By Scott Rhoades and Mike Wizynajtys –
I’ve been flying planes with internal combustion engines for a couple decades now and the thought of going electric causes me to hyperventilate. I’m trying to learn about electric flight but there just seems to be a bazillion things one needs to understand just to get that first electric plane in the air and I find the reading on the subject so friggen boring.
Having ventured into electric helicopters recently I learned a few things but for the most part I took the easy route and relied heavily on HCH member Mike Wiznajtys, because he has this electric stuff pretty much figured out. Once Mike had me and the heli on the right road I thought how cool it would be if he wrote an article or two for the newsletter, demystifying electric flight.
When I asked Mike he gave the subtle look that simply says; are you nuts? Well little did I know at the time the enormity of what I was asking. Although instead of flat out saying no, Mike later emailed me a PDF he had found labeled, “Everything you wanted to know about electric powered flight”.
This file is a 72 page long e-book. It’s basically a compilation of input from internet forums. What I intend to do is present parts of this book in a series of articles over the next few newsletters.
Not sure how many articles this is going to take because the idea here is just to relay electric power basics, not delve not ‘Everything you wanted to know’. Also the wording in this document can be somewhat confusing to the true neophyte. So as a true neophyte I’m taking the liberty of doing some serious rewording as the proverbial light bulb goes on with each passing ah ha moment during my learning process.
Without further ado here’s the first installment of what you need to know to enter the realm of electric powered flight.
AMPS vs. VOLTS vs. C rating
This discussion is intended to clear up a few terms and concepts of electricity as it applies to electric airplanes. The beginning of this may come across as a little dry but if you stick with it I promise it will all come together later.
First, think of electricity like water. Volts = pressure, while Amps = flow. Volts is like pounds per square inch (psi.), however it says nothing about how much water is flowing, only how hard it is being pushed. Just like the water pipes in your house right now. If no faucets are open there is no flow (amps), however the water is still under pressure (volts). If you open a faucet you have flow (amps) so amperage is similar to gallons per minute of water that is going through the pipes.
Ok you’ve likely heard amps and milliamps, so what’s the difference? The difference is just moving a decimal point. 1 amp (short for ampere) = 1000 milliamps. So a milliamp is 1/1000 of an amp.
If we look at amp hours or milliamp hours (mAh), this would be (flow) over (time), in other words how long can those amps be sustained. This is used as a way of measuring how much electrical capacity a battery is capable of, just like how many gallons of gas is in your tank. Although mAh says nothing about flow or pressure. Let’s take a 7 cell NiCd pack that provides 8.4V (pressure). The motor will draw electricity from the pack at a certain flow rate, or amps. If the pack is rated at 650 milliamp hour (mAh) that means it can deliver a flow of .650 amps (650 milliamps) for one hour. If you draw it out faster, it will not last as long. Let’s bump up those numbers and say the pack is now powering a motor that is pulling 6.5 amps (6500 milliamps) then the battery will only last for 1/10 of an hour, or about 6 minutes.
What is C in relation to batteries?
C ratings are simply a way of talking about the maximum charge and discharge rates for batteries. 1C, = 1 times the rated mAh capacity of the battery. For an example let’s go back to our 650 mAh pack. A 1C, maximum charging rate is 650 milliamps. 2C would be 1.3 amps (1300 milliamps). 3C would be 1950 milliamps… I think you get the idea.
When looking at motor batteries they are often rated in both discharge C and charge C. So if we take a 1600 mAh pack that is rated at 15C discharge. This means it can deliver a maximum 24 amps (15 x 1600 mAh /1000 = 24 amps) without damaging the battery. The same battery might be rated at 2c maximum charge rate, so you can charge it at 3.2 amps (2 x 1600 mAh / 1000 = 3.2 amps) without damaging the battery.
Let’s look at some other C rating examples just to make sure the concept is clear. Take a 2200 mAh pack that is rated for 20C discharge, that means you can pull 44 amps. (20c x 2200 mAh /1000 = 44) If you have a 3300 mAh pack that is rated at 30C discharge, that means it can deliver a maximum 99 amps. Easy stuff right?
When determining what size pack to use, first you need to know what your amp draw will be. Then you have a choice to use a pack with a higher C rating or a pack with a higher mAh rating to get to needed amp delivery level.
Motor batteries vs. receiver batteries:
Some batteries can sustain high discharge rates. Others can not. Those used as transmitter/receiver packs typically are made for low amp (flow) rates, while those made for motor packs can sustain higher rates.
Having a 600 mAh pack does not tell you if it is a motor pack that can put out 6 amps, or if it is a transmitter/receiver pack that would be damaged if you tried to pull power at 6 amps. Clearly a motor pack could be used for a transmitter/receiver job, but a transmitter/receiver pack should not generally be used as a motor pack.
It is best to size your battery packs so they run somewhat below their maximum C rating. You will stress them less and they will last longer. For example, if your motor needs a pack that can deliver 10 amps, getting a 1000 mAh pack that is rated for 10C ( 10 amps ) it meets the spec, but it is running at its limit. A 15C rated 1000 mAh pack would be better, or perhaps a 1300 mAh 10C pack. In either of these cases, the pack will be less stressed and should handle the load much better over the long term.
That’s enough for this installment. In part 2 we’ll talk about sizing the power system.