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Project "Flexible Solar Panel Modification"
The purpose of this project is to demonstrate how to modify a portable, flexible solar panel pack (that charges batteries) so that you have some idea as to how much the batteries have been charged.

There are a variety of portable solar panel packs that you can take with you so that batteries can be charged (such as an all day outdoors event - hiking - boating - ecetera) enabling you to use small electronic devices for extended periods of time. Unfortunately, I've found it to be rather disappointing that there are no battery charge indicators built into many of those portable systems.

Significance of the Modification
The advantage of being able to tell when batteries that are being charged are actually ready for use, save a lot of hassle with (1) guessing, (2) risk overcharging your batteries thereby reducing their lifetimes, and, (3) wasting time moving batteries from the charger to the electronic device to test them...and then move them back again if the electronic device isn't operating as expected so the batteries can be charged longer.

The modification that I'll show you how to do with a flexible solar battery charging pack, using approximately $7.00 of parts, will provide a visual cue to indicate the amount of charge in batteries. What is best with this is that the parts used consume hardly any space or weight and fit on the end of the double AA battery holder.

Let's Do The Modification
In this tutorial I'll be modifying a 2-panel Silva Techno 4 Flexible Foldable Lightweight Solar Charger (produces approximately 3.5VDC output in full sunlight) that charges two "AA" or "AAA" batteries at a time. The eyelets for this item make it easy to rope or tie onto surfaces (such as a backpack) since it was designed for outdoor use.


  1. Place the solar charger as shown.

  1. Flip over the solar charger. The end facing you should be the end that you will be soldering the modifications to. To verify this, look for two small holes in the battery holder nearest you. The side of the battery holder you will modify is the side that the solar panels are connected to.

  1. Perform a continuity test on the tact switch (to make sure it works) and as well the red led.
  2. Solder a 3/4 inch long, red colored, 22 gauge wire end to one of the leads of one of the tact switch.
  3. Solder the positive lead of the red led to the lead of the tact switch that is diagonally positioned from the lead that you soldered the red wire to.
  4. Solder a 3/4 inch long, black colored, 22 gauge wire end to the negative lead of the red led. You may want to clip the other two leads of the red led since they are not used and could get in the way later.

  1. Solder the positive lead to the positive post of the battery holder (outside).
  2. Solder the negative lead to the negative post of the battery holder (outside).
  3. Carefully adjust the position of the wires, tact switch and red led so the components are flush within the surface area of the side of the battery holder. Make sure there is a small gap between the tact switch and red led and the side of the battery holder (3/16 inch is sufficient).
  4. Place the solar panels towards the sun or near a light so that you can press on the tact switch and have the red led illuminate (if it does not illuminate either (1) the solar charger is defective, (2) the light source is not strong enough, or, (3) there is a faulty soldered connection that you'll need to fix.

  1. Apply the silicone sealant between the components and the side of the battery holder. You may also want to apply the silicone sealant around the sides of the tact switch and the red led so that those components will be more stationary (and less likely to separate or break off from the silicone sealant if they are bumped).

  1. After the silicone sealant has dried, it is time to perform another test of our charge indicator modification.
  2. Insert either charged "AA" or "AAA" batteries into the battery holder and press on the tact switch. The red led should brightly illuminate.
  3. Congratulations! Now you have a compact way to tell when the batteries are charged - when the red led is dull or not illuminating at all you know more charging is needed and when it is bright the batteries are ready to be used.
  1. Why is this modification so simple? The biggest reason this modification is so cheap (approximately $7.00), is that the red led used (which produces red light) operates at a voltage of 2.6 - 3.3VDC. This means that the red led will only create bright illumination at 2.6VDC. Most single batteries are rated at 1.25VDC; a pair of them would create 2.5VDC...that correlates to the voltage required by the red led to operate at it's typical rating. The maximum voltage of 3.3VDC of the red led provides a buffer in the event the batteries contain higher voltage so that it does not burn out.
  2. When you perform a test of how much charge is contained in the batteries, be sure to cover up the solar panels; otherwise the power created by the solar panels will be picked up by the red led instead of the batteries.
  3. In order to prolong the life of the solar panels, be sure to remove the batteries when the solar panels are not charging them (such as when the sun goes down). If you do not do this, you risk having the power from the batteries go into the solar panels and damage them (this is the case with any solar panel that is not protected from reverse voltage). Damaged solar panels will produce less power and it will take longer for batteries to charge.

Project Materials
[1] 2-panel Silva Techno 4 Flexible Foldable Lightweight Solar Charger
[1] 5.0mm High Tact Switch #275-0002
[1] 4-pin High Brightness Red LED 2.6 - 3.3VDC #276-0020
[1] 22 Gauge Stranded Copper Wire #278-1224 (only an inch for red and black conductors are needed)
[1] DAP, Clear 100% Silicone Aquarium Sealant

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