Batteries

The power for the R/C system is usually sourced from a battery. In my situation, a heavy, large Sealed Lead Acid (SLA) battery would cause instability. Nickel Metal Hydride NiMH batteries are superior in many ways and readily available in pre made packs.
 
The required system voltage.
 
The proposed Hitec sail servo HS–785HB is quoted as both 4.8v and 6v. Increasing the voltage increases the available torque. 11kg.cm @ 4.8v 13.2kg.cm @6v.
 
Hitec Service Dept says that the servo will run from 4.2v to 7v which equivalent to 4 and 5 cell NiMH power packs.
 
My choice is for a 5 cell NiMH pack giving a useable voltage range of 5 to 7v.
It is recommended that NiMH cells are not discharge below 1v per cell.
 

Maximum Continuous Discharge Current. (CDC) = 7.5A

The system consists of four sail servos and receiver.
 
4 Hitec sail winches (1.8A stalled) + receiver (32mA)
 
The maximum worse case system current has to be calculated as the battery must be able to supply this current even for only a short time.
Standard NiMH batteries have a CDC of 3C ie 3 x battery capacity.
"That’s correct for sub C cells but if you get into smaller sized cells then in my opinion you are pushing it ... "
Roger @ strikalite
 
Newer high quality, current types can supply a CDC of 10C
 

Average Discharge Current (ADC) = 0.95A

4 Hitec sail winches (230mA running) + receiver (32mA)
 
The ADC is the current used when running under normal conditions; this depends on how often you use the controls and the power used by the servos controlling the sails.
 

Battery Capacity = 3500mAh

950mA x 4hr = 3600mAh use 3500mAh
 
To calculate the battery capacity, multiply the ADC by your required running time.
 
The spec sheet for GP 3500mAh C size, gives a time of 5 hrs to discharge at 950mA (0.3C) down to 1v.
 
Battery capacities are usually below the manufacturers published rating by as much as 10%, The battery comparison sites below can help. Otherwise its pot luck
 

Cell Size

You would think that the bigger the cell size the more the power/capacity.
GP is a well known battery maker; they produce a 2200mAh battery in AA, C and D sizes. I am informed that the two larger sizes use the same AA size but in different can sizes. Similar to using battery converters!
 

Battery Choice.

 
"This is a high rate discharge cell and is made to a higher specification than the average cell."
"If you use standard AA batteries at high discharge currents it diminishes their life considerably, you may only get around 200 cycles.
The other cell we would recommend is the GP 3700mAh, but it’s on the expensive side."
 

Wire Sizes

Electrical wire used for R/C circuits should be flexible multi strand to withstand vibration.
SWG – Standard Wire Gauge (AWG – American ..)
 
Hitec/Futuba servo leads use 22swg 60 strand 0.08” = 0.33mm²
 
The nearest equivalent metric instrument wire is
7/0.2 –1.4A = 0.22mm²       24swg = 0.21mm²
16/0.2 – 3A = 0.5mm²         20swg = 0.52mm²
55/.01 – 25A = 0.43mm²     21swg = 0.41mm² Extra flexible
 
Wire size converter
Maplins
 

Fuse Switch PCB

To stop the birds nest of wires connecting the battery, cutout switch and inline fuse, I made up a small PCB (47 x 22 mm). A link was added so that the circuits's current could be measured. Rapid sell a cheap Mulitimetre for £7.50, its leads can be clamped in the Link connector block.

Total cost was £2. Components from www.rapidonline.com. Stripboard 33-0505. Fuse Holder 26-0167. 10A Fuse 26-2497. Connector Block 21-0002.