In part one of our dummy’s guide to electric motorcycles we learned about general terminology and motor types. Now, Brett Gober, a self-described “knuckle-dragging redneck” that we discovered leaving suspiciously intelligent comments on HFL, takes us through the difficult stuff: batteries. — Ed. >
Electricity is weird. It isn’t that hard to learn about, but there are some counter intuitive things going on sometimes, and it can definitely be confusing. Hell, the idea of flow from one pole to another in a circuit has two conventions “conventional flow” and “electron flow.” We say that charge is flowing from positive to negative, the actual reality is that positive is ground and the negative pole is the one with the excess electrons; the electrons are flowing from negative to positive. When Ben Franklin was observing his little experiments with electricity, he got the polarity wrong. The wrong polarity stuck, they couldn’t change the convention, and it’s been confusing people for 200 years. That’s how it stands today.
Polarity does not matter. Electricity works exactly the same, backwards or forwards. If you hook up a motor backwards, does it make negative power? No, it makes negative torque, negative revs, the damn thing spins the other way. It is essential to understand that polarity does not matter if you want to even try to understand AC.
Let’s go back to the old “it’s like voltage is the size of the pipe, and amperage it the flow of water through it” metaphor, which I’ve always disliked because it does not explain the difference between hi-voltage/low-amperage and low-voltage/high-amperage very well. Use the pipe metaphor and high-voltage looks like a placid river (big pipe, little flow) and low voltage looks like a fire hose (small pipe, lots of flow). I assure you, a lightning bolt is anything but a placid river. Nikola Tesla kicked Edison’s ass because Edison was fiddling around with 12v Christmas-tree lights while Tesla was lighting off discharges of millions of volts, just to see what happened.
If you want a metaphor that we can all relate to, try this one:
Voltage=engine displacement, amperage=carb size
A 50 volt/300 amp system is a 50cc engine with a huge carburetor, like one off a big-block V8. Hauls butt, but it keeps overheating and you can’t get any more top end out of it. You could put two Holleys (600amps, two motors) on there if you wanted, but gosh, the thing already pulls stumps, what are you looking for?
A 300 volt/50 amp system is a 300cc engine with a teeny-weeny carburetor, the one left over after you decided it was a good idea to put a 4-barrel Holley on your fiddy. Goes fast, but you really wish it had more punch and it can’t pull fourth and fifth anymore. You could punch it out (600volts), but gosh, it is pretty fast already and maybe if you muck around with gearing it will run right.
That makes things a bit clearer, don’t it? Keep in mind, it is just a metaphor and like all of them, flawed, but it is why I look at the KTM Freeride spec sheet and say “Holy Crap, 300v!”
They bumped-up the displacement and I am sure you guys all know what that means.
The good, the bad, the ugly:
Before I can start digging into batteries there are a couple of basic vehicle dynamics things we should look at. OK, so rather than just looking at motors, lets look at the whole system for a moment. On any EV there are three main components: batteries, controller and motor.
The batts act as the fuel tank and the engine at the same time. They are the source of both the energy (how far you can go) and power (does that thing do wheelies?). The bad news is that batteries don’t store a whole lot of energy, the good news is that they can make pretty good power.
The controller regulates power to the motor. You can simply hook up a motor directly to a battery, but better stand back while you do it because it is going to go “twack!” The controller sorta does what a carb or fuel injection system does on an ICE, but it is a weak analogy. Suffice to say that the name, for once, is an accurate descriptor; controllers give you control, it is directly connected to the throttle/twistgrip.
The motor acts as the transmission; it is the interface between electrical power and mechanical power. Electrical power is not mechanical power (the stuff motorcyclists are interested in) until it is converted to torque and revs. Motors make this conversion quite well, this is why multi-gear transmissions are not a necessity on an EV, a motor is quite capable of making torque in a wide range of revs. You could put a multi-gear transmission on a motor, but it acts more like a hi/low range more than the multi-gear transmissions we are used to in ICE vehicles.
Math, it’s gonna get ugly:
We are going to have to do some math here, but stay with me, I’ll boil it down to something understandable.
Gas and energy:
Gasoline is a great medium of energy storage. One gallon of gas weighs about six pounds and can be calculated as having ~33kwh (kilowatt hours) of energy. This means, if combustion was 100% efficient, one gallon of gas could produce about 44hp for one hour (33kwh/0.746=horsepower hours). Combustion is nowhere near 100 percent efficient; in gas engines you see efficiency ranging from about 12 percent (two-strokes) to 25 percent (a really good 4-stroke). At 25 percent a gallon of gas (6lbs) can make 11hp for one hour, convert it back to kilowatts (11hp-per-hour*0.746=8.25kwh), and there is your energy-density (8.25kw/6lbs). Now toss lbs for kgs (6lbs/2.2=2.7kg). (8.25kwh/2.7kg=3kwh-per-kilogram), and you have the useful energy content after combustion. Doing the numbers like that is a little wacky, 3kwh/kg is way too high, it would mean that a motorcycle running on flat ground at 35mph would get 100mpg+, possible, but only for little engines. The reason being that big engines only make good efficiency numbers under high loads. Asking a 50hp engine to only make 3-5hp involves a lot of waste, the efficiency goes back down to ~10 percent and realistically we need to add the weight of the engine in there somewhere. The real number is somewhere between 1.5kwh/kg and 3kwh/kg. If your eyes are glazing over from all the numbers I am tossing out, don’t worry about it, all I am trying to do is show:
