Electrickery Demystified pt.2

Electrickery Demystified pt.2

The reliability of electric flight systems can’t be understated when the model is hovering this far from the ground!

In part 1 we hopefully de-mystified a little about motor and propeller selection, aircraft types, and speed controllers, so lets tackle that other old chestnut – batteries, and finish off with a bit about radio gear.

First off I think I should expand a little more on how I select a battery for a particular model, because as they say there is more than one way to skin a cat and this applies to getting our required wattage too! I like to try and keep the Amp draw down to around 45A maximum and the best way to do this is by increasing the cell count. EG: if we want say 800 Watts, then on a 3s Lipo ( approx 10.5V ) and a given motor and prop combination, that would require a whopping 76A. However if we go up to a 4s pack ( 14V ) this comes down to 57A and with a 5s ( 17.5V ) a far more manageable figure of 45A.

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Remember of course though that the RPM will go up proportionally due to the Kv factor of the motor as discussed last month and propellers will have to be changed to stay within our 800 Watts requirement. This makes it important therefore to decide on which battery and prop you will be using before choosing a motor that has the right Kv to swing that prop at the revs we want. Of course cost is proportional to pack size, but this is offset somewhat by longer duration through lowering the Amps consumed. Also the space available to fit the battery has to be considered – combining several smaller packs into one final battery configured in perhaps a more convenient shape is one way to go and how to combine different packs to achieve this is covered later on in this article. This also has the extra advantage of allowing the final battery to be split back up into the separate packs for use in different models.

NiMH packs still have their uses especially where their weight can benefit the C of G

Although I’m now completely Li-Po-fied these days, I appreciate that for various reasons some people still use or need nickel based cells, so I’ll outline the differences, and then use Li-Pos as the comparator for further discussion. I do believe they are the way forward for the majority of electric flight aircraft – for now at least! Incidentally, I know that other Lithium cell types are available, but lithium polymer is most commonly used so we’ll stick with that.

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CELL COUNT

All electric power sources, irrespective of type, basically do the same thing – supply a set voltage for a given amount of time, and then are either re-charged or discarded. We will concentrate on re-chargeable cells. A battery is a collection of cells, arranged in a particular configuration in order to provide a multiple of volts, or duration of voltage supply, or both. Nickel based cells have a nominal (average) voltage of 1.2V and lithium polymer cells are 3.7V – again nominal. Putting nickel cells in series – i.e. starting with the positive terminal of cell 1, and joining its negative terminal to the positive of cell 2, which has its negative terminal joined to cell 3 positive and so on, we arrive at four lots of 1.2V added up to give us 4.8V. In other words, a typical flight pack for an i.c. powered radio receiver. Generally speaking we don’t connect this type of cell in parallel, which involves connecting all the positives together, and then all the negatives together, to produce 1.2V still, although the duration, or time for which this battery can supply the volts is four times greater than an individual cell.

Two 3-cell packs, this is a common size and packs like these can fly a whole variety of models

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SERIES AND PARALLEL

One of the great things about Li-Po cells is that we can happily join identical cells together in either series or parallel, or indeed a combination of the two. This is where the pack identification designations come from. If a Li-Po battery is described as 3s 2p this means there are 3 cells connected in series (which gives 11.1V) and that this is connected in parallel to another identical pack – giving 3 in series, and 2 in parallel, or 3s 2p. Just to practice a little, a battery marked as 5s 3p would consist of 3 batteries, each of 5 cells in series (17V) connected in parallel with each other. So if we want to increase the volts, we join in series, and if we want to increase the duration or current (amps) available, we join in parallel. Combining both increases both. Easy huh? The other great thing is that the voltage available and duration of the battery is much higher like for like in weight, than the older nickel cells.

C RATINGS

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Now the other thing that confuses many beginners is something called the ‘C’ rate. Put simply this is the same as the capacity of the battery, or what is often called mAh which means milliamps (thousands of an amp) per hour, with a multiplier applied for either charging or discharging (in other words actually ‘using’). For example. If a cell (or battery) is described as 11.1V 2500 mAh, this means it can theoretically supply 11.1 volts, at 2500 milliamps (2.5 amps) for 1 hour.

Matching the battery to the current draw is important – this 30C 2400mAh pack is rated for 72 amps maximum and so is very confortabe with the 30amps demanded by the motor in this Multiplex Twister

If the pack is classed as 10C this (again, theoretically) means it can supply its power 10 times faster, or if you like, the current supply capability of the pack is 10 times greater than the mAh figure. So in this example, 10 x 2.5 amps, which is 25 amps. The downside is that this means it will only supply this for 1 hour divided by 10, so six-minutes of power is all you get before it is exhausted! Now it should be stated here that in practice, one should treat manufacturer’s claims of very high ‘C’ rates with a certain skepticism.

Apart from anything else, running the packs at their stated maximum ‘C’ rate will shorten their useful life overall. I have always found it good practice (and common amongst those people who care about such things) to try and operate my packs at about half their claimed maximum ‘C’ rate. So if I need a pack to supply 11.1V at 40 amps, I would look for a pack which claimed to have a capacity of say 4000ma and a ‘C’ rate of 20. The ‘claim’ is that this pack can actually supply its 4 amps at 20 times that rate (80A) but personally I would run it at half that rate (10C) = 40A.

Some packs even claim ‘C’ rates as high as 30C – but remember if you actually did run them at that, they would supply their juice for just two minutes! (1 hour divided by 30). Furthermore, the more you push your batteries, the less likely they are to be able to maintain their specified voltage, and this is also a factor in the quality of the battery cells. I have known some cheaper packs to virtually collapse when asked to supply their stuff at high ‘C’ rates. In this case the power available will fall very short of expectation, and using the Ohms law in relation to our desired power (wattage) you can see the effect of this falling voltage quite dramatically.

