Reversing the flow– when cars power houses

“You don’t have to plug it in” Toyota keeps insisting in their commercials about the Prius. A conservative company confronted with the problem of marketing a disruptive technology  to a conservative audience, opted for doing the exact opposite of what every other automobile manufacturer is doing: convincing you that their product is just like any other car. While every one is spinning tales of personal liberation and spiritual enlightenment, Toyota says this is just another car. You drive it like any other car, never mind this “hybrid” mumbo-jumbo and there is no fussing with a power cord.

The problem is owners want to mess with power cords. Toyota management may want to brush up on their copy of Crossing the Chasm, because the people buying into the technology are still early-adopters at this stage and they are interested in pushing the limits of the technology. Already CalCars demonstrated plug-in conversions and Google has a fleet of converted Priuses drawing power from the solar panels lining the roof of the parking lot for buildings 45-46 in Mountain View campus.

The capability goes both ways: they can either draw power from the grid (or off the grid from renewable sources, in the case of Google) or they can supply the grid. The economic viability rests on a type of “price arbitrage”– charge at night when electricity is cheaper due to lower demand, then supply the grid during the day when it is more expensive with all those air-conditioning units whirring away.

But that vision is a few years out for a vehicle rolling off the assembly line in bone-stock condition. Without voiding the warranty or paying for a conversion that costs more than the car itself, there is no easy way to tap into this resource. Undeterred, a community of tinkerers continue to push the limits while holding on to the warranty. One of the more interesting projects is PriUPS, a pun on UPS for Uninterrupted Power Supply.

The standard answer to unreliable utilities and hurricane-prone regions is a portable generator. But a car is effectively a portable generator. When the engine is running, the alternator supplies current. (Nevermind the battery commercials showing sports cars equipped with their brand shredding rubber: the battery starts the engine but is largely out of the picture under normal driving.) Tapping into this with an off-the shelf inverter is standard. Up to ~200W can be drawn directly from the cigarette-lighter adapter– is that an anachronism now that few people use it for that purpose?– and upwards of 2000W by hooking up to the battery itself. This means that in an emergency or power-loss, a car could power basic house-hold appliances.

In principle the Prius is great for this application. First it is equipped with a large bank of batteries distinct from the ordinary one (called “traction battery” in Toyota language) that can supply more juice. More importantly, it has automatic power management to sense when the battery is running low and run the engine to recharge. This is a big improvement over running the engine constantly or manually stopping/starting. Ordinary car batteries are not designed for deep-cycles so they can’t handle being nearly discharged completely and come back to life. The Prius traction battery has no such problem.

But the engineering department had other ideas and drawing power from the traction battery was not high on their list of priorities for the car. (Besides– what would happen if potential buyers got wind of that capability? It’s no longer “just another car” and they would run away screaming, intimated by all this technological complexity, according to Toyota thinking.) PriUPS project details all the hoops to jump through to get this working. Detailed descriptions makes for good reading in their own right, but the short version is this gentleman managed to get 2400W while the engine was running 40% of the time in burts of 3 minutes. 5-6KW is the upper limit extrapolated from there, more than enough to keep a residence fully functional, minus air conditioning.

Interesting enough Honda, Toyota’s biggest competitor at home, has a healthy business in small, portable generators from home back-up power. Exactly the type that could be replaced with a Prius in the garage.


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