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Is there a DC charging system available that can be powered by a stand-alone PV system?
I am aware that one could charge from a 240v AC system, but I would like to avoid converting to AC and then having the charging system convert back to DC. There must be a considerable loss of efficiency.
Hi Barry – that’s a good question – and the answer to it revolves around two key issues:
- What are the purposes of AC vs DC charging and
- Would a direct DC to DC system for electric vehicle (EV) charging would increase the overall efficiency of charging an EV with solar.
To begin with, DC charging is designed for rapid charging of the EV battery at high currents and the control of the charging is principally carried out by the EVSE (Electric Vehicle Supply Equipment) itself. The issues of using the DC output of a home battery system I have previously looked at here.
AC charging in comparison is designed for trickle to overnight charging at lower currents. This form of charging is principally controlled by the EV on-board charger in ‘consultation’ with the EVSE (BTW: I have covered the needs of charging an EV on solar outputs here and here).
At first glance – it would therefore seem that AC charging an EV would be better suited to the limited output of a home solar system.
On the other hand, trying to charge an EV directly from a DC solar output presents many layers of issues for any potential system design.
To name a few:
- You will need much more in the way of electronics to control the EVSE and car charging as compared to an AC EVSE (making it more expensive);
- you still need electronic DC voltage regulation of the solar supply output to the EV, as solar systems (a) have different output voltages depending on the design of the system, and (b) have highly variable voltage outputs related to the amount of solar radiation they receive. (eg in the morning, evening, when clouds pass over, etc, etc, etc!). This means you are still not eliminating conversion losses entirely as you will need a DC to DC converter in place of a DC to AC inverter;
- even with a controlled voltage, you will still need to control the DC charging current to provide a steady supply given solar systems have highly variable outputs as the sun rises, clouds pass etc. This is at odds with the design of current DC fast-charge systems (making it a special design EVSE that is not currently made).
Moving on to the second part of your question: as you mention Barry, the down-side to using an AC EVSE on a solar system is the potential losses through doing the DC to AC and back to DC in the EV.
However such conversion systems are now very efficient – solar inverter (DC to AC) efficiency is in the realm of 95-97%, and in-car charger efficiency (AC back to DC) is also comfortably over 90%. Taken together, it means total losses are actually not all that great.
Also, given that in a home solar DC to EV DC charge situation, you are really only eliminating one step and not both – you will be gaining only a little in efficiency overall.
Additionally (unlike DC EVSEs) there are already AC EVSEs on the market that offer the ability to track solar power output and ramp EV charging current up and down to not exceed it. (For a single phase system, AC EVSEs can easily vary from 1.4kW through to 7.4kW and on-board EV chargers are designed to comfortably cope with this).
- Direct DC to DC EV charging from a solar supply is at odds with the natures of both DC charging (being a ‘fast-charge’ system based on providing high currents through a carefully controlled DC voltage), and solar supplies (where current supply is limited and DC voltage varies considerably).
- Whilst losses do exist for DC to AC and back again exist, they are not large and by doing a ‘direct’ DC to DC charge, you are still only eliminating one step, not two;
- AC EVSEs by design are ‘trickle charge’ systems and far better suited to home solar system outputs than DC charge systems;
- There are already AC EVSEs available that are optimised for use with solar systems, whilst no direct DC to DC systems for home solar systems are currently available.
Of course, all this is subject to change.
The EV charging world (and electricity world in general) are at the lower end of their respective innovation curves: so it’s a matter of ‘watch this space’ where it comes to battery storage system (and possibly even vehicle-to-grid) developments.
However, if you currently want EV charging that maximises the use of the solar output from an off-grid or grid-tied system – AC charging is currently your only option for an efficient, cost effective off-the-shelf solution.