Written by: Inverters R Us Customer, G.V.
I own a Toyota Prius. It has a 12-Volt battery, but it’s not under the hood; it’s in the passenger compartment, next to the spare tire. That makes it easy to permanently install an inverter next to the battery. It’s a bit tight, but you can fit a small inverter between the battery and the spare tire, and you can attach it to the body of the car with self-piercing metal screws after drilling small pilot holes in the body.
I did this for the first time several years ago with my first Prius. I replaced that car this past Summer with a new Prius and, soon after taking my new Prius home, I installed an inverter in it. The installation requires sawing away a piece of the styrofoam baskets that hold the tools. It felt a little strange taking a saw to my brand-new car, but it was for a good cause.
The Prius engine, as in most hybrid cars, does not idle in the conventional sense. The high-voltage traction battery powers the car under normal conditions without the engine running. The engine starts from time to time and runs for a few seconds, as needed to keep the traction battery charged. The engine in my Prius runs for only about thirty seconds every five minutes or so when it’s in “Park” with no accessories turned on.
The Prius has a 12-Volt electrical subsystem for compatibility with conventional cars. That way, it can use standard components such as light bulbs, windshield-wiper motor, etc. that are made for automotive use. However, unlike conventional cars, it does not have an alternator mechanically coupled to the engine for recharging the 12-Volt battery. Instead, there is a DC-DC converter that emulates the behavior of an alternator. Whenever the car system is “on” the DC-DC converter draws power from the traction battery to charge the 12-Volt battery; the voltage across the 12-Volt battery is kept at a constant 14.1 Volt when the car is “on”, as would be in a conventional car with the engine idling. In the Prius, this is so regardless of whether or not the engine is actually running.
I had to decide how big an inverter I could attach to the 12-Volt battery without overloading the 12-Volt system. In a conventional car, the 12-Volt battery has to be large enough to start the engine, and the alternator must be powerful enough to recharge that battery; but, in the Prius, the 12-Volt battery is much smaller because it does not have to start the engine. How much power can I safely draw without damaging the system? I approached the problem in two different ways. First, I located the circuit diagram for the Prius and I found that the 12-Volt system is protected by a 150A fuse. Certainly, I should not exceed (or even get close) to that limit. Second, I estimated how much current might be drawn, under normal conditions, by the car’s subsystems: headlights, audio system, rear-window heater, A/C fan, etc. If I don’t use any of those while I am using the inverter, that power is available for the inverter. I decided that I could probably draw nearly 1000W without trouble. This corresponds to a current of about 85A at 14V, assuming 85% for inverter efficiency.
I bought an 1100 watt inverter for around $100, the price even included heavy-gauge wires!
As I mentioned, the inverter can be permanently installed in a Prius right next to the 12-Volt battery. This is good because the wires can be made very, very short: one wire is 6″ long and the other wire is 10″ long. With such short wires, the voltage that the inverter sees, even under heavy load, is virtually the same as the voltage across the battery; essentially, the inverter always gets a solid 14V when the car is turned on. This gives the inverter a good operating margin and makes it better able to handle surges with high efficiency. One thing to be careful about: one should remove the styrofoam baskets when operating the inverter to make sure that there is adequate air circulation for cooling.
I completed the installation this past Summer, shortly after buying my new Prius and… two weeks ago, on October 29, hurricane Sandy arrived (I live in NJ, near the coast). We were lucky not to suffer major damage, but we lost power, together with some six million of our neighbors.
When the lights went out, I had already set up my Prius in our detached garage, with a cable running from the inverter to our breaker box. All I had to do was to start the car, throw the transfer switch in the breaker box, turn on the inverter, and voila’ the lights were on. Well, to be honest, before getting to that point I had to turn off quite a few things in the house. After all, 1100W is not exactly a lot of power for an entire house; but in the following days we were able to have a relatively normal life with refrigerator, hot water, heat, lights, internet and television, all on 1100W!
We didn’t leave the house for over a week, but through our television we were able to see the incredible devastation that had swept our area. It was sobering and very sad to see how many people had lost so much. Our immediate neighborhood was lucky because no-one suffered major damage or injuries, but we were out of power for many days, and ours was the only house with lights on.
On the second day after the storm, we invited our neighbors over for dinner. People brought food that was spoiling in their refrigerator. Everybody was happy to meet their friends, enjoy some warmth, recharge cellphones, and watch TV. Most of our neighbors had not yet seen images of the storm’s damage. They were stunned.
After a couple of days of outage, some of our neighbors got generators. And then the gas shortage came. That’s when I realized that the Prius-with-inverter set-up had a major advantage over a generator that I had not envisioned: I had filled the Prius tank the day before the storm, and, by the time power was restored several days later, I still had almost half a tank of gas left. And I had kept the inverter on without interruption, 24 hours a day, through the entire power outage. My neighbors with generators were going through a five-gallon can of gasoline in less than a day!
The one inconvenience with my inverter was that, whenever I overloaded it, it turned itself off to avoid damage, and I had to manually reset it. This is normal behavior for an inverter, but it happened frequently, especially at the beginning as I figured out what loads it could and could not handle. We have a detached garage, which is good because I could leave the car turned on without fear of fumes entering the house; but resetting the inverter involved going from the house to the garage every time it happened. I had to do it many times during the height of the storm, and I confess that being outdoors at that time was rather terrifying.
So, when things went back to normal, I went on the inverters-r-us web site and I found, to my delight, that they sell inverters with a remote switch!! I quickly ordered one. I made sure that the size is small enough to fit in the space available in the Prius. Now, when the next storm comes, I will have power and I can trip the inverter as many times as I want without having to go out in the storm to reset it!
Click here to see some PSW inverters that will work with a Prius!