I've had two people ask me, in the last 7 days, about the merits of using lipo batteries in transmitters and receivers so I thought it might be a good time to do an article on battery technology, and power systems generally:
LIPO batteries are wonderful things.
They can store a huge amount of power for their size and weight (say 200watthours per KG), whereas a NIMH battery designed for frequent cycling will typically store about 70watthours per kg.
And LIPOs can deliver their power really quickly, and be recharged really quickly, whereas NIMH batteries are happier delivering their power over hours rather than minutes, and being recharged at, say, 1/10 of their rated capacity - overnight before you go flying.
So LIPO batteries are magic at running electric motors and, with care, will deliver years of useful service (or one flight if pushed too hard).
I think that most people who use them will know that LIPO batteries also have a dark side, which is that, if something goes wrong during charge, discharge or due to a crash, they are VERY unstable and will quite happily burn your workshop to a pile of cinders. I have seen a lithium battery go into flame and it took about 1 second (no exaggeration) to become so hot that it looked like it had come out of a blacksmith's furnace and, after sitting under water (salty water) for 10 hours, it took less than 5 seconds to go into flame state again. This is a chemical fire, incidentally, and pretty hard to put out, hence the need to store LIPOs in a serious metal case and never leave them unattended when charging.
They also have another downside, though, and that is their energy delivery, shown in the graph below. Basically they can deliver most of their power flat out until they are about 98% discharged, at which point the power drops off the edge of a cliff and you get nothing. That's great news for running an electric motor (you don't want it gradually slowing down throughout a flight, but not great news when you need a reserve of power. There is also about .3 of a volt (yes, 3/10ths of a volt) between "almost full" and "too late", which is a very small margin of error and something that most on-board battery checkers can't cope with.
Now, in small electric planes, foamies, gliders etc, the ESC has a built-in safety system to cut motor before things get critical, at which point the voltage recovers slightly, and you still have power to the receiver and servos to make your landing. I wouldn't trust this in larger electric planes so wherever there is space and capacity for a little extra weight, my receiver will be run from a NIMH 5 pack (Eneloop and now FDK 2500mah batteries). That means that on my large electrics, the LIPOs are doing nothing other than powering the motor, and ALL my flight controls are handled by NIMH, which have a very nice, rounded discharge curve, so my on board battery checker can tell me if they are struggling.
Would I use a LIPO in my transmitter? We'll, I'd be putting a live firework into $500 of equipment that I hold in my hands, in order to save a couple of hundred grams in weight, I get about 14 hours run time on NIMH which is adequate for most days out, but then I'd also be risking my plane with LIPOs in the transmitter because the transmitter voltage display is nowhere near accurate enough to give me a warning well before it got to the critical drop off point ringed in red, below. So no, I wouldn't, and I don't really care that some radio manufacturers think it is ok (or, perhaps more importantly, cheaper) because I suspect they think that in the same way that airlines, mobile phone companies and fancy electric car manufacturers regularly get caught out by lithium technology.
I really like LIPOs. I have 2 ammunition boxes full of them that are used in a variety of models. But I like NIMH technology too. It's horses for courses. I like hot glue, especially for fixing the grandchildren's toys, but I wouldn't use it for fixing a firewall prior to attaching a 5kw electric motor.
I hope this is useful.
I thought the following might be of interest, and might save a few planes, and a few bush walks.
I was recently flying a model that is several years old, and has been stored for at least a year. The RX battery was about 3 years old and had charged up normally on the Futaba trickle charger, but the model's battery indicator behaved oddly - showing a full battery as expected after just one flight but, when I moved any control surface, the indicator dropped very quickly to low battery, and then settled back to full when I left the sticks alone.
We have heard a lot about the dangers of big petrol engines and how to avoid getting sliced up by that big angry prop. While electric motors have always been regarded as dangerous and there are plenty of examples to prove that, we typically regard electric motors as a lot smaller and less powerful than petrol engines. In the majority of cases this is certainly true but even a small $10 motor can do nasty things to human limbs.
The one big risk/dvantage/feature of electric motors is that they can jump from "dead" to full power in a split second and without warning through a simple move of the throttle stick. This is quite different from petrol or nitro engines which cannot start on their own purely from a push of the throttle stick.
I do fly exclusively electric models and over time I have developed a healthy respect from these electric power houses. What I have also developed are a few simple rules which I follow and which I believe are at least partially responsible that in my modelling career I have never suffered even a scratch from a turning motor or prop.
You all know about the "do not power up an electric model in the shed unless the prop is removed" and these type of rules. I won't repeat these here but I want to show you some simple technical solutions on how to avoid these "oh shit, I didn't want to..." moments.