1. It is difficult to figure out exactly the useful energy content of gasoline.
2. It is easy to figure out the energy content of batteries (more on this later).
3. It is difficult to rationally compare energy content of batteries to the energy content of gasoline.
Looking at a battery and asking ‘How many gallons of gas is that?’ requires an answer that will have to make a number of assumptions. You can ballpark it (it seems like ~8-10kwh of batt = 1gallon of gas), but there is so much fudge-factor involved I am loath to trust the numbers.
Gas and power:
Gasoline has excellent power density. If you are willing to toss efficiency out the window, the limits on how much gas you can pour into an engine are, to make a huge understatement, high (think jet turbines, fuel is used at a rate of pounds-per-second). T
he limit is the size of the combustion chamber, pour more fuel into a small engine, it does not make any more power, just blows fuel out the exhaust (running rich). Something like the BMW S1000RR is making impressive power density (good power-to-weight), but it’s only impressive because it’s a one-liter engine, if it was two liters, meh. We look at power density in gas bikes as an overall spec, power-to-weight, of the whole motorcycle. I am only putting this in because, with batteries, we look at the power density of JUST the batts sometimes and it’s difficult to compare that with gas. At least until the whole bike is built, then power-to-weight is something we can compare directly. Enough, lets look at batts.
Batteries and energy, this is depressing:
Batteries use a couple of specs to describe energy and power. All are rated by amp-hours; how many amps the cell can deliver for one hour. If you have a 10ah battery in front of you and you put a load on it, it will crank out 10amps for one hour. The cell also has a voltage rating, it is a range of voltage, but cells are called out at a nominal voltage, which makes life easier. So, lets call our 10ah battery a 12v nominal. We can multiply voltage by amperage and figure out that the battery has 120 watt hours (it will make 120watts for one hour, 0.12kilowatt hours) of stored energy, divide kWh by mass (in this example, let’s say 1kg or 2.2lbs) and you come up with 120 w-per-kg (a far cry from 3kWh-per-kg), a measurement of stored energy by weight. It will make 120 watts for one hour. It will also make 240 watts for half an hour if the power rating is good enough. Keep going down that road and you can also see the battery make 7200 watts for one second before it is dead. This brings us right to the other spec that we have to know, how much power the battery can make.
Batteries and power, hmm, not so bad:
Batteries are rated a little like motors as far as power goes. There is a continuous rate, and a peak rate. It is called the “C” rating. A 1C continuous rating a battery can, in fact, discharge its amp-hour rating in one hour without bursting into flames. Using our 12v/10ah batt from above, a 1C continuous rating means that it discharges completely in one hour without heat problems. If the battery is a 2C rating, that means it can discharge its amp-hour rating twice as fast (twice the power!), in our example you can pull 20 amps out (2C times 10ah), but since there isn’t any more energy available, you will only see that 20 amps for half an hour.
That takes care of continuous power, but what about peak? Most batteries can momentarily deliver power at a much greater rate than their continuous rate. It is rated the same way, by a multiple of its amp-hour rating, C. The company manufacturing the battery will say “1C continuous rate, with a 10C peak of 60 seconds,” the time stipulation is what keeps the battery from overheating. The more powerful batteries will actually specify a number of peaks, with different time stipulations on each: “1C continuous, 10C for 60sec, 30C for 30sec, 50C for 1sec.” With our 12v/10ah/1kg batt, a “50C 1sec” rating would allow a burst of 500a. 12v*500a=6kw, 6kw/0.746=8 horsepower, which is damn good for something that weighs 2.2lbs (1kg)! Power-to-weight is something batteries can do, but, as a disclaimer, very few batteries have a 50C discharge rate and a battery that runs flat in 1.1 seconds isn’t all that useful.
Power-density(specific power) is measured in watts-per-kilogram (w/kg), our example cell would be 120w/kg at its continuous rate(1C), 6000w/kg at that crazy one second rate(50C).
What a motorhead looks for is the 3-5 second peak power, that is the number that tells us if the battery can boogie. I’ll go into why that’s the number we care about right after a little more about batts.
Chemistry, or why we shoulda stayed in school:
Batteries are using chemistry to fiddle around with electricity. It would be much easier if there was just a magic box that we stuffed electrons into, after all the little guys are nearly massless objects, but no, the only way, for right now, to store electrons is through torturous chemical backflips and trickery. The various industries that use batteries are creating the kind of demand that has battery companies fighting tooth-and-nail to come up with better stuff, the innovation over the last ten years has been astounding. I am not going to give a complete run-down on every battery chemistry out there, there is so much variation in construction techniques and manufacturers that it wouldn’t be helpful. I can do a general overview.
