AA Battery Solar Charger

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 cells in the sun. I have used it to keep my
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, this time varies according to the battery type.

Specifications

Open Circuit Voltage: about 4.0V

Closed Circuit Current: about 25ma (depending on the cell types)

Charge Current: < 25ma (depending on the cell types)

Charge Time AA cells: approximately 1 full day of direct sunlight

Charge Time AAA cells: approximately 1/2 full day of sunlight

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Theory

Each of the solar cells develops about 0.5 volts across itself when in full sunlight. The string of 8 cells puts out around
4V with no load. When the cells are connected to the batteries, a current will flow which will charge the batteries.

In most cases, an 8 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 cell panel, connect jumper J2 and disconnect J1. For a 7 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 to build the
circuit.

For operation in cloudy weather, it may be useful to add one or two more 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.

Construction

Lay out the 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 plexiglass 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 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 cells, use a small soldering iron and only touch the cells for a few seconds at a time. The 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.

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
plexiglass. The 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 plexiglass.

Alignment

None required unless you count pointing the panel at the sun.

Use

Insert two cells in the battery holders, point the device at the sun, and let it charge. Do not leave AAA sized batteries to
charge for more than a few hours, overcharging may cause the cells to overheat and emit electrolyte goo, this will shorten
the cell lives. Larger cells need more charging time. 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 they need to reach a full charge.

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, remove the battery at night to prevent discharge through the solar cells.

Parts

SC1−SC8: single solar cell, .5V, 20 to 50 ma output each in full sun

D1: 1N5818 Schottky Diode

Battery Holder AA or AAA battery holders (2)

Perf Board for mounting solar cells

PC board solid piece for mounting battery holder

Plexiglass cut to size

misc hardware, wire

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Parts Sources

Jameco 1−800−831−4242 http://www.jameco.com/

Digi−Key 1−800−DIGIKEY http://www.digikey.com/

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