|VoxSolaris: The Voice of the Sun|
|The VoxSolaris Variable Voltage Project|
The usual method of converting DC to DC at another voltage is to use a switch mode converter of some description. Such a device can be designed to supply a fixed or variable voltage output, accept a fixed or variable input and if need be, handle several inputs and outputs at the same time. The flexibility of a switch mode converter is perfect for many applications including the two we are most interested in, electric cars with or without a range extender and off grid power solutions. The problem for us was the efficiency. A good design would get of the order of 95 percent efficiency which sounds very good but that still means, to state the obvious, that you are paying a toll of 5 percent for the sake of having the correct voltage. In an off grid scenario that could make the difference between needing to fire up a generator and not doing so. And in an electric car it seemed particularly wasteful.
In an electric car with a permanent magnet DC motor the speed is controlled purely by setting the voltage. The accelerator sets the required torque and thus the required current. The required voltage at any given time will be that needed to overcome back emf (proportional to speed) plus that needed to give the required current on the basis of the ohmic resistance of the curcuit as a whole. In a typical arrangement a fixed voltage battery pack would form the input to a switch mode converter which by means of rapid switching, would charge short term energy stores (i.e. capacitors). Then the required voltage, calculated every 10 milliseconds or so from a reading from a sensor on the accelerator, would be assembled again by rapid switching, from the charged energy stores.
But in an electric car we already have stores in the form of battery cells so we only need to assemble the output. To ensure even discharge, the cells would have to switched in on a rotating basis. The output voltage is granular as it is essentially always a multiple of the cell voltage but for an electric car with a high voltage motor this granularity is inperceptable. With this approach, one half of the switching, the energy stores and the associated resistive losses with their rapid charge/discharge, are eliminated. This cut the toll to 2 percent or less and provided a means by which cells could be voltage tested while rotated out allowing superior battery management. When the output voltage of a cell falls below a safe threshold it is excluded from further dischage but regular testing provides a picture of cell deterioration over time and cells can be excluded from charging use if they fail to accept charge efficently. This contrasts sharply with batteries comprising a fixed number of cells connected in series where, depending on the type of cell used, one 'dead' cell can seriously undermine the performance of the battery as a whole.
The Retro Experiment
By assembling the output from cells rather than short term energy stores, less switching takes place. Instead of switching at 20KHz, typical of switch mode converters, switching only takes place every 10 milliseconds in response to the accelerator sensor readings. And in most cases, when a cell is switched in it is left in for many 10 milisecond cycles at a time. The slower switching rate did little to improve the effeciency of the voltage assembly process. The The power transistors still exhibited the characteristic voltage drop and consumed much the same power. But in the slower switching, of one of our committee members with a self confessed mistrust of electronics and of in-car electronics in particular, saw an opportunity to add a manual overide to the otherwise completely computer controlled system. At his request we benchmarked the transistors against solenoid operated mechanical switches that were switched on by a pulse and switched off by a pulse, rather than relays that are energized throughout the on phase. To our surprise, the switches performed better than we expected. The switches took too long to switch (about 1 millisecond) which looked horrible on the oscilloscope and distracted power output. But in terms of electrical efficiency, this ultra-retro experiment comfortably beat the transistors. The member's motion for a manual overide was voted out but experiments involving combinations of mechanical switches and power transistors are continuing.
The above describes how translation losses between battery cells and the variable voltage load, in this case an electric motor, are reduced. But the method of matching voltage by switching in an appropriate number of cells can equally be applied to the charge cycle if the input is DC. This is of relevence when considering range extenders, regenerative braking and other possible inputs such as the incorporation of solar body panels. With regenerative braking, the output voltage is cut below the back emf and tracked lower as the car slows. We have assumed there are no circumstances in which the brake and accelerator would be applied simultaneously so when the brake is applied the accelerator is ignored and treated as though the foot is off. Solar body pannels are just set to charge cells that are rotated out from the task of providing current to the motor.
The more complicated input to handle is that from the IC engine range extender. For highest efficiency two conditions need to be met. Firstly the IC engine needs to run within the most fuel efficient part of its operating range as much as possible. Secondly, while there are circumstances where it is appropriate to have the range extender recharge the cells, the power is better directed to propulsion motor as the cells don't give back more than around 85 percent of what is put in.
While the origins of the project owed much to an interest in more efficient electric cars, the project aquired the name 'Variable Voltage' to acknowledge a wider application base. It is a comprehensive system for matching multiple power sources that may or may not have stable voltages to multiple power sinks that may or may not have stable voltage requirements. As well as a means to manage power in a car, its other, somewhat simpler objective is to manage power in stationary micro-generation scenarios with or without a grid connection.
The project is controlled by software from HadronZoowhich is written in C++ and runs on the Arduino processor. The first part of this is complete but is aimed at prototyping. Before we publish anything we will need to test out several variations on the design and conduct formal field trials on the more promising designs. We are hoping to have this done before the middle of 2017. Pending legal advice we will at that point publish the circuit diagrams, configuration documentation and the software under a general public license.