Lead-acid, the good old days:
The only good thing about lead-acid batteries is their power-density. The energy density is piss-poor. A good lead-acid batt can easily deliver current at a 50C rate. The energy content is somewhere around 20-30w/kg, which is terrible. They make good ICE starter batteries, the newer absorbed glass matt cells are much better than wet cell batts. If you still have a wet cell in your gas bike, do yourself a favor and drop a sealed AGM in it, they last a lot longer and have better cranking amps.
Don’t build electric motorcycles with lead-acid cells, please!
Hmm, a little better than lead-acid. Fair power-density, you can see 10-20C rates, not as good as lead, but still pretty good. Better energy density than lead, 30-80w/kg. Huge problems with memory and heat. Very careful battery management systems required, and even then cell cycle life (the number of charge/disgarges a cell is good for) isn’t all that good. Remember all those ni-cad packs in battery powered drills? They work OK, but nothing great. Fairly cheap, despite being made of nickel.
Don’t build motorcycles with ni-cads/nimh, maybe 5-6 years ago, but not now.
Lithium-ion/lithium-polymer, the good stuff, for now:
This is what all the ‘modern’ electrics are using. Within this basic category are about 20-30 or more variations.
They all share similar specs, poor to damn-good power density, 2C-50C rates. All have good energy density, ranging 100w/kg to 180w/kg. Between four and ten times better than lead-acid. Common chemistries are: lithium manganese oxide, lithium cobalt oxide, lithium iron phosphate, lithium vanadium and lithium titanite. Tackling the pros and cons of every single li-ion chemistry is like drinking from a fire hose. Generally, the ones with good power-density have lower energy-density and the ones with really good energy-density have crappy power-density. There isn’t a single chemistry with both, yet. They are all much better than anything else. The biggest concern for vehicle application right now is safety. The power dense cells, the ones we want, have an unfortunate tendency to occasionally burst into flames. Think Dell laptops. Most of the innovation in the last five years has been towards making it safe to look away after you plug a cell in to charge. It is sorta OK to build motorcycles out of them, but you better have your battery management down-pat or the cells will fail.
Better chemistry for better riding:
If you believe the press releases, there is a nano-turbo-transhypertitanate battery right around the corner. Truth is that energy storage is just plain difficult. Optimistically, we could see battery energy-density double in the next ten years. It is definitely not going to happen any faster than that and, pessimistically, it could take 50 years. The power density is already good, not great, but not the huge technical challenge that increasing the energy-density presents.
Back to Power, WTF is happening when you pull the trigger?
So knowing a bit more about batteries and
motors, lets take another look at one of these bikes. We will pick on the KTM, they are big boys and can take the abuse. Spec on the battery they use is 300v, 2.5kwh. Continuous power on the motor is 8kw, peak is 22kw. Those are nearly all the numbers you need to know to figure out quite a bit about what the bike can do. Hmm, need top speed, 42mph. I’d like to know how quickly it gets from 0-42mph, for argument’s sake lets say it’s three seconds.
Remember when I said we want to know what the 3-5 second power rate is on a battery? Mostly it is so that we know that the batts will be powerful enough to force the motor to get with it. We only care about the three second rate because the bike only goes 42mph, thats all the time it takes to accelerate to that speed. A lot of folks look at specs like this and assume that 22kw (peak power)/2.5kwh (energy-capacity) = 8.8 minutes run time. That would be true, but only if you were accelerating at full load the whole 8.8 minutes. 8.8 min*60=528 seconds. 528 sec/3sec=176 full power launches. That is enough to go do some backyard racing. It ain’t enough to go run a 50-mile loop in the woods with your 4-stroke buddies. 20 mile loop? Probably, it depends on the elevation change. 100-mile poker runs are out of the question.
42mph is slow. Can they gear it taller? Yeah, but remember that whole continuous power thing? When you gear-up a bike you increase the amount of power it can use, going faster uses energy at a faster rate. 30mph is the point at which aerodynamic losses start kicking in, at 60mph you use four to five times the power that you do at 30mph. It runs down quick and, realistically, it can’t pull that much more gearing. To go faster you need to make the bike more powerful.
The higher voltage is better, but it has a restriction plate on the “carb.” Its 22,000w/300v=73amps. Pathetic, I can make 73amps with my little finger. Double that amperage, you can pull all the gearing you want.
Weights and Measures:
To conclude, sorry about all of the KW=HP=MPH=KPH=WH=KWH=mumble-mumble.
I am American, and as such I think in mph, mpg, and all the rest of the horrid imperial measurement system.
I run a CNC, so I end up switching from fractions to decimal-inches to metric all the time. The most confusing part about most of this is the number of standards there are, and having to switch back and forth all the time. If you followed all the number crunching, well done, give yourself a pat on the back.
If you would like to learn more about electricity theory in general, go pick up a copy of this book. I don’t think there are any other electricity manuals endorsed by Dave Barry.
– Brett Gober