo in it. I use around 80-90 Mah per 10 minute flight. I will recharge after 4 flights.

14 lb Laser 200 with 92'' wing DLE55 3D plane. I run 2 HS7940's, one HS7945 and one HS7950 on flight controls and one Hitec Digital standard HV on throttle. I have two 1600 Mah LiPo's and I use 600 Mah per flight. Also the ignition is also run off these batteries thru the receiver with a opto switch. I recharge every 3rd flight

GP spite fire with HS-85 digital servos on flight controls and a HS65 HB on throttle wit ha OS 30 FS on it. 3lbs flying weight. I run a HP 800 Mah LiFe in it and pull about 40 mah every flight with it. I recharge every flying day as I can fly all day on it.

WM 46 size mustang with Digital standard servos and a Saito 125 and a Hitec retract servo I ran the HP 800 or a Hobbico 1100 Mah LiFe. every flight pulled around 150 Mah. Recharged every 4th flight

GP revolver 46 with OS 120AX 145 MPH plane. Servos were Hitec digital standard on ailerons and rudder and HS7985 on elevator and HS-65 HB on throttle. I ran a 1100 Mah LiFe and it pulled around 170-180 Mah per flight.

On all control surfaces they need to be very free to move or your causing undue load on the servos and will not center correctly. The control surface needs to drop under its on weight or you have a bind somewhere unless mass balanced.

ORIGINAL: mattnew

Nope, thats fair, and with today's digital electronics even more important since a dip can cause a reboot

I'm an engineer by profession... I like to understand the system I'm building.. hence the questions. Sure I can go get dual 2500mah LiFe battery packs and put them in my .90 sized plane and be reasonably confident that I'll be well into overkill for powering a receiver and 8 servos.... and its probably not even that big a weight issue either.... but it doesn't contribute to understanding the system I've built and/or knowing its limitations.

ORIGINAL: BuschBarber


If you are measuring the Current Draw of an individual servo and momentarily stress the control surface, you can get a good idea of how much Current the servo is drawing in a Worst Case Scenario. Check all your servos that way, add up the numbers, and use an Rx battery that has more than adequate Capacity. The results are valid.

Using a radio with Telemetry will give you In Flight information.

ORIGINAL: airraptor


keep doign that you will be just fine.....NOT lol Momentarily stress the control surface.. boy that is accurate so some servos have a stalled drain of 4-4.5 amps. so 4 servos stalled is 16-18 amps. so your thinking should have what size battery? it works for you but may not work for everyone else.



All charge and discharge your battery several times. this way you know its performance. fly the airplane 1-2 times and recharge. see how much was put back in. then fly for 3-5 times if can and then recharge. see how your using each flight. now use a battery that lets you fly as much as you want for the day or before your next charge that is double your useage. Simple huh

ORIGINAL: BuschBarber


All 2.4 radios are susceptible to sudden voltage drops which can be caused by servos drawing more current than the batteries can handle. While the batteries may seem to be adequate under normal circumstances, there are situations where servos get jammed by stuck control surfaces, binding linkages, etc.. You need to stress the servos on the ground to estimate the worst case scenario so you can install batteries that have the Capacity to avoid these voltage drops.

Cycling NiCad and NiMh batteries was necessary to ensure that they were providing their rated capacity and in good condition. Unless you know what kind of a current draw will cause the voltage drop, you don't know what capacity batteries you need. It is also recommended to use at least 6v Rx packs rather than 4.8v and use a voltage regulator, if you have servos that are only compatible with 5v.

Lithium Rx batteries do not need to be cycled like NiCad and NiMh packs. Chargers, like the FMA 10S, will inform you as to how your batteries are performing and how each cell is performing within a pack.

The capacity you choose for your Rx batteries should be determined by how many servos you plan to use, analog or digital, speed and torque necessary, and your style of flying.

A wise pilot is going to measure the amount of current each servo may draw and that of the entire system.

ORIGINAL: airraptor


dude read what I said and not what you think I said. I didnt say you had to cycle them. I said to cycle them so you will know how they perform. I have been in this hobby since the 70's and I have seen batteries that are bad from the get go. I have seen it all.

A wise pilot huh... see your assuming that you are a wise pilot and saying that I am not. A wise pilot will get to know his equipment and know the proper operation of all his equipment not matter the method used to get there. each person has their own way of doing things. I know some guys that will pull all his servos apart clean and debur all the gears clean and resolder all components in the servo before use. again we all do it different. Just dont promote your way as the best and only way as you may mislead someone in to doing something wrong or carry to much battery. Oh and by the way I fly JR/Spektrum so I know all about its requirements.

ORIGINAL: dirtybird

If are an engineer I can tell you how to calculate it.
Use the quadratic equation. The result is the square root of the sum of the squares of each component.
In other words measure the stall current of each servo, square it and add all of the squares from all servos together. Then take the square root of the result.
This will give you the three sigma value or 97% of the time your current draw will be less than your calculated value. If you want that last 3% just add all of the currents together. I can guarantee that will be overkill though. Your servos are never going to be all stalled at the same time.
Its a lot easier to just fly it and measure it.
BTW the receivers sold by XPS have a light that gets set if the receiver reboots. Its cheaper than an oscilloscope.
If you like I can design a circuit for you that will detect a momentary loss of voltage and turn a light on. I am surprised nobody sells one.
I should add how to calculate your lowest voltage point. Just get the internal resistance of your battery from its specification. Then multiply that times your calculated current and subtract the result from the lowest useful voltage your battery will have. You get the lowest useful voltage from the chart usually contained in the specifications. This is the lowest voltage your receiver will see if you keep the battery sufficiently charged.

Take a look at this

http://www.xtremepowersystems.net/pr...hp?prod=XPS-TT