WA8LMF Mirror of WB4APR Website - 21 July 2008 FD Priuis Power

Field Day Prius Power

Bob Bruninga, WB4APR

Field - Power - To - Go!

This page describes a Prius Power project to combine high power solar, with thousands of watts of backup gasoline driven generator power in the Prius for providing emergency power for field operating events.

Emergency Power for Field Day: Every year Ham Radio operators across the USA take their communications equipment to the field for a national operating contest to demonstrate emergency preparedness by operating for 24 hours on Emergency Power. . Historically, this involves noisy gas powered generators for high power stations, or solar power for lower power operating stations.

The photo to the right shows several youth at the Anne Arundal Radio Club Field Day in 2004 participating in a demo of Amateur Radio Satellites using simple chicken-wire antennas.

PRIUS POWER BACKGROUND: The Prius has a 12 volt electrical system for normal car accessories, a 220 Volt 7 Amp-Hour Battery for propulsion assistance, a 50 kW motor-generator, a 50 Kw DC/DC converter/inverter, a 2nd motor-generator, and a 76 Hp gas engine all integrated into a seamless power system. . When the ignition is "on" (called READY) in the Prius, a built-in 100 amp DC/DC converter is powered up to provide 12 VDC to all normal car circuits and accessories. . In this READY state, the Prius engine will warm up, and then shut down. . It can remain in this state indefinately and provide plenty of electrical power and, as the energy system is depleted, the engine will automatically start to maintian the charge on the battery. . It can maintain this "READY" state indefinately until the gas tank is empty. See more on my Solar Plug-in Prius

12 VDC Power: The 100 Amp DC/DC converter is water cooled and can provide plenty of DC power to external loads. . This is by far the easiest way to use the Prius as an emergency power station. . But there are disadvantages as well for distributing this power to a larger field event:

  • 12 volts requires big heavy cables to handle these currents
  • 12 volts can only be distributed a few feet without substantial voltage drop
  • The Prius 12v battery is very small compared to a regular car battery
  • Its small battery can only power loads for a few minutes. .
  • Because of the small battery, the car must be in READY standby so the inverter is on.
  • In READY state, the Prius consumes about 200 W of wasted power even with no load.

    220 Volts DC: The larger 220 VDC 7 Amp-Hour Prius battery is about 10 times bigger than the 12 volt battery and provides 200 to 240 VDC depending on state of charge. . Thats about 1.5 KwH of battery capacity. . See the 220 VDC outlets on the back of my Prius in the photo at right. . If one can figure out how to use this power at this voltage, then distributing power from this 220 VDC power source has several advantages:

  • For the same power as 12v, wires at 220 VDC carry only 5% of the current
  • For the same power as 12v, wires at 220 VDC can be 20 times smaller
  • For the same power as 12v, 220 VDC can be distributed 20 times further.
  • Some devices can operate on 220 VDC directly (heaters, etc)
  • Many modern electronics systems are dual power, with 115/220 VAC power options.
  • Many dual-power 115/220 VAC power supplies when set to 220 VAC, will work just fine on 220 VDC!

    220 VDC Disadvantages: The Prius will still draw the same 200 Watts of wasted overhead power if left in READY standby for indefinate operation as it does when left in READY mode providing power at 12 V. . But with the much larger capacity of the main 220 VDC power system, for small loads, the Prius can be powered down, to save that inefficiency and allow it to run external loads much longer. . But there are disadvantages to 220 VDC:

  • Operating with the ignition off (not in READY), however, requires vigilant operator monitoring. Running the main 220 VDC battery down below 40% depth of discharge can get the Prius control system in a very unhappy state from which it may not recover without Toyota intervention!

  • Playing with 220 VDC is dangerous if not well respected. However, DC is safer than AC and the 220 VDC is only slighlty higher than the peak 168 volts of a 120 VAC circuit. Further, this 220 VDC system is floating with respect to ground, and avoids the ground-fault dangers associated with conventional one-side-grounded 120 VAC systems.

    Further down the page are examples of using this 220 VDC power. But first, there is another source of power in my Prius as shown above.

    SOLAR-POWER: . What's unique about this project is the addition of solar panels to the Prius to not only provide fre energy to offset the cost of gas, but also to provide an effective application for solar power in the first place. . The solar panels shown here can provide as much as 215 peak Watts of power good for about 1.7 KWh of free electricity when parked for 8 hours in the Arizona sun. . Unfortunately, only 800 Wh or less is available from the average Maryland Sun.

    PURPOSE - Power-to-GO: . The purpose of this conversion is to put solar panels where they can be more useful than stuck on the roof of my house. On my hybrid they may gain 10 to 20% in MPG. . But like many other solar energy projects, this investment has an indeterminate cost-effective break-even point, because it solely depends on how much you need power when you are not plugged into the grid. Solar Power portability gives peace-of-mind where there is no gas or plug-in electricity available. . (more on my Solar Prius)...

