Choosing a sail servo1 – What battery?

[Tony Bell 2Participant @tonybell2]

I’m new to R/C, the variables seem endless. As an engineer/designer I can start at both ends of the problem.

GRP Sail boat type unknown.

1.12m lg, 25.5cm beam, Mast 1.5m,  No Motor. Sails controlled separately.

Main – 6040cmsq, boom lg 41cm, attachment points at 16 or 26.5cm from gooseneck due to deck layout

Jib – 4630cmsq, boom 37cm, attachment point variable.

I bough a Hitec  HS-785HB drum winch as it looks competively priced for the Torque £25/13kg.cm

Batteries choice factors – weight, size, voltage, capacity, current drain capability.

The Lead gel cell is not suitable as its mass will have to be above the waterline so reducing stability otherwise size/weight is not a problem

LiPo type appear to be for light weight, high current applications and are expensive.

This leaves NiCd and NiMh

The one piece of data I would have expected to be readily available is the max current drain of both batteries and servos.

My best estimate for the current drain is 4 to 6A  

2 Hitec sail winches (1.8A stalled) + rudder (Futaba S3003 Amp?) + receiver (32mA)

Battery voltage for the servos can be either 4.8 or 6v, It seems silly not to go for 6V as the available torque is 13.3 as against 11kg.cm = 21% increase.

Should I use NiCd or NiMh?

 What Ahr should I use?

I notice that some servos have a power feed from the battery, on/off switch and connector to the receiver. (The Hitec HS-785Hb does not). How does this system work?

Tony

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[ashley needhamParticipant @ashleyneedham69188]

Tony. I am not a sail person so will answer in a general fashion. The bigger the battery the better,,,to a point. If you have space and weight allowance, go large. Nothing worse that persistantly running out of juice after only a short time. I have no idea what a sail winch or rudder servo draws, but i should imagine that a ….ooooh….3300MaHr battery would last all day, or at least a full afternoon.    As far as I am aware the Nicads are being phased out, due to cadmium being poisionous……so NiMh are the only things to go for.   I only have experience of servos being fed power via the reciever, and if your servos are of this type then the battery voltage would have to be that specified for the reciever. Ashley

[Len OchiltreeParticipant @lenochiltree67043]

Yipes what Qs,

All I know is that I can run a 42" Thames Sailing Barge for a couple of hours on 4x nimh AAs operating a Sail arm servo and a rudder servo.

hope this helps.

Len

[ashley needhamParticipant @ashleyneedham69188]

Common AA rechargeables come in now at around 1800/2000 Ma/hr, and I dont suppose Len has bothered to splash out on hi-capacity 2500 cells. the norm only a couple of years ago for AA cells was about 900/1000/1300 Ma. Maplin do 2000 Ma AA cells, and plastic holders for varying numbers of batteries if you want to buy loose ones and make up a pack    Ashley

[Tony Bell 2Participant @tonybell2]

Thanks for info.

After much searching I have found a good source of NiMh battery packs, but they, like all suppliers fail to supply one vital piece of data – Max current drain. I have emailed them. Will post the data when received.

I like to know such things, I teach science, maths, DT, business.

 With regards to the number of leads coming from the servo, I can only assume is the servo voltage and current draw is so high that they cannot be drawn from reciever + battery configeration. That leads to another question.  What is the max current drain of a receiver, nobody publishes such data.

 New to R/C, but v old to Industrial design, sales and marketing management, I find the total lack of data astonishing. Lots of people know a lot of thing but why is it all hidden. Thanks for this Forum and your good selves

Tony

[ashley needhamParticipant @ashleyneedham69188]

Tony,  Reciever drain is minimal.. I am sure I saw a figure of 40 mA (ish)  somwhere. As far as I know most servos operate directly from the reciever and draw thier current from them. With such tiny motors in a servo I cant see the drain being very much at all. A "normal" reciever and a couple of "ordinary" servos will operate for hours and hours on a 4-pack of duracells, so I think a 2000 mAhr capacity AA set on NiMh batteries will last all afternoon, at least.  Ashley

[Tony Bell 2Participant @tonybell2]

The following is the result of my research.

 The most helpfull website I found was Strikalite.co.uk, they advised me to use their King Cobra 6v 3300mAh NiMH 4.5C battery pack SC

The worst case senario is the Maximum Discharge Current calculated from the servo stalled current values. 1.8A sail servo, 0.6v rudder, 0.3 RX = 5A  It will handle 15A = 3.300 x 4.5.  It’s size is SC = Sub C. This requires a higher spec than the normal NiMH batteries (max 1C discharge rate).

