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[DominicS]

Im fairly new around here, but I've been playing with radio controlled boats and hovercraft for a few years now, but I have a few simple rules that they must follow, so they are that


1) The models musy be small enough to fit in a  carrier bag
2) The power will be suppplied by single cell Li-Ion or LiPo batteries
3) All motors will be brushed rather than brushless.



Now I realise that may be a bit unusual, but over time Ive standardised how handle power within my models, so thought it may be of interest to others how that has evolved. I'll do this in 2 parts, the first being off the shelf items and the second part being about "home made" speed controllers


So first of all we have batteries, I use both Lipo and Li-Ion. I don't think for the amount of power that I use in boat that current output of a Lipo cell has any advantage to that of a Li-ion battery, so my smaller capacity batteries (between 220mAH and 750mAH) are Lipo and the higher capacity batteries (between 3AH and 8AH) and Li-ion. With my batteries between 6AH and 8AH the cells are wired in parallel so can be treated the same as single cells. With smaller models weight is always an issue and things need to be as light as possible, so thats why I've got such a range in sizes as I'd like my models to run for up to half an hour. One of my models has a continious current consumption of 0.5 amps, but one of them also has a continious current consumption of 17 amps. Shown below are the batteries I use, with the smaller ones in the top left being the Lipo cells and the rest being the Li-Ion cells.



With the cells I use,I prefer to use branded ones, and with the Li-Ion batteries I use 18650 and 21700 cells (if you didnt't know the first two numbers are the cell diameter and the rest is the length, so an 18650 battery is 18mm diameter x 65mm long).  With the cells, there is always the compromse of capacity, discharge current and cost.  Under high loads, a lower capacity cell with a higher discharge current may deliver more power than a high capacity cell due to the higher capacity cells having a higher internal restistance which means the output voltage drops more that the high current cells.


The 18650 cells I use are Sinowatt 30SP cells

https://www.nubattery.co.uk/sinowatt-30sp-18650-battery

and the 21700 cells I use are are Samsung 50S cells, but they may no longer be available,so these days I'd be tempted by these.

https://www.nubattery.co.uk/eve-50pl---21700-battery---5000mah---125a

Once you've got your batteries you need to charge them, and thats one reason why I decided to use single cells is that you can get really cheap chargers based on the TP4056  charging modules like this

https://www.ebay.co.uk/itm/800250874422


if you solder on a usb cable and a battery connector you have a working 1 amp single cell charger, and if you change one resistor on the circuit board,you can reduce the charging current so the one of the left charges at 1 amp and the one on the right charges at 0.5 amp. I dont use the 1 amp version anymore as I've got IMAX B6AC charger for the li-ion batteries and will charge them normally between 2 and 4 amps depending on battery capacity, but I still use the one on the right on the smaller lipo batteries.



On the model boats I dont use battery management systems, as the limit the amout of power being delivered and cause a voltage drop, but I do use them for some display models as they can be used instead of a fuse and when the battery voltage gets too low they stop working to prevent damaging the battery.  I use them in a meccano crane and a battery powered air compressor and these have contininious current rating of 2 amps.

https://www.ebay.co.uk/itm/156563096286


So next we come to connecting everything up and on a small low voltage boat its a compromise between wire flexibility, voltage drop and weight as copper in the cable is roughly 8 times the weight of the water its displacing,so its best to keep things as light as possible without too much voltage drop.



I use 3 different types of connectors:-


1) JST 1.25mm micro connector (on the left below) I use these for connecting motors that have a continious current of less than 0.5 amps, and I also use the three terminal versions for connecting the receievers and servos together.

2) JST BEC connectors, I use these on the batteries that are less than 1AH capacity and for motors that will draw less than 3 amps continious current.

3) XT30 connectors for connecting batteries above 1AH capacity and motors that will draw more than 3 amps constant current. One of my hovercraft draws 17 amps constant current and they are fine with that.



then it comes to wiring, with the JST micro connectors I just use the wire that they come with. For the other wires up to 3 amp constant current, I use 22awg wires, up to 5 amps 18 awg, up to 10 amps 16awg, and above 10 amps 14 awg.

and finally we come to the speed controllers and I have 3 bought ones that I use show in the photo below



1 amp speed controller shown on the left both after being wired up and this takes the power from the servo connection to the receiver as the current provided to the motor will be less than 0.5 amps, it also show speed controller as bought.

https://www.aliexpress.com/item/1005012569516901.html

a 2.7 amp speed controller the one in the middle, and it shows the xt30 connector as it will be used with one of the larger batteries, but the motor uses about 2 amps maximum, so using the JST BEC connector for that.

https://www.aliexpress.com/item/4000637701600.html


and a 20 amp speed controller (although I wouldn't use more than 5 or 6 amps though them continiously), this has been modified by removing the on/off switch and bypassing the BEC circuit as  the battery voltage doesn't need to be reduced to power the receiver and servos.


