Stand-Alone
Sound Digitizing Device
Lansing,
Michigan
September 27, 2002. We left Dallas for Okemos, Michigan. Dr. Stuart
Gage and our small team are quite happy with the overall performance
of the pilot Sound Digitizing Device (SDD). So far the system is
very stable; no known outages, the quality of the sound is very
good, and it is connected wirelessly with spread spectrum radios,
providing MP3 audio streams to an MSU Internet site http://cevl.msu.edu
as the "NSF prototype machine".
The next logical step would change the power system of the SDD to
a completely stand-alone system, independent of utility power. As
installed, the system ran on standard 110V AC supplied from an outlet
on the Gage's home. Utility power is usually not feasible for true
remote deployments, but tackling the independent power task during
the last trip was not viable at the time. We did not have enough
proof of concept experience to warrant the expense of the solar
powered system, and there were some minor adjustments to make to
the system to improve its operation in the way of file naming conventions
and ability to access the system from a remote location.
During
the initial installation phase, we worked around the network limitations
of the existing network at the Gage home where our pilot system
would be proven. This trip would focus on providing better communications
with the SDD for remote administration and updates, provide another
option for streaming and restore the network operation to the same
virtual operation that existed before we first installed the SDD.
To enable
the system to be powered from the sun, we took steps to decrease
the power consumption by removing the video card requirement from
the Linux boot sequence. We acquired two large 100Watt solar panels
and two 90Amp hour batteries to get the system through the long,
dreary Michigan days that are fast approaching in fall and winter.
The solar
panels and batteries are from West Marine http://www.westmarine.com.
The solar panels are supplied with 7Amp rated charge regulators.
The panels can produce 6.7Amps each, so the entire power system
is placed in parallel. The panels are connected to the charge regulators
independently, and each is connected to its respective battery.
Finally each like-polarity battery terminal is tied together - positive
connected to positive and negative connected to negative, so the
batteries are in parallel.
Solar
Panels and batteries staged for installation. The SDD system is
still in its temporary mounting location.
Guard dog "Cowboy" supervises the installation.
Batteries are capable of truly incredible amperage, so protection
of all wiring is very highly recommended. If there is a fault, such
as a short in a device powered by the batteries, a fuse rated at
or below the maximum current rating of the wire, in series with
the device (load) and the battery will protect the system from damage.
In the case of a fault, the fuse will open, preventing current flow.
In most cases, it is highly desirable to locate the fuses in a location
easy to reach, and near the point of power origination, such as
at the battery terminals if possible.
Mike
making battery connections. The charge regulators are not plugged
in at the solar panel end for safety during this point of the installation
In this
installation, the charge regulators are acting as circuit breakers
between the solar panels and the batteries per the manufacturers'
instructions, no fuse required here, but highly recommended if the
charge regulators were not designed to open the circuit during a
fault condition. Additional fuses are placed in other locations
to protect the wiring. One fuse is between the positive poles of
the two batteries to prevent a fault in case one battery has a severe
internal problem. This link will open reducing potential destruction
of the wiring between the two batteries. A second fuse is placed
between the batteries and the load. If there is a major fault on
the load, the current drawn from the batteries will not exceed the
current capacity of the wire. A third fuse is placed between the
main fused link and the SDD computer platform. This was left over
from the previous installation, but it is not a bad idea to protect
the DC-DC converter of the SDD system. In case the DC-DC converter
has a fault, it will not self destruct to the point it would if
it were on the main fuse. The fuse should be smaller and open before
major damage is done.
See figure
1 below:
The illustration above is a standard way of installing a power system.
The source (solar panel) is protected from a battery fault by the
charge controllers. The batteries are protected from one another
by the fuse between the (+) positive terminals of each battery.
The batteries (in this case a source for the load) is protected
by a fuse on the positive lead of the load. Also shown is an additional
fuse inside the load providing protection specifically for the SDD,
this could be repeated for each load, such as the Pre-amp and the
radio.
The solar
panels are rated at 100 Watts each in direct sunlight. Wattage is
calculated as P=E*I, where P=Power, E=Voltage and I=Current. If
E provides a charging voltage of approximately 14.4V, then I= P/E,
(100/14.4) or 6.9 Amps. Since we have 2 solar panels, we might see
2 x 6.9, or 13.8 Amps charge current at peak sunlight. Efficiencies
result in a slightly lower charge current, so plan on a few percentage
points less.
Since
Michigan is cloudy many days of the year (I grew up in Michigan,
I know it is cloudy quite a bit) and knowing the solar opportunities
are limited, this solar powering system needs to be far more robust
than a system installed in a sunny region such as in Colorado, Nevada,
New Mexico, etc., where the sky clear most of the time. However,
some reduced amount of charge current is still generated by the
solar panels even in cloudy conditions. In our case, our load draws
only 1.2 Amps, including the SDD, Pre-amp for the microphone and
the spread spectrum radio. This is a fairly efficient system! 1.2
Amps should be generated on the most dreary days, short of a solar
eclipse.