6v (5 cells) was chosen as it increases the torque by 20% over 4.8v (4 cells)

 The average current was then calculated using normal load values. 3 servos @ 230mA + RX  @0.03A  = 0.7A. Capacity is 0.7A x 4hr usage = 2800mAh + 20 % buggerance factor = 3400mAh

 If you go to my website Jolie Brise you will find a fuller explination + sail servo comparison chart, cell sizes, wire size and various interesting links (how to mod a servo).

Thanks to Roger at Strikealite for his help, it’s far more complicated than you think.

The Japanese have invented an organic battery which can be powered by any liquid even urine!

 Tony

[60wattParticipant @60watt]

Hi Tony.

The 2.4 Ah AA pencells would do for me.

When I last looked at nicad pencells in an engineering supply catalogue,they were 50 milliohm per cell where scr and sce sub-c sized cells were in the order of 2 to 5 milliohm.These things were used for fast electric racing classes which had a sail duration of 2 to 5 minutes!

The gauge of the standard battery harnesses is 22awg(23swg) which is around 55 milliohm per metre.Standard servo cables come in 22 or 24 awg. Receiver and servo circuits work perfectly well with the variation in voltage.The digital chips in Futaba receivers run from a 2.5 volt regulated supply.

Tom

[Tony Bell 2Participant @tonybell2]

Hi Tom

 Your comments about milliohoms per cell are interesting as I have never seen such values on manufacturer’s specs. What relevance are they in choosing a 6V battery pack and how do you use them in calculations.

I prefer to rate cables by Amps as it is more relevant to electronic circuits and easier to buy. The metric 7 strand instrument cable is suitable for use in boats as they are not subjected to the high strains of aircraft. As the cables a short the voltage drop calculations are not necessary.

I was concerned about the high current that could pass through the reciever in the stalled condition. Hitec made this comment  "There are step downs in the reciever to keep things from burning up."  Servo voltage is 4.2v min to  7V max.

Tony

[60wattParticipant @60watt]

Hi Tony.

You can see the effect of combined resistances if you connect a scope to the power rails of a receiver and make a servo move from one end of travel to another.

Receiver rails are beefed up by solder.According to the data I have, 0.1 oz Cu per square inch printed circuit  tracks are :0.025" wide = 1.0 Amps ,0.050" wide = 2.0 Amps

The voltage regulated section in a non-bec receiver draws a negligible current.The supply is passed directly to the servos.

On a bec receiver the voltage drop across the regulator is minimal and the component that carries the load is a high current smt transistor that uses the circuit board as a heatsink.You could check the volt drop with a one amp load.

Sailwiches with a separate power source  probably exist because that was the most economical design solution when RC equipment and components were primitive.

Tom

[Tony Bell 2Participant @tonybell2]

[Geoff CropperParticipant @geoffcropper73342]

Hi All,       I have a 48in loa american schooner which has 5 sail sheets attached to one winch line so all 5 sails move together.      I used a lego 9volt motor unit with one axle and two wheel hubs and two plastic discs.       The winch line is wrapped round the axle at one side between the loose plastic discs which are held on with the hubs at each end of the axle.         The drive unit is powered by 8 AA nimh batteries of 2100 mah.    2×4 battery holders connected together( to ballance the boat ) and gives 9.6 volts.       The winch would pull the masts down if there wasn’t two micro limit switches to stop the winch.(Maplins).        The receiver has 4 AA nimh batteries of 2100 mah. giving 4.8 volts to drive the servos         The sail servo operates two micro limit switches to operate the sail winch.

[Dave MilbournParticipant @davemilbourn48782]

I notice that some servos have a power feed from the battery, on/off switch and connector to the receiver. (The Hitec HS-785Hb does not). How does this system work?

Tony

Have a look at the Sail Winch diagram page here http://www.action-electronics.co.uk/pdfs/P96.pdf  (Page 6). The connection shown to P96 can be read as “to the receiver”, unless you want to include a P96 to vary the throw and speed of the winch.

Component Shop have an “entry level” 2500mAH SubC NiMH pack which will cope with a 30A current discharge. Their newer high-capacity SubC cells will happily handle 50A!

Dave M

 

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