Motorhead Memo: Capacitor Flux

Quoth Doctor Emmet Brown, “The flux capacitor is what makes time travel possible.” Naturally, at a critical time in time, the DeLorean/time machine fails to start. Supposedly because the flux capacitor, discovered by Doc when he slipped and hit his head on the toilet while trying to hang a clock, had lost its charge… of something like 1.21 gigawatts! This, of course, leaves our hero stranded and makes for, soon thereafter, a damn good story about getting Back to the Future. It’s classic science fiction, as long as you don’t think too hard about the battery. The battery has to start the DeLorean so it can go 88 mph to start the flux capacitor and enable time travel. How’s that work, anyway? Kinda like the latest electric/hybrid cars, with a regular battery for starting, a bank of monster batteries to run, and capacitors to do lots more. Well, while we’re thinking about that, there’s some scientific fact creeping up on us that makes Doc Brown’s deal seem plausible. Let’s call it a capacitor “flux.” Some of us, especially with old motorcycles using points ignition, might know the basic capacitor by it’s other name: condenser. But capacitors aren’t so basic anymore. Come to think of it, neither are batteries, so we’ll start there.

Battery “charges”

Technically, “battery” is a military term describing an array of weapons functioning together. The reality is a little different for the battery we know and love. The device that may (or may not) actually enable you to start your engine, play with your cell phone or run that pacemaker, among many other things, is an electrochemical contraption intended to store an electrical charge. The bazillion nuanced iterations of the basic idea only have that much in common. Beyond that, about all you can say is they have been with us since long before the devices they power and they have never, ever fulfilled their initial promise. Whether lead acid, AGM, lithium, nickel cadmium, or any of many internal compositions, the big breakthrough, the battery we want and have been promised, has not yet appeared, after close to 200 years of incremental development! But don’t take my word for it; ask the electrical technicians of the Boeing Dreamliner, or the folks at Tesla Motors, or check with experts in almost any field that relies on them. Indeed, for most movable, portable applications, or even as backup for critical grid outages, batteries are still the only game in town. A brief list of pros and cons, well known and unwavering, might clarify continuing issues in the imperfect battery, yet explain why the damn things are still the lesser of evils. Face it; batteries are heavy, short-lived, run hot, can be explosive, are full of acid, environmentally nasty, and unpredictable if not downright unreliable in service over time. (Forget cell phones and computers; these facts remain a brutal obstacle for electric vehicles in the future, including Harley’s LiveWire.) On the other hand, where they still hold the edge over any other electrical storage option, for now, is their “energy density,” an ability to store electricity for reasonable periods of “run” time (making electric vehicles possible at all). The other measure, “power density,” is the ability to deliver or absorb electricity, rapidly, which is where batteries lose out. Mostly, this is because the whole concept of a storage battery relies on a chemical reaction, while capacitors do not.

Lightning in a bottle

In one of history’s major ironies, none other than Benjamin Franklin proved that electricity can be stored without a chemical process in his famous experiments with linked-up Leyden jars. What he invented was an early form of capacitor. By any other name (capacitor or condenser) the types we are most familiar with as motorcyclists have been busy for decades storing and releasing electrical energy at incredible speeds, to make sparks at contact points and occasionally to eliminate batteries! Old-timers recall those days when you could tell when a condenser was going out of capacity by examining the faces of points for pits and craters. Some of us were racing then as well, and getting rid of a heavy, vibration-sensitive, potentially troublesome battery was always a good idea for a race bike. At least as long as you could kickstart it! Yeah… those little blue beer can (or nowadays “energy drink”) looking things, that lived in coiled springs under the seat were capacitors that used two charged plates separated by an insulator to get the job done. Better living without chemistry. In those days, demands were simpler, and so-called “battery eliminators” were adequate, even though using them on the street meant dim lights at idle and a horn that didn’t work for crap. Of course, a neat thing about it was you could use it, wired in parallel, along with a battery, which gave you a “backup” plan for limping home if the battery was failing or had failed. The advent of electric starters, turn signals, EFI and a host of on-board electronics unimagined 40 years ago put paid to the battery eliminator/capacitor as we knew it.

Those old capacitors we used in lieu of a battery were clearly “power density” devices, as anyone who used them will attest. You couldn’t have the lights turned on when you kicked the engine over, let alone power an electric starter, but they could be “charged” with a couple kicks, last nearly indefinitely and like their smaller condenser cousins, actually worked better at high rpm. The reason? First the nature of how they worked, not providing an electric current, rather as an ultra-fast electric field generator. If you look at the alternator system of a bike with an oscilloscope as you kick it through, you see that voltage doesn’t stay constant; it isn’t a straight DC voltage, but more of a pulse DC. Putting out an AC sine wave, alternator current goes through the rectifier, which essentially inverts the negative half of the waveform. Meaning, instead of the wave looking like a positive pulse, and then a negative pulse, to a capacitor it acts as a series of positive waves, going from zero volts to 12+ to zero to 12+. Once the bike is running this occurs so fast it doesn’t matter, but at low rpm when kicking it’s a big deal to ignition coils because the voltage drops, collapsing the magnetic field inside of them, creating the spark. They fire when electricity stops flowing. Capacitors act as a very fast charging/discharging “medium.” When the voltage goes up, they charge, when the voltage drops to zero, they dump that charge essentially evening out the waveform and letting the coils build up a good spark. Think static electricity as it builds up walking on a rug then zaps you when you touch a doorknob… it’s very close.

