It’s quite cheap and easy to build your own photovoltaic system with battery backup nowadays. Solar inverters with a battery interface are mature technology and can be purchased easily online, but there still is no solution to use your electric car battery as additional storage.
Making use of the huge battery we’re carrying around every day has many advantages, the most obvious is to power our houses during outages. But they’re rare nowadays, and the best use for this tech would be to charge the car at work with solar energy during the day, and use that energy to power the home at night. This can easily zero out the energy import from the grid.
I’ve already solved the “charging only from solar power” problem by building Prism, my entry for the Hackaday Prize 2019: https://hackaday.io/project/166859-prism Now it’s time to step up the game: here I’ll explore the options to power my house with the energy stored in the car's
If you’re a smart engineer or even anyone with a working brain cell, you might be asking why car manufacturers are not making their charger to be bidirectional already:
After all, (almost) all the required power electronics for a 7kW (or more!) converter is already included in the car charger! It’s just a matter of replacing secondary side diodes with a bunch of mosfets and writing some software to make it bidirectional:
A standard cheap EVSE could then negotiate the energy transfer and connect the car to the grid. There are only a few drawbacks for this approach:
carmakers will not be able to sell more overpriced hardware to the users
grid configuration and regulations are different from location to location (this can be solved by sending the configurations and parameters to the car via the overengineered ISO 15118)
some car batteries may have a voltage that’s too low for the DC/AC converter when they are discharged
Turns out, a few carmakers are starting to offer V2L (Vehicle to Load) to power utensils but still forbid V2H (Vehicle to Home). In this category we have the Mitsubishi Outlander, the Ford F-150, the Hyundai Ioniq 5 and the Kia EV6. Look at that cool adapter!
The crafty hacker can now make a suicide cable and power its house, yay!
Car manufacturers are hesitant to allow users to use the battery energy for anything other than driving for various reasons, like battery warranty or not wanting to compete with other products they’re selling like the Powerwall.
Only the Nissan Leaf historically offered bidirectional energy transfer capabilities since 10 years ago, and a few bidirectional chargers popped up to support this using the CHAdeMO connection standard. CHAdeMO allowed CAN bus communication between the vehicle and the EVSE, and bidirectional power transfer could be requested.
It must be noted that this is simply the same procedure as DC fast charging: the car and the charger negotiate a session, and the car directly connects the battery bus to the terminals. In a DC charging session, the charger would start the current flow to the car. But the battery voltage is directly available at the connector, so we can just connect any 400VDC load and we’re good to go! Unfortunately all modern cars interrupt the session and disconnect the contactor as soon as they detect energy flowing out of the battery, so that’s a no-go. But there's still hope! There’s a standard called ISO 15118 that describes the new communication method for DC charging (CHAdeMO is deprecated). I will rant about this standard in a followup post, but suffice it to say that it supports many Bidirectional Power Transfer modes! Unfortunately no car has implemented it yet, so we’ll have to wait at least for a software update if EV manufacturers are generous enough to provide one.