The Electric Motorcycle, Part 4

Battery-charging systems are complicated!

Brammo Empulse battery pack

Power from overhead power lines is AC, so some system must be used to convert that to DC at the voltage required to charge the vehicle’s battery. Or a dedicated high-current DC charge point must be found that is plug-compatible with your vehicle. There must be protections against short circuit (ever had a rectifier diode fail on your car’s alternator?), over-voltage, over-current, and over-temperature. All this information must be a part of the charging system software.

Decisions must be made: Are we happy with the low power charging from a household 120V/15-Amp plug? Wattage is Volts times Amps, or 120 times 15 equals 1800W. If our vehicle is carrying a 14-kWh battery, we can roughly figure charging hours by dividing 14,000 by the realistic 1500W we can pull from the wall outlet without popping the circuit breaker. The result is 9.33 hours. If we need to charge faster and our system can accept the higher voltage, we can plug into a drier/stove socket supplying 240V/30A equals 7200W. Because the charging process is not 100 percent efficient, actual charging takes a bit longer than this simple arithmetic suggests.

The above two options assume that there is an on-board charging circuit, equipped with a transformer to convert the line voltage to the battery charging voltage, and a rectifier diode array on a heat sink (energy conversion is never 100 percent efficient, and the wasted energy becomes heat) to convert AC to DC.

Now there is the question of whether the charger is to be carried on the vehicle or to be part of a fixed charging point. An argument for the on-board charger is that it “knows” the battery with which it is teamed and can provide the necessary above protections. Nobody wants to be sued, so some charge points err on the safe side, only charging vehicle batteries to 80 percent.

Saving more vehicle weight and offering faster charging are dedicated DC charge points that “talk” to your vehicle’s charging software. This makes it unnecessary to carry the charging electronics (diode array, heat sink, transformer, associated wiring) on the vehicle.

Another option is to use parts of the vehicle’s own traction electronics, as it already carries a muscular diode array for the purpose of converting the battery’s DC into the traction motor’s three-phase variable frequency AC. This potentially allows high-current charging without having to carry much extra equipment.

Finally, there is the battle of the charge points, as various systems vie to become the worldwide standard for electric vehicle charging. Having invested big money in its version, each player is determined that its system will not end up as the “Betamax” of this competition.

Brammo Empulse cutaway

The US-based Society of Automotive Engineers (SAE) standard is its J1772Combo, adopted by US and German automakers. The quite different CHAdeMO standard is supported by some Japanese automakers. Tesla’s “Supercharger” system provides a half charge to Model S cars “in as little as 20 minutes,” which offers one model a future for interstate electric vehicle travel. You drive to the next “Supercharger” station (there are presently 116 of these) and you have tea and look at your phone during the 20-to-30-minute half charge. This charging is free to Model S owners who have chosen the 85-kWh battery.

Li-ion batteries do not like to be over-discharged or charged too fully, and both charging current and voltage must be limited. Compromises must be accepted between the life of the battery and how much of its capacity is used. Seeking to store more energy, we are tempted to charge and discharge a battery more fully than is best for long life.

Excessive heating can result if Li-ion batteries are charged at too high a voltage, with unwanted chemical reactions such as production of gases (remember, Li-ion batteries have to be sealed to prevent ingress of atmospheric moisture). If charge is delivered faster than the battery can accept it, the surplus energy is consumed in “parasitic reactions” that may damage the cell. This is the reason battery electrolytes contain fluorine compounds (like lithium hexafluorophosphate); fluorine’s very strong chemical bonding resists these parasitic reactions. Fluorine appears for the same reason in lubricants for extreme temperatures.

Li-ion batteries generate little heat early in the charging process, which allows rapid charging to half charge, after which the charging process slows and more heat is generated. Returning to my airliner-loading analogy, empty seats fill quickly, but once passengers have to climb over each other to reach the dwindling number of empty seats, the process slows down, and “heat” is generated. The ease and rapidity of the first 50 to 60 percent of the charge is what allows rapid charging at Tesla’s “Supercharger” charging stations.

More complex charging methods are possible, such as charging in short, intense pulses, followed by a recuperation interval during which bad outcomes like the growth of lithium metal dendrites or generation of gases may be reversed. This can be aided by even having a pulse of discharge, followed by another intense charging pulse.

Fortunately, charging is automated to provide the necessary basic protections, so you need not sit up with your electric motorcycle through the night, monitoring charging current and voltage, and placing an affectionate hand on its battery pack from time to time to be sure it’s not getting too warm.