With the ever growing presence of electric vehicles such as the Nissan Leaf or the Tesla Model S, there becomes an increasing demand for electric vehicle charging stations. Normally a charging station, or commonly referred to as an electric vehicle supply equipment (EVSE), takes AC (alternating current) power from the grid and supplies it to the EV. The EV then has an on-board charger which rectifies the AC into DC to power the car. We want to explore ways to utilize photovoltaics, DC (direct current), to bypass this rectification process in order to improve efficiency in a direct DC-DC charging process. We are implementing this system at the Grange Hall for workers who own an EV to receive a partial charge while working.
We will be utilizing Sun Power E20 435 solar panels for our solar array to power the charging system. By creating two strings of five solar panels in series we are able to generate, ideally, 364.5 VDC with 11.94A. This accumulates to 4.35 kW and with San Luis Obispo having on average 5.5 full sun hours, this would charge a typical EV battery with 30kWH enough to drive home.
We plan to connect our solar panel system to a readily available ChaDeMo plug to attempt to charge a Nissan LEAF. While our original plan involved attempting to simply charge through the AC port, ultimately establishing an off-board charging system through the DC port would be easier.
Our primary challenge is the communication between the electric vehicle’s system. We have primarily experimenting with a Nissan LEAF yet no real life tests have been done. We know that the system will be able to accept the range of 200-450VDC as seen on a RAV-2-LEAF direct charge and that the system will also accept the current we input. However the greatest challenge lies in the instability of our current and voltage. As our source of energy are solar panels, there is great variability of voltage and current depending on time of day and weather. We hope that the voltage will not vary much which would mean the solar panels themselves would act as a controller. Yet, there is still question about the variability of current. Further testing is required.
Current DC Appropriate EVSE’s
There are currently a variety of different DC compatible plugs used today. The largest two competitors or the SAE J1772 CCS (combo charging system) as well as the Japanese company ChaDeMo. Currently we are mainly interested in the ChaDeMo charging system as this is the DC system compatible with a Nissan LEAF.
SAE J1772 CCS plug (left) and ChaDeMo plug (right)
Currently the cost of a ChaDeMo product is approximately $2500 which is quite expensive. Furthermore, there are many rumors that the ChaDeMo product itself may be discontinued as the future of electric vehicles require some sort of standard system. As many more companies are beginning to comply with the Society of Automative Engineers (SAE) standard, they will begin to switch their DC charging protocol to the CCS. This prompts us to try and acquire a cheaper alternative simply for experimental purposes. Thus a 3D printed model with attached electrical components is being implemented to hopefully allow us to establish the same connection.
On-Board Charger of Electric Vehicles
Our original plan of action to accomplish DC-DC charging was to still apply a charge through the AC port. Most EV’s have an on-board charger that transforms AC from the grid to DC to power the battery. A typical block diagram is shown:
However, solar panels provide DC and there lies the crux of the problem. The on-board charger accepts AC while we provide DC. Potentially, the system of the vehicle will reject our input and simply not charge. We hypothesize that running DC through the converter would be able to bypass the rectification process. This occurs as the typical four bridge diode set-up in a typical rectifier prevents AC from going backwards. However, running DC will only take one path essentially still producing a voltage. Further testing is required to confirm.
After realizing that the on-board charging system of an EV was much more complicated than simple rectification processes (for example it actually goes through multiple stages of control and rectification process), DC through the AC port would be unwise without significant modification to the internal system itself.
Persons Involved in Project:
Ryan Lau- 2nd year Physics major at Cal Poly who is passionate and interested in the different aspects of physics.