If we
do not have charge controllers or solar panels with reverse blocking
diodes, our next worry is night drain. At night, solar panels without
built-in blocking diodes turn into resistors or effectively a load
connected to the batteries, draining the batteries in parallel with
the intended load (the SDD). To prevent night drain, blocking diodes
are usually added to the system between the solar panels and batteries.
The anode of the diode is connected to the solar panel positive
lead, the cathode to the battery positive lead. The negative lead
of the solar panel is connected to the battery directly. The diode
must be rated a bit higher than the maximum solar panel output,
such as 10 Amps for a 7 Amp panel.
In our
case, we don't have to worry about night drain from the solar panels.
Our solar panels have blocking diodes built in. Look for this feature
when buying larger panels. We do have the constant drain of the
load day and night however, consisting of the SDD, Pre-amp and Spread
Spectrum radio, a 1.2 Amp constant drain, a drain of 1.2 Amp-hours
(1.2 Amps per hour.)
In Lansing,
Michigan, we are just a few degrees south of the 45th parallel,
so it gets dark fairly early in the evening and light late in the
morning during winter. We may also have a problem with snow on the
panels. So we must plan on a fair storage capacity during these
periods of no charge. The batteries are rated at 90 Amp-hours each.
When connected in parallel, the battery bank yields about 180 Amp
hours of capacity. The load draws 1.2 Amps, so in one hour with
no solar power, our battery storage will be depleted to 178.8 Amp
hours reserve. In 150 hours or 6.25 days, our batteries would be
dead. A good number for "dead" should not be zero volts,
but rather 0.9 volts per cell as a minimum voltage, or 10.8 volts
across a 12 Volt battery. Discharging a battery beyond this level
can result in internal cell polarity reversal, a mode fairly terminal
to battery life (no pun intended).
Low
light levels require large panels To get us through the dark Michigan
winter.
After
a 14 hour night of load drain with the SDD, we will have consumed
16.8 Amp hours of current. This would take roughly 1.28 hours of
direct sunlight to replace. However, we are still draining while
charging, the load did not go away! The effective charging power
is not 13.8 Amps, but 13.8 minus the load of 1.2 Amps, or 12.6 Amps.
13.8(Charge
rate) - 1.2(Load Rate) = 12.6(Effective Charge Rate)
16.8
AH consumed / 12.6 Amps (Effective Charge) = 1.3 hours
This
assumes we have enough power from the solar panels to break even
at minimum for the daylight hours. Tests at the Gage home, severely
shadowing the solar panels with our body shadows indicated we should
have more power than break even unless it is very, very cloudy.
Solar
energy available along with the efficiencies involved in charging
result in charge times longer than the 1.3 hours reflected above.
For this, a fudge factor could be used to provide a little margin,
or
12.6
Amps (Effective Charge) x 85% = 10.71 Amps for a new Effective Charge
rate.
16.8
AH consumed / 10.71 Amps (Effective Charge) = 1.56 hours direct
sunlight required
A safety
note about batteries:
1) Batteries
give off Hydrogen as a bi-product of the charging process. If a
spark ignites the hydrogen, you could experience a hydrogen explosion
causing serious injury, even death. Although the battery design
itself makes an attempt to minimize the risk of an explosion, it
can still happen with almost every type of battery, since water
(H2O) is usually a major component of the electrolyte, and the charging
process (electrolysis) frees a small portion of the hydrogen and
oxygen atoms from the water molecule (electrolyte) into the two
separate elements.
a. Minimize
your danger risk by reducing the chance of hydrogen production.
Do not work on batteries while they are charging or discharging.
If a battery is sitting idle with no load or charge current, the
hydrogen production will be minimized.
b. Wear
safety gear. Eye protection is a must.
c. Do
not enclose your batteries in a sealed container. This tends to
trap the gasses and can contribute to an explosion. Batteries should
always be in a well-ventilated area.
d. Use sealed Gel-cell batteries whenever possible. The hydrogen
created by sealed cells is minimal, and greatly reduces the chance
of an explosion.
e. Never
use tools that are longer than the distance between the poles of
a battery without being properly insulated. Insulate longer tools
with tape or shrink tubing. If by chance the tools fall across the
battery poles, the chance of a short is minimized.
f. Use
fuses generously. Fuses are cheap insurance against mishaps and
failures.
Left
to Right: Michael
Willett, Stuart Gage, David Hughes Jr.
Michael
Willett
Senior
Technical Assistant and Collaborator
