The following article sets out how you go about converting from the standard 3S lipo configuration that the majority of T-Rex owners use to a high voltage setup. Within this article I'm going to convert to a 4S configuration, which is generally not considered high voltage but the principles laid out here are good for running 5S or 6S as well. I will also cover creating a 5S pack from a 2S and 3S pack..

So the first question is why would you do this? Well, if you go to my videos page and look at the Heliup videos the T-Rex in those videos is running a 5S setup. Basically higher voltage equals more power but it also requires different motors and different lipos. Power is definitely one element of the decision to run high voltage, however, there is more to this than just power. As you increase the voltage you reduce the current draw, this is generally good news for your lipos as lower current draw equates to less strain on the pack, which in turn equates to a longer lasting and cooler running pack (or packs).

My own argument for running 4S came from the announcement of the Thunder Power 4S 2000mah pro-lite. Here for the first time was a 4S pack with the same capacity as a 3S pack and the same weight as a traditional 3S pack. Not only this but it is also of a similar size to many 3S packs currently on the market. Additionally I was fast coming to the conclusion that the T-Rex is in fact not a 3S helicopter but because of size and weight better suited to 4S configuration.

OK, so there are some reasons on the positive side but as always there are negatives. First you are going to need a charger that can handle 4S or more (although if you combine 3S and 2S packs to make a 5S then you can still get away with a 3S or less charger). Secondly unless you have a Kontonic 40-6-18 ESC your are going to need a separate BEC to handle the higher voltage as all other ESCs on the market don't have a BEC capable of supporting 4 servos, gyro, receiver and voltage above 3S configuration. Thirdly that separate BEC may introduce some interference or glitching to your helicopter.

So having decided to go 4S you have a purchasing decision, either buy the Kontronic 40-6-18 or a separate BEC. If you already have an ESC then getting a separate BEC is the cheaper option. I will be using the Medusa Potencia 3.5A BEC for this article. Here is the full specification detail for this BEC :
5V 6V
Electrical Specifications Min Max Min Max
NiCD/NiMH Cells 10 33 10 33
Li/LiPo Cells 3 10 3 10
Input Voltage 8V 45V 8V 45V
Output Voltage 5.0V 5.15V 5.9V 6.1V
Continuous Output Current No Min 3.5A No Min 3.5A
Peak Output Current No Min 4A No Min 4A
Number of Standard Servos (typical) No Min 8 No Min 8
Physical Specifications
Width 30mm
Length 50mm
Thickness 13mm
Weight w/o wires 13g
Weight w/wires 20g
The loading of micro servos, gyro and receiver on this BEC will be well within it's capabilities. However, it should be noted that it is very important to understand the loading requirements when fitting a BEC. Few manufacturers give details of peak current that a servo will draw but measurements have shown that four full size digital servos can peak at over 4.5A which would result in a BEC failure. Fortunately the T-Rex does not use servos capable of drawing this level of current so we are quite safe using a 3.5A BEC.

The BEC has two input wires and a single output cable that plugs directly into a spare receiver socket in the same way a servo connects to the receiver. In some cases channel five on your receiver could be used if it isn't in use as a remote gain channel for your gyro. If you don't have a spare receiver socket then not to worry as this is easily resolved with the use of a servo Y-Cable.

The Y-Cable output should plug into the throttle channel on the receiver, the ESC and BEC connect into the input sockets of the Y-Cable. The live power wire on the ESC side of the Y-Cable should be cut, this in effect disables the ESC BEC allowing the separate BEC to do all the work.

Once this is done you have effectively done all the hard work to enable running a high voltage configuration. Be sure to mount the separate BEC well away from the receiver. Also place a ferrite ring on the BEC output cable to remove any unwanted radio frequency interference generated by the BEC.

You can now connect your 4S (5S, 6S) lipo and the voltage conversion to 5V will be handled by the separate BEC whilst power to the motor and throttle control is still handled by the ESC. Some things to note are that the higher the voltage the faster the motor spins and the smaller a size pinion you will require. It is for this reason that 2.3mm shaft motors are best suited to HV configuration as it is easier to source the 8T or 9T pinions required for a 2.3mm shaft. Small pinions in the 3.2mm size are much harder to locate.

For this article I'm going to be using 4S and 5S packs. I will go into how you can combine several packs to create 5S or 6S packs and also give some detail on how to manage this 'created' pack. The 4S packs to be used for this article are the PolyQuest 4S 1800 slimline and the Thunder Power 4S 2000mah pro-lite. The 5S packs are Kokam 1250mAh in 7.4V and 11.1V combined to make a single 5S pack.

OK so lets take a look at some photos of how to wire the Medusa BEC to the ESC. I am using the Hyperion Titan 30A ESC for this article, one particular useful feature on this ESC is that it has a BEC switch which allows you to enable or disable the onboard BEC, which means that there is no need for any cutting of wires to disable the BEC as detailed above. If using a Castle Creations ESC then you definitely have to do some wire cutting to disable the BEC as previously described.
The above pictures show the BEC input wires soldered to the input wires of the ESC. This means that when you plug in the lipo it immediately supplies power to the external BEC. The Titan 30A ESC BEC switch is in the off position so the Titan internal BEC is doing no work at all to create a regulated 5V supply to the receiver.

In my configuration I have used the Y-Cable to interface together the output from ESC to control the motor, with the output from the BEC to power the receiver. Also I have fitted a 'glitch no more' from RCHover.com, instead of a ferrite ring. The purpose is the same, to remove unwanted current ripple or radio interference.

