Download AA Battery Solar Charger

AA Battery Solar Charger
For higher power solar systems, see the CirKits solar charge controller circuit board kits.
AA Battery Solar Charger
(C) G. Forrest Cook 1999
Introduction
This almost trivial circuit may be used to charge a pair of AA or AAA sized rechargeable
battery cells from sunlight. The circuit has been used to keep a Palm Pilot and walkman
radio running perpetually. This is an unregulated charger, proper charging is achieved by
placing the unit in the sun for a known amount of time, and the time varies according to
the battery type.
Specifications
Open Circuit Voltage: about 4.0V
Closed Circuit Current: about 25ma (depending on the solar cell types)
Charge Current: < 25ma (depending on the solar cell types)
Charge Time AA cells: approximately 1 full day of direct sunlight
Charge Time AAA cells: approximately 1/2 full day of sunlight
Theory
Each of the solar cells develops about 0.5 volts across itself when in full sunlight. The
string of 8 solar cells puts out around 4V with no load. When the solar cells are connected
to a battery, a current will flow and the battery will charge.
Two versions of the circuit are shown in the schematic; the 8 solar cell panel with a diode
is the recommended circuit. The diode prevents the battery from discharging through the
cells at night and the 8th cell boosts the voltage up enough to compensate for the voltage
drop across the diode. For an 8 solar cell panel, connect jumper J2 and disconnect J1. For
a 7 solar cell panel, connect jumper J1 and eliminate SC8 and D1. Typically, the jumpers
are not necessary; they are shown in the schematic to illustrate two ways to build the
circuit.
For operation in cloudy weather, it may be useful to add one or two additional solar cells.
It is a good idea to temporarily insert an amp (microamp) meter in series with the battery
to measure the charging current in various light conditions.
Since solar cells are current-limited devices, it is possible to use the circuit as-is to charge
a single battery cell. If one cell is all you ever need to charge, five solar cells and a series
diode will be sufficient for the task.
Construction
Lay out the solar cells to determine the size of the circuit board, allow for about 1/4"
(1cm) of extra space around all four sides. Cut out one piece of perforated circuit board,
one piece of solid PC board, and one piece of 1/8" clear Plexiglas in this dimension. File
all 3 pieces to achieve smooth edges.
Drill 2 holes down the center line of the 3 pieces while holding them together allowing
room for the screws to pass between the solar cells. Mount the two battery holders on the
blank piece of circuit board with screws or silicon rubber glue. If the solar cells don't
have wire connections, solder thin wires to the cells. Wire-wrap wire works well for this.
Be careful not to overheat the solar cells, use a small soldering iron and only touch the
cells for a few seconds at a time. The solar cells should be secured to the perf board with
a drop of silicon rubber on the back side, or they can be held in place with the wires of
the solar cell if you have the right kind of cell. Wire all of the cells in series, plus to
minus, connect the two end wires to longer wires that go to the diode and battery holder.
Typically, the positive connection is the metal on the back of the solar cell and the
negative connection is the wire grid on the blue (front) side.
Using a pair of 3/4 inch 6-32 machine screws and nuts or washers, make a sandwich of
the 3 boards. Use the nuts or washers to make gaps between the board layers, it is
important to prevent any contact between the solar cells and the Plexiglas. The solar cells
are very brittle and will break under compression.
If you want to make the panel waterproof, cut 4 thin strips of solid circuit board or other
plastic to fit around the sides of the sandwich. Glue these boards to the sides of the
assembly with silicon rubber. Apply a small drop of glue to where the screws go through
the Plexiglas.
Alignment
None required unless you count pointing the panel at the sun.
Use
Insert two rechargeable cells in the battery holders, point the device at the sun, and let
batteries charge for a few hours. Larger cells will need more charging time. The solar
array should be placed in direct sun; it should not be shaded in any way. It might be a
good idea to monitor the battery voltage during the first few charge cycles to get an idea
of how much time is needed to reach a full charge.
Do not let the rechargeable cells overheat. If the charger is left outdoors in the summer,
the excess heat can cause the cells to leak out their electrolyte goo, ruining the cells.
Operating the charger indoors behind a window may help to reduce the heat. Operation
behind a window will also cause a drop in the charge current, resulting in a longer charge
time.
This circuit works with rechargeable alkaline cells, NICD cells, or any other rechargeable
that has a potential of 1.5V or lower per cell. If you build the 7 cell version (no diode),
remove the cells at night to prevent discharge through the solar cells.
It is advisable to connect a volt meter across the battery with a pair of alligator clips to
observe the battery voltage as it charges. If you have a lot of batteries to charge, it is best
to charge cells that are matched by brand. If possible, use cell pairs that start with a
similar voltage, this allows both cells to finish charging at the same time.
The NiCd Memory Effect
Keep in mind that the so-called NiCd "memory effect" is largely an urban legend that
started from a legitimate early 1960s Nasa experiment involving first generation NiCd
cells charged and discharged within a very tight voltage range. The two biggest killers of
modern NiCd cells are overheating during charging, and reverse voltages applied to the
weak cells as the result of the complete discharge of multi-cell NiCd packs. The NiCd
cells in cheaper appliances such as cordless phones and portable vacuum cleaners will
last a lot longer if they aren't left on the charger 24 hours a day.
Overheating can cause the loss of electrolyte, resulting in lowered cell capacity. Reverse
voltage can cause conductive dendrites to grow in the cells making them self-discharge
more rapidly. So called "memory effect" dischargers can actually cause the reverse
voltage problem if used on multiple cell packs. The weakest cell in a pack will go to zero
volts, then negative volts as the stronger cells discharge. Discharging can be a good way
to insure that all cells are charged from the same starting point, just be sure to limit the
minimum discharge voltage to around 1V per cell. The BatteryUniversity.com has a good
article on the behavior of aging NiCD cells, and tips on cell restoration.
Parts
SC1-SC8
single photovoltaic solar cell, .5V, 20 to 50 ma output
each in full sun
D1
1x 1N5818 Schottky Diode
Battery Holder 1x 2 cell AA or AAA battery holder
Battery
2x AA or AAA NiCD or NiMH rechargeable cells
Perf Board
1x for mounting solar cells
PC board
1x solid piece for mounting battery holder
Plexiglas
1x approx. 1/8" thick, cut to size
misc
hardware, wire
A company called Electronix Express sells a solar cell array (part #08SLC07) that
contains 10 cells and a built-in diode, the array will work nicely for this project and costs
around $7.
Parts Sources
Jameco, 1-800-831-4242 http://www.jameco.com/
Digi-Key, 1-800-DIGIKEY http://www.digikey.com/
Electronix Express, 1-800-972-2225 http://www.elexp.com/
http://www.solorb.com/elect/solarcirc/aacharge/index.html
Reverse engineered circuit diagram of a popular mass produced solar light. The light
automatically turns on when there is no light falling upon the solar cell, so the parts count
is minimal. The charging circuit is not particularly efficient since the battery must charge
through a plain diode.
Another Reverse engineered circuit diagram of a mass produced solar LED light. This
light also automatically turns on when there is no light falling upon the solar cell but this
circuit waits until it gets bit darker outside than the one above. Although battery charging
is through a schottky diode, some charging efficiency is lost in the 330 ohm resistor
during daylight time.
Reverse engineered circuit diagram of yet another commercially produced solar garden
lamp. This one depends upon a CDS photocell to turn the light on at night, and although
more complex than the circuits, it allows the battery to charge more efficiently.
A reverse engineered circuit diagram of one of the more commercially produced solar
lamp. This one is very similar to the one above, but it is brighter and contains an extra
capacitor.