The first and most important rule I follow religiously for every electric model I have, is program and use a throttle hold switch on the transmitter! This is a simple on/off switch disabling the throttle stick on the transmitter. I always use the left shoulder switch for this. If the switch is in the up position, the throttle stick does nothing and if the switch is in the down position the throttle stick is active. Always before connecting the main battery I visually check the switch and make sure it is in the up (disabled) position.
The benefit is, that even if the battery is connected and the model ready, bumping the trottle stick has no effect. Most of the inadvertent motor starts and accidents with electric models are caused by the pilot accidentially bumping the trottle stick while carrying the model or working on the model. Can't happen if you have a throttle hold switch and use it!
I only enable the throttle with the model on the flight line and ready to take off and disable the throttle after landing before picking up the model. So far, this simple rule has done a great job and I can honestly say I have never inadvertently powered up an electric motor also I did have a few occasions where I did notice I moved the trottle stick while carying the model.
Every newish transmitter has a throttle hold function (sometimes with a different name) and on most older transmitters this function can be emulated with a mix or something similar. So if you haven't done it yet, go and grab your tx and add a throttle hold function to all your electric models! It could save your fingers or prevent your model slicing up your unsuspecting mate which just happens to be in the wrong place at the wrong time!
If you need help with your transmitter, ask myself or anyone of the more technical members.
Throttle hold is my personal most important safety rule and on most models I consider this, combined with common sense and the written safety rules, as sufficient. However, as models get bigger and motors grow with them, I'm feeling sometimes quite uneasy with just a little tx switch between me and that big prop mounted to a multi-kw electric motor.
The solution has been clear for a long time - do not power the motor until you are on the flight line and ready to go! Much easier said than done as often batteries can only be accessed by removing the wing or through a hatch on the bottom which make it impractical to plug in the battery at the flight line. The next logical way would be to add a switch like the common receiver switch. Good idea but switches capable of handling the high currents of a halfway decent size model don't really exist or are way too big and heavy. The only practical way is to use one of the common connectors, for example a deans connector as a switch. The way this works is that you cut the positive wire going to the ESC and add a deans or anderson socket. The socket is accessible from the outside and when you are ready you plug in the matching connector and close the electric circuit. Difficult to describe but the picture should help.
You can make an arming switch yourself or buy it from places like this http://www.flyelectric.com/arming-switches.html.
They are a good solution and work well but the major shortcoming is that they are limited in the current they can handle. Looking at the picture of the above commercial available arming switch suggests that the wire can probably handle about 40A and this is just about good enough for a small plane. Of course if you make your own you can use heavy duty wire and improve the current handling capability but still, deans plugs and the anderson plugs used in this example are only rated for 40A (anderson) and 60A (deans). These are the manufacturers ratings and while they can certainly handle more, this is not the solution for really big models.
Let's use the example of my Pilatus. With 2.7m span it is not exactly a small model and while it is actually very lightly built (<7kg ), it has been built as a glider tug. The motor in the Pilatus is a Scorpion HK-5035 with a "motor chief" gearbox attached. It runs on 12 LiPo cells and with the 26x18 Fiala prop it is capable of delivering 6.5kw while drawing 145 Amps and producing 23.3kg thrust!
I do admit that I went probably a bit overboard with that power system and I rarely reach half throttle in flight. Nevertheless, it is a good example of what a modern electric motor is capable and it is probably understandable that I feel much safer having some way of additional safety switch.
The arming switch above is no solution as deans plugs would melt at 150A and apart from that, soldering the wire gauge I use in that installation to a deans plug is almost impossible. Building the arming switch with the big 6mm bullet connectors would be possible but rather cumbersome and not exactly good looking.
The solution comes from Emcotec and is a solid state power switch capable of handling up to 240A.
They come with a magnetic switch which is installed in the fuselage and not only is easy to use but also looks great.
As long as the magnet is in place, the motor is visually and elecrically disabled. Pull on the string to remove the magnet and the plane is ready to go.
Disarmed and armed:
Before I had the Emcotec switch I kept the batteries disconnected until just before the flight. This meant I had to fiddle with the battery connectors inside the fuselage every time. Now I can connect the battery in the pitch area with the knowledge that the motor is still disabled and I can safely push the Pilatus to the flight line before pulling the safety switch.
But there is always a little disadvantage and in this case it is the price. This switch will set you back $110 from Australian distributor scmodels.com.au. The good news is, that this is considerably cheaper than the price when bought directly from Emcotec in Germany! Smaller and cheaper versions are available and if you are looking for a good safety switch you should probably visit scmodels web site.
I'm not pretending to be the safety guru who knows everything and makes no mistakes but I'm a technical guy and I like to employ the technical solutions available. If you combine that with a big dose of common sense and a little respect from the power of electric motors you have a good chance to finish your modeling career with a complete set of fingers ;)