https://www.aliexpress.com/item/1005008173918192.html






you may have noticed I tend to talk about current consumption, and for the smaller models with currents up to 3 amps I use a cheap USB current and voltage display, with the usb bits removed, JSP BEC connectors added and put in some clear heatshrink. they dont weigh much so can be used on the smallest of boats

https://www.ebay.co.uk/itm/389054682679


and for larger currents, I just use analogue meters like these

https://www.ebay.co.uk/itm/188307651655





I also have a ATorch DL24 load tester, I use this mainly for discharging the batteries to a storage voltage of 3.5V and can alter the current depending on the capacity of the battery, but its also useful for load testing components like speed controllers at full power, or seeing the voltage drop on a battery due to internal resistance at high loads. its easier to do on a workbench without moving propellers and staying away from water, although I can test mine in a kitche sink.


https://www.aliexpress.com/item/1005004567336099.html

if anyone gets to here, I hope it makes sense, but this is what I use and it works for me.

[DominicS]

I started to make my own speed controllers by accident, and this started with the modifications to an old Taiyo Edge hovercraft to get it working with modern radio control gear. This is the first install showing a couple of 20 amp speed controllers on the prvious post, and the cam and mircoswitch which formed the on/off switch for the lift motor. The issue here was that the lift motor needed 4 amps to work it, but a suitable microswich would have been to heavy and bulky, so I used a mosfet to provide the power switching.






and that worked very well, the main thing for me was to use a mosfet with a low resistance and low threshold (operating) voltage. I only very occasionally dable in electronics and after a bit of looking used a LR8726 mosfet as they are also readily available.

https://www.infineon.com/assets/row/public/documents/24/49/infineon-irlr8726-datasheet-en.pdf

I then thought about using  forward only speed controllers (rather than bi-directional ones) to reduce the power loss from the battery to the motors. I did try a couple for aircraft, but found that they were not suitable for spring loaded centre off stick transmitters as their off position didnt match that of the speed controllers. So I tried to make my own and I first tried one of the 1 amp speed controllers from the first post to trigger the mosfet. It worked to a point but most of the proportional control had gone, and I thought this may have been due the quality and speed of the drive signal from the 1 amp speed controller.


I then found elctromagnetic servo drive modules. these are used to operate the control surfaces on ultra small radio control planes and use electromagnetism rather than motors and gears to operate.

https://www.aliexpress.com/item/1005001939708493.html

and these gave a square wave ouput from 10%



to 90% (and full power)



so with this module, a mosfet, a 1k0 resistor and a 10k resistor I could make my own speed controller.



and in its simplest form is wired like this (if the receiver has a alternative suppply from the battery), with the battery connection to the left and the motor to the right. The vertical 1k0 resistor limits the current from the module to the MOSFET, and the 10K resistor is to ground the gate terminal on the mosfet to make sure it doesnt randomly provide power to the motor.



if you wire it like this you can also power the receiver (and servo if required)



now for comparison, I did some testing today with a high capacity, high discharge rate li-ion battery at 3.8V and with one of these speed controllers below powering a 10 amp load, it was droping 0.38V across the speed controllers connection to the wires.



and the single mosfet speed controller at 5 amps was dropping 0.08V and 0.18V at 10 amps. Ive also tested a 3 moset speed controller with the mosfets in parallel to reduce the current in each one and at 10 amps that was dropping 0.03V, which means that the  power being dropped across the speed controller is less than the power lost in the connecting wires and connectors.
One advantage of making your own speed controller is that you can adapt the wiring to suit your needs. On another hovercraft I needed to have a tiny waterproof speed controller, so this was wired up, with a receiver on the left.





then folded up and sealed into a cut down 10cc syringe




and then fittted into a model

[roycv]

Hi this is great all your info has been hovering behind my ear for some time now.  What about the use of standard servos are there low voltage ones?  But perhaps your hovercraft are all electronic?
I was planning to use 18650 cells but put off by charging etc.  I had not thought of running all from a single cell.  With this as a possible problem I opted for using NiMh sub-c 4/5ths size at 2.8Ahr capacity.  They came from Ali..... and have proved excellent.  And I now do the welding and wrapping myself.
The power to weight ratio sorted out my 16 inch loa Swordsman, which now planes and performs for about 40 minutes on 6 cells.
Any more info welcome I shall archive your post for later reference.
Thanks
Roy

[DominicS]

small servos are readily available that run off 3.7v such as the SG90 servos like these

https://www.ebay.co.uk/itm/168572224687


it looks like the MG90S servos work down to 4.8v (if you read their specs) but Ive used them down to 3.5V without issue. Smaller servos are also readily available to work down to 3.5V. Another option may be t use a boost converter to increase the voltage to 5V. Ive recently been using this one

https://www.aliexpress.com/item/1005009708365128.html


to increase the voltage to 5V that the mosfet works from while the receiver and servo module work at 3.7V. Shown below is me testing this setup, and to get the 2A at 5V it uses 4A at 3.7V. I had to fit a larger heatsink and cooling fan, but for the brief use of power to operate a servo these may not be required.



and the final installation and wiring. the two boost converters are on the right of the photo with a heatsink and cooling fan on top.