More 12 Volts
The Prius has a 12V system that is anemic at best. Non-hybrid vehicles
rely on a powerful alternator to charge a big 12V battery, which in turn is
relied on for critical functions such as starting the car. Because the
Prius has a much bigger traction battery for starting the car and other
functions requiring a lot of electricity, the 12V system is much attenuated,
with a tiny battery. This can be a handicap when a lot of 12V power is
required. Normally this extra 12V power would be used for running such
accessories as inverters and communications equipment. While the rest of
this PriUPS site handily takes care of the inverter requirement, 12V at hundreds
of watts can still be useful, and isn't truly available from the Prius.
While scanning eBay one day, I came across an interesting device from a
company called Vicor. This company manufactures power conversion modules,
and specializes in DC-to-DC converters. The particular module on offer was
a 150 watt module, and it converts 300VDC to 12VDC with good efficiency and 150W
output at 12VDC. These modules can be paralleled to increase the power
capability. They are also available in 300 and 600W versions.
Although the input voltage rating is 300V, they are specified to operate from
375V down to under 200V, i.e., not only will they work at the lowest voltage of
the traction battery, they will also work with other hybrids such as the
Highlander, that have a higher voltage battery. These modules are
amazingly small - they weigh only a few ounces, and measure about 2-1/4" by
1-1/2". They must be mounted on a heat sink, but only generate about 30W
of heat at full power.
||Here's the module before any connections have been made.
This is the first one I got and I had some fun testing it, as you can
read in my blog entry
||Although communications equipment is normally specified for "12
Volt" operation, the actual operating voltage is normally between 13V
and 14V, since this is the nominal battery voltage while it is being
charged by the car's alternator. The two resistors at the lower
right are calculated to trim the module output voltage up to 13.2VDC.
|In order to get the desired power for my 100W
transceiver a 23A supply is specified, which is almost 300W. To
get this power, I paralleled two of the Vicor modules. (The yellow
wire connects the PR pins on each module to the other one. This
automatically allows them to share the load equally.)
||The heat sink and fan assembly is one that is normally used to cool a CPU
chip on a PC. I had some lying around, and I tapped mounting
holes in it and mounted the Vicor modules using 4-40 screws and Berquist
Sil-Pads for thermal conductivity. The heat sink and fans normally
have to dissipate a lot more power, so the assembly remains cucumbric
under load. Note the fuses in each side of the DC line.
|The power inlet has a makeshift bridge rectifier
soldered to the leads. Even though this module can supply 300W at
12V, the input current is negligible from the traction battery - less
than 2 amps, so the primary wiring needn't be heavy. The purpose
of the bridge rectifier is to protect against reverse polarity and thus
allow use of any handy IEC line cord instead of having to carefully
paint + and - on one.
||Here's a close-up of the fan end. Not coincidentally, the fans
run on 12V, so I'm powering them from the modules. The connector
on the right is for the 13.2V output that goes to the radio equipment.
During testing I found that this gets a little warm, implying a bit of a
voltage drop. It's best to keep the power supply as close to the
load as possible since the heavy current flows on the load side.
I'll shorten this cable when I do a permanent installation.
||The plastic case was just something the right size I had lying
around. It serves as a makeshift mounting for the power inlet, and
as a protector for the circuitry and any hands that would contact it
accidentally. For a permanent system it would
be better to use a metal enclosure for the whole thing, and punch holes
in it for air circulation.
||Here's a side view, with a label I scribbled. The modules are
out of sight, so I put the specs where I could see them without
This module assembly is just about right to power any ham radio transceiver
that runs on "12V." It weighs well under 2 lbs. There is almost no
limit to the amount of power you can get at 12V since multiple modules of
different sizes can be connected to the traction battery for different
applications. The Prius can easily power an emergency communications setup
for a police or fire department!