Standardising your connectors makes life much easier, These 4mm banana types are probably the most popular for UK flyers

If a 3s (11.1V) pack is supplying a decent 10.5V when under load, of say 10amps – that’s 105 watts (10.5 x 10), however if the battery can’t hold that under pressure, and drops by just 1.5v to 9.0V output, at the same 10A, then your wattage falls to just 90, a drop of 15 watts, almost 15%.

So you can see, keeping the voltage up is paramount in maintaining your power levels and is, in most cases, a better way of getting power than lower volts and higher amps (as discussed at the beginning of this article).If you remember in our discussion on motors and ESCs (last month), it is amps that kills these things not volts alone.

DISCHARGE

A very important factor in Li-Po cells is the amount that they are allowed to fall to in terms of final voltage after use. This figure is generally accepted to be an absolute minimum of 2.5V and better still higher, at 2.75 or even 3.0V. A fully charged Li-Po cell will read 4.2V when fresh off the charger (incidentally you must, must use a dedicated lithium capable charger for charging lithium cells) and discharge must terminate when 2.5V absolute minimum is reached (per cell) under load. This is where the choice of ESC is important, and good ones will ensure that the LVC (Low Voltage Cut-off) is set correctly to protect your particular battery combination.

IN A NUTSHELL

When it comes to charging, then Li-Pos should normally only be charged at a maximum rate of 1C – so a 2000 mAh pack will be charged at 2000ma (2A). Slower charging may well be advantageous in prolonging cell life, although this is not actually proven as far as I am aware. There is a lot of discussion elsewhere on internet forums regarding the procedures for charging and balancing, and I suggest you hunt these down if you want more detailed information about things like cold weather charging, and storage levels etc.

2.4GHZ radio gear has transformed electric flight by eliminating the glitches sometimes seenRADIO EQUIPMENT

It might be said that there is no need to have a section dedicated to this, just because it is electric flight, but I would disagree. There is little doubt that high powered electric powertrains can cause more problems with your radio equipment than i.c. powered models. One of the prime areas for concern is that of shielding the receiver from the stray electrical ‘noise’ which can be so effectively produced by fast switching devices like motors and ESCs. This noise can find its way back down the main wiring and even servo leads – especially longer ones. Fitting ferrite rings close to the receiver end of servo leads can sometimes help, but good practices in all areas is essential. These include not extending the main battery or motor wires if at all possible, and where it is required, only extending the motor to ESC wires, and not the battery to ESC leads.

High voltage spikes can be produced at the ESC and the battery effectively shunts these spikes out, but this is compromised if the leads are lengthened, and these spikes can then feed back into the ESC input stages and cause damage. Always try to mount receivers as far away as possible from motors and ESCs, and ensure that your aerial is in good condition, and again, routed away from sources of electrical noise.

The better quality (and usually therefore more expensive) ESCs and receivers perform better in many respects and cheap ESCs have been found to be lacking in this area. Good quality – preferably dual conversion receivers have often cured an otherwise glitchy model, too.

Electric models suffer vibration far less than their i.c. equivalents so simple receiver installations like this are perfectly satisfactoryWhilst we are on the subject of radio, I should mention more on the role of the ESC. Although it is primarily there to act as a throttle to your motor, many of the ESCs you will encounter deploy a device known as a BEC (battery eliminator circuit) and this is simply a regulator built onto the ESC circuit board, which reduces the high voltage of your battery pack down to a level that suits your receiver – normally 5V.

Badly designed BECs can introduce problems by way of insufficient voltage / current being available when needed, and can also themselves cause noise. When you move into the areas of higher voltage batteries, and larger currents, most ESCs will deploy something called OPTO isolation, and they do not have BECs. Instead, your radio is powered by a separate small battery, completely isolated from the main battery. The OPTO feature of the ESC completely isolating the main battery and throttle sensing wiring from the receiver through clever use of infra red transmission and receiving devices.

I can’t finish this section without mentioning the new kid on the block – 2.4GHz. Many will argue that a good 35MHz set is perfectly capable of providing a completely glitch free and reliable radio link and in most cases good quality sets will perform well, but frankly, in my personal opinion, 2.4GHz is a prayer answered when it comes to electric flight, especially in some of the more extreme installations such as EDF (electric ducted fans). I tried almost every thing possible to cure a little EDF model from the occasional glitch when still using 35MHz, but to no avail. Granted, it was nothing serious, but enough to make me a little nervous each time she flew.

Since installing my 2.4GHz radio, I can honestly say that I’ve never had a single hint of a glitch, and even when deliberately trying to induce a problem (for research purposes only) by attaching the receiver right on top of the ESC. My attempts were to no avail and everything worked perfectly as normal.

Many people find electric flight difficult to understand compared to i.c., and in some ways it is. However there is more to i.c. powered craft than many appreciate. The beginner is often baffled by things such as two-stroke or four-stroke selection, engine size, bore, stroke etc. Mounting an i.c. engine can often prove just as tricky as installing an electric motor, especially when setting up throttle travel and end points. The relentless march of technology and increased demand for electric equipment has seen prices tumble over the last few years, and there really is no better time to try your hand at it. Given sufficient money, any i.c. powered model can be converted to use electric power, and in some cases outperform the previous powerplant. Start off with something simple and inexpensive, and enjoy the clean and quiet revolution that is electric flight.

Missed part.1? Click here to read it.

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