Efficiency & Waste Heat
Efficiency is a dimensionless parameter expressed in percent. It is the ratio of power output divided by power input.
Inefficiency in any system produces waste heat.
The majority of internal combustion engines convert only about 20 to 30 percent of their fuel's heat energy into useful rotational power at the end of the crankshaft. The lion's share (70 - 80%) of the original chemical energy is waste heat which leaves via the exhaust and cooling systems.
Electric vehicles have several sources of waste heat, described below.
Battery chargers for electric vehicles are typically switched-mode power supplies. They convert AC power from the utility into DC power to charge the battery. I have measured the efficiency of the 5.7's charger at 87% and the ePure's charger at 93%.
The round-trip efficiency (energy in versus energy out) of a single lithium-ion cell is about 95 - 96%. For comparison, this number for lead-acid cells is only about 75%.
However, when a number of li-ion cells are connected in series, the efficiency gets a little worse. The individual cells that make up a battery pack are not all identical. There may be slight differences in internal resistance, actual capacity, and rate of self-discharge. A BMS is necessary to compensate for these differences.
By design, the BMS wastes energy.
The BMS attempts to bring all the individual cell voltages to within about 10 mv of each other. It does this by resistively discharging the higher-voltage cells to the level of the lower-voltage cells (although there are safety protocols in place to prevent overdoing this and making the battery useless).
More advanced BMS technology uses active circuitry to remove charge from the higher-voltage cells and redistribute it to the lower-voltage cells. Although much more efficient than a simple passive discharge, the active process is not itself without losses.
Motor efficiency is the ratio of mechanical power output divided by the electrical power input. Very generally, electric motors have peak efficiencies in the 90-ish percent range. However, efficiency is not a single number and can vary greatly throughout the motor's operating range. (See the ePure's Motor section for a typical efficiency map.)