Having installed your radio, with or without Y-Cable and verified that your radio is still working properly and receiving a nice 5V supply it is time to look at motors. Many motors will run in 4S or more configuration, however the limiting factor tends to be the availability of a suitable pinion. The smallest pinion for 2.3mm shafts is usually an 8 tooth pinion (8T). If your motor has a very high kv rating (RPM per volt) then it is likely you won't be able to run 4S or higher as it won't be possible to get a small enough pinion. For motors in the 3000-3500kv range an 8T or 9T pinion normally suffices. The higher the voltage the faster the brushless motor will spin

For the purposes of this article I'm going to use the AON 3000kv motor with the 2.3mm shaft, I will be using the 9T pinion for the tests. The AON 3000kv is a fairly docile and low amp draw motor when using 3S power so it will be interesting to see how it transforms as we up the voltage. The packs I'm going to use for this test can be seen below. The 4S power is provided the the Thunder Power Pro-Lite 4S 2000mAh pack. The 5S power is provided by two Kokam FMA 1250mAh packs. One is a 3S and the other a 2S with them wired in series (as shown in the photo) to make a 5S pack. Both these packs are similar in weight and so provide a good comparison of inflight performance. Wiring liposin series means taking the red from one pack and plugging it into the black from the other as shown in the photo below right. To create a 6S pack you would just wire in series two 3S packs in a similar fashion. Because of weight considerations you do need to go for smaller mAh packs but the lower amp draw that will be required to run the motor means you can maintain a reasonably good flight duration before the packs are expended. One other thing to note is that wriring packs in series like this is fine for discharge but during charge the packs should be charged separately. This introduces you to the two-three hour charge time required to run a 5S or 6S pack as two separate pack charges are required.

Pictured below the packs are the power graphs showing how they performed on the bench when subjected to a full power, full pitch simulated climbout. This is using the Align Carbon 325mm blades and +10 of pitch on the blades.
It's quite interesting to compare these two charts. Both are using a 9T pinion, ideally when going to 5S it would be prudent to change to an 8T pinion, this is why we have a slightly higher amp draw on the 5S test as the motor really isn't running efficiently as I have reduced the throttle curve to stop the head speed from reaching insane levels. The key data is the RPM that is held during the full power / full pitch.

The 4S powered AON manages to sustain 1920RPM at the end of 30 seconds of full power / full pitch. Remember this is with the Align carbon blades, which typically take a huge amount of power. The AON 3500 on 3S is comparable to this result and widely regarded as a powerful motor on 3S. However, compare this to the AON 3000kv on 4S and you have a slightly more powerful result but at the expense of only 16.5 amps. This is far better for your pack and of course will result in longer duration flights with the same power as if you were running the AON 3500.

If we take it up a notch and look at the 5S power chart. Here we see a higher amp draw, which is down to running inefficiently and not using an 8T pinion, dropping the pinion to 8T cuts 2-3 amps from the overall amp draw. Now look at the power produced, 2160RPM was sustained for an entire minute. This is better than any 3S motor tested (including the awesome Medusa). The amp draw is not peaking over 20 amps either. Of course the Kokam packs are only 1250 mAh so they are actually working a little beyond their 15C capability, but knock 3 amps off using an 8T pinion and you are back inside specification.

Flight testing these two different setups has been most interesting. Having AON 3500 type power but getting 15 minute flights has been typical with the 4S AON 3000kv configuration. If you look at the graph you can see that hovering throttle typically draws 10-11 amps, so conservative flying could even get beyond 15 minutes. Consider what flight times could be achieved with the standard wood blades which take less amps to swing than the Align carbon blades. I really like this combo as it gives good power but great duration. Plus the pack is well below specification in terms of the power it is being asked to supply. This is a great combination for power and care of the pack. Additionally you can get away with using a 20A ESC. Not only this but the Thunder Power 4S pro-lite 2000mAh pack is no heavier than a fairly standard 3S 2000mAh pack.

The 5S combination has been delivering a typical 10 minute flight but power is explosive during those 10 minutes. The Kokam packs are being pushed to the limit and it really needs an 8T pinion to get things back inside specification. Having the abundance of power of this 5S combination also changes the flight characteristics of the T-Rex. Powering out of vortex ring situations is effortless with this combo and collective response in general is much more immediate. Which means less bobbing up and down and a much smoother flight from a collective pitch perspective. Power manoevers are also much easier accompolished and maintained without lots of headspeed reduction.

The only down side of both of these combinations is that the AON 3000kv ran quite a bit hotter than in 3S configuration but it never reached temperatures that were outside of it's specification. The AON heatsink ably deals with this extra heat.

I was given the expectation that introducing the separate BEC would cause a lot more glitches, nothing could be further from the truth and my glitches have actually been less than some of my 3S configurations I've run in the past. Careful attention needs to be given to placement of the separate BEC to keep it away from other sensitive components but provided this is done there is no reason for suffering more glitches than you would in any 3S setup using the ESC internal BEC.

I hope this has provided an insight into the benefits of high voltage setups, over the coming weeks I will be testing various different high voltage combinations to give some insight into what can be expected of various motors that will support this type of configuration. This will be documented on the motors page against each motor tested as usual. In the meantime I hope this has de-mystified the ins and outs of high voltage setups and will allow more people to experience this more versatile way of powering the T-Rex.