Ultra-super caps

For motorcycle applications as an old-school battery eliminator, those blue can types (Lucas 2MC type, as they are usually known) were rated at roughly 4,700 “microfarads,” each microfarad being one millionth of a farad. (This unit of measure named after Michael Faraday is so huge it is to electricity as “light years” is to distance. Hence the micro.) The modern thinking for the same use (in motorcycles that can use ’em) is closer to 22,000 microfarads or more, for nice bright lights at idle and upwards of 35 volts to keep the charging system from cooking. All well and good for folks who intend to use them in their old kicker sleds and again; in conjunction with a battery, why wouldn’t you? But, looking to the future instead of the past, we can now anticipate the use of super caps in our modern machines, not instead of a battery but as a battery! Or maybe a hybrid battery, actually…

So, here’s the deal… this basic nature of capacitors makes them more attractive than batteries for a great many future applications, especially in vehicles. As always, that means overcoming downsides and/or modifying the upsides of capacitors, which has been going on with great forward strides for the last 20 years or so. (Way more progress, in fact, than there has been with battery technology in the same span.) We are now to the point where so-called “ultra” or “super” capacitors are here and happening! What matters most with all capacitors is “capacitance” (the ability to store a charge). Let’s stick with the fact that super/ultra/double layer caps have so much more capacitance they approach being useful as batteries, largely because the plates in them are coated with a spongy material which means way more surface area to soak up a charge. As an example, a regular D cell battery has a capacitance of around 20 microfarads. Whereas a super cap might handle more than 300! Here—right here—is where things get a little sticky, because there’s voltage involved. So, where a regular capacitor of 20 microfarads should be able to handle 300 volts… a super cap can only manage 2.7 volts! Put another way with all its amazing abilities in other areas, a super cap can only store about five percent as much energy as a comparable lithium-ion battery. It’s a closer race when compared to small (say motorcycle-sized) lead-acid batteries… but still a hurdle, when it comes to replacing the old-fashioned battery with cutting-edge super caps. The tradeoff of speed for staying power remains. But what if the tortoise and the hare join forces?

What goes around comes around

A symbiotic relationship is defined as cooperative and mutually dependent… hopefully beneficial. It’s easy to see how the positive characteristics of batteries and super caps, melded together, could be real eye-opener in terms of electrical performance. In fact, used in electric vehicles, super caps could provide the power needed for acceleration, while the battery provides range, then recharges the caps between surges. The trend in fuel-saving tech for automobiles also plays this game with so-called “stop/start” cut out of the engine while sitting at stoplights. Super caps take care of that, then get recharged as you drive. So, it would seem that batteries and super caps already make a good duo, so why not take it further?

Like the old-school capacitors wired in parallel with a lead-acid battery on kickstart bikes, using lithium-ion and super caps combined into a single unit—a “hybrid” or “modular” battery—might just be a great thing. There are at least two companies pursuing the notion as this is written, one here in the U.S. and one in India. The American company, Maxwell Technologies, is currently focused on their successful super cap-based “ESM” (Engine Start Module) for diesel engines in over-the-road semi-trucks. Adding one to a bank of batteries relieves them of any starting function. Never needing a jumpstart and making it easy to restart a rig (to comply with anti-idling pollution laws for diesels) makes this a big damn deal and a godsend for truckers. Better fuel economy, less wear and tear, longer battery life, extreme temperature functionality, and guaranteed electrical performance (to the tune of 100,000 cranks) have actually led to ESMs as a factory option on new Kenworths and Peterbilts.

The other player offering tomorrow’s technology today is Chheda Electronics (www.masterignition.com) who just went ahead and did it! Their product is almost exactly what the doctor (Brown?) ordered for motorcycles! Nothing for huge Harleys yet, but they are already building hybrid batteries for small motorcycles and scooters, even a few electric-start applications! The super caps are made into a purpose-built casing, which in turn plugs into an equally purpose-built VRLA (sealed) lead-acid battery, so the two become physically one unit. The super caps start the motorcycle and the battery re-energizes the super caps—neat! Can’t wait till the design trickles up to a Harley-sized version! Flux indeed… Doc Brown would’ve loved it!