Residential Solar Battery Backup
Location: 1517 Beall Ave.
Wooster, OH 44691
Contact: Barry Romich, P.E.
225 watt solar array
12 volt 210 amp-hour battery
300 watt and 1200 watt power inverters
The primary goal of this project was a solar charged battery backup for powering a wood stove blower and other items during failure of commercial power. The secondary goal was reduction in fossil fuel-based electricity use.
The architectural effect is that of a mansard roof or awning that offers some protection from rain and snow as well as shades the picture window from summer sun.
The solar panel array was assembled from five 45 watt solar solar panel kits (Item No. 90599) commonly available from Harbor Freight (www.harborfreight.com). The solar panel kits include three 15 watt panels, a steel mounting frame, a voltage regulator, two 12 volt 5 watt compact fluorescent lamps, and connection cables. The steel mounting frames were used, but the position of the steel mounting clips in the aluminum solar panel frame extrusions was changed to accommodate the mounting arrangement. (The intended mounting seems to be placement on a horizontal surface.) 3/4" x 3/4" x 1/8" aluminum angle was added at the lower end of the panels to tie them together for greater stability.
The panel kits were mounted on a treated lumber frame consisting of three 10 ft. vertical 4 x 4s bolted to the side of the house and two 16 ft. horizontal 2 x 6s spaced to match the holes in the steel frames. 12 degree wedge spacers were cut from 4 x 4s to bring the surface angle to 45 degrees, the approximate latitude of Wooster, Ohio.
Each panel comes with a covered two conductor cable with 1/4" ring terminals. The three cables for each kit were bundled together and tie wrapped to romex stables nailed into the 2 x 6s. Speaker wire with polarity indication (#18) was used for connection to the panel cables. 1/4" stainless steel hardware was used for the connections and they were covered with shrinkable tubing. The wires were then inserted into 3/4" split corrugated plastic cable protectors (Auto Zone). 3/4" rigid plastic conduit was used to go through the exterior wall of the house and down the inside wall.
Inside the house, two lead acid batteries were placed on a reinforced stand. They are wired in parallel. (For information on connecting multiple batteries in parallel, visit: http://www.smartgauge.co.uk/batt_con.html.) The batteries were selected with advice from the local battery distributor. The absorbed glass mat sealed batteries eliminated the need for venting hydrogen gas, a natural byproduct of the charging process with flooded cells. With traditional batteries, the options were to leave the batteries outside where perfomance deteriorates with cold temperatures or keep them inside with a vent system that would assure that the gas goes outside.
Since the solar panel kits were originally purchased, the design of the regulator was changed by the manufacturer to include a digital battery voltage display, a 5 volt USB connector and a 12 volt automotive power connector. The above photo shows both the original regulator design and the newer design. As of this writing, the newer design was not yet available in local Harbor Freight stores, but could be purchased online. Each of the five sets of panels is connected to a regulator. All regulators are parallel connected to the battery. Click here for a .pdf format wiring diagram. (For information on connecting multiple batteries in parallel, visit: http://www.smartgauge.co.uk/batt_con.html.)
Two power inverters are connected to the battery. The smaller 300 watt unit has a small fan and is normally left running. It is connected through a normal automotive power connector. The larger 1200 watt power inverter would require over 100 amps from 12 volts DC for full output and is wired to the batteries using heavy cables and a 100 amp DC circuit breaker. It is not usually turned on since it is not normally needed and the fans generate more noise.
The system is presently being used for small lighting loads, both 12 volt DC and 120 volt AC compact fluorescent lamps. It is also used for charging small items (mobile phone, pocket PC, etc.). During power failure an extension cord will be used to connect the wood stove blower motor and any other loads deemed necessary. The wood stove blower takes 35 watts. If this were the only load, a fully charged battery would power the blower for two to three days, assuming no interim solar charging.
A system for logging charging current is planned. An EL-USB-3 Thumbdrive-sized USB-connected Voltage Data Logger was purchased from Saelig (www.saelig.com). Since it records over a range of 0 - 30 volts DC, an amplifier to convert DC current to voltage is planned. The voltage logger can be programmed to sample at predetermined intervals.
A Tenma 72-7750 Digital MultiMeter has been purchased. It includes an RS-232c optically isolated output with logging software for use with a PC.
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