    And if you can take solar power where you need it, then its value can be far greater than 10 cents per KWH. . And if you need power in the field, and don't have any, then solar power is cheap at any price. . Just consuming it in the Prius is an equivalent payback of 25 cents per KWH which is better than the 10 cents at home. The incentive for this conversion comes from the simple economics of solar power:

  • Solar power with battery storage is rarely cost effective at home when competing with 10 cent per KWH electricity!
  • Solar power *is* cost effective if it provides electricity where you do not have any.
  • A Prius *not* in your driveway needs energy and is a great place to stuff free solar power to offset the cost of gas
  • And/or you can plug your house into your S-PHEV during blackouts.

    SOLAR 220 VDC BATTERY: . To use this solar power for driving the Prius, an added battery is often used to provide greater range on all-electric power. . Another advantage of this extra battery is that it allows for greater dynamics on a cheaper battery while not having to worry about damaging the very expensive OEM Prius battery. . This battery also provides a nice place to stuff any extra solar power without any risk of overcharging or over cycling the OEM battery. . The photo above shows the 9 Amp Hour cells prior to being wired together in series. A special controller shares this power with the Prius system for driving. . But it can also be used for powering external loads with less worry.

    220 VDC POWER CONNECTORS: . One of the first issues with distributing 220 VDC power is finding a safe connector that is innexpensive and readily available yet, cannot be confused with any other electrical connector. . This connector must be unique to prevent anyone from plugging in any non 220 VDC device. . The connector we came up with was a standard two-prong power plug but with two PREVENTION devices to prevent inadvertant plugging in of other devices. The connector is shown to the right:

    A standard two-prong socket is used, but without the usual 3rd ground pin. . This blocks the use of any 3 wire 115 VAC power cord. . In addition, a 3/16ths inch nylon protrusion is fixed to the center of this receptical inbetween the two prongs. . This blocks the use of any 2-wire 115 VAC devices. . To make a mating two prong plug, a standard replacement plug is used but the center is drilled out to pass this blocking pin as shown in the photo. . On the right, this center pin protrudes into an unused, and isolated area of the hard plastic plug construction. . These connectors cost under $2 each for both the plug and the receptical.

    Lightweight Power Cords: . Because of the much lower current demand at 220 VDC, a #18 zip cord can easily carry plenty of power over hundreds of feet. . In the photo above, the original 25 feet of large orange extension cord were removed and about 120 feet of #18 lamp cord will fit on this convenient spool system. . The current carrying capacity of #18 is as high as 10 amps, but I limit mine by fuses to only 5 Amps. . Again, the outlets are blocked from any inadvertant use by the uninformed by the dual protection-blocking system. . Only switching power supplies or other loads that can operate on 220 VDC are provided with these matching plugs.

    220 VDC Switching Power Supplies: . The reason distribution at 220 VDC is simple is beacuse almost all modern power supplies are designed using switching regulators to elminate the large, heavy and bulky 60 Hz power transformers. These switching power supplies work on DC input that is obtained from 115 VAC or 230 VAC via an input rectifier as shown above. . These supplies when set to the 230 VAC position will also work well on 220 VDC.

    LAPTOP Power supplies: . For simpliity, most laptop switching power supplies are also dual voltage (115/230 VAC) but to avoid the hassle of a switch, the supply is simply designed to operate on any voltage between 115 to 230 VAC directly without a switch. These supplies can often be plugged directly into 220 VDC.

    Any experimentation you do with your Prius or your Electronics is at YOUR OWN RISK!

    These notes are only intended to share my results and for no other purpose!


    Bottom line: . If you are going to invest in solar power anyway, you may as well put it on your car where you can use it to offset the cost of gas and where you can take it with you wherever you go or might need it. Field events, emergencies, or just watching the kids soccer match (with the Prius electric A/C)... This array has been designed to just fit the size and shape of the Prius while having a good voltage capacity for easy charging.

    ANALOG INSTRUMENTATION: . A retro analog instrument panel was added to keep track of the electrical power going into and out of the Prius and added Solar battery and solar panels. . There is also an added tach and vacuum gauge for the gas engine. . The left meter is 150 to 300 volts and the right one is -50 to 0 to +100 amps. . The APRS ham radio below the dash integrated with GPS provides tactical situational awareness of the positions, status and messages of similarly equipped Amateur Radio mobiles in the surrounding area. . The APRS network is linked to the internet and so you can always see where I am.


    Bob Bruninga, WB4APR Naval Academy Satellite Lab

    WA8LMF Mirror of WB4APR Website - 21 July 2008