Solar Walkway Lights Solar Panel Replacement

I recently put in a series of solar powered walkway lights in front of my house. They're equipped with a fairly powerful LED, and correspondingly have a large solar panel (compared to other solar powered walkway lights). I've found that these lights work really well when they are exposed to direct sunlight for a significant portion of the day, but when they are partly or completely shaded, they only charge enough to run for an hour or two after sunset. To fix this problem, I decided to replace the stock solar panel with a larger one. The original solar panel provides 5V to the charger circuit which in turn charges the 3.7V LiFePo4 battery that powers the LED at night. The charger circuit also switches the direction of current flow through the battery; when there is enough sunlight hitting the solar panel that its output voltage is sufficient to charge the battery, the circuit charges the battery. When the solar panel's voltage falls low enough, the charger circuit switches the battery to powering the LED. There is a certain amount of hysteresis built into the system so that the light doesn't flicker on and off when the power level is right around the cutoff/turn on point. Because the circuit requires a solar panel that nominally provides 5V in full sunlight, this is the kind of panel that must be used. I bought my replacement solar panels from a store on eBay that sells them for about $4. This adds about 50% to the total cost of the walkway light, but vastly improves its function. replacing the original solar panel with a larger one enables is to charge in less light, which in turn lets the walkway light fully charge even when placed in a fairly shady location. For the walkway light right in front of my front door, I used a larger 2.5W solar panel to let it charge even though it is completely shaded by a large bush right next to it. This is the walkway light shown below that has a larger solar panel attached to the top.

Step 1: Supplies Required

Parts:
Walkway Light
Solar Panel (any 5V solar panel should work here; the larger the solar panel, the dimmer the light the walkway light will be able to charge in) This is the item I ordered from eBay: http://www.ebay.com/itm/200855961823?var=500129297657&ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649
Assorted wire and soldering supplies
Drill and small drill bit
Flat head screw driver
Waterproof glue (I used LocTite FixIt glue, but any strong glue for outdoor use should work)
Clamps to hold down solar panel when gluing it on

Step 2: Light Disassembly

To take the electronic component of the light off, twist the black top counterclockwise until it clicks and pull it off. Before doing anything else, remove the walkway light's battery cover and battery. To split the top part apart, stick a flat head screwdriver into one of the slots and pry the metal tab towards the edge. Next, stick a screwdriver in between the metal and plastic parts, and pry them apart. You can then pull the two parts apart.

Step 3: Desoldering the Solar Panel

The new solar panel will be glued over the original solar panel, so the original solar panel will need to be desoldered from the charger circuit.

Step 4: Drilling the Walkway Light

A small hole must be drilled into the top of the walkway light to allow the wires from the solar panel to pass through to the charger circuit. The top is fairly thin steel, so it is relatively easy to drill through with a small drill bit and a hand drill.

Step 5: Soldering Wires to the Solar Panel

Next, solder wires to the pads on the solar panel. The wires don't need to be particularly thick; I used 28 gauge solid core wire. The current from the solar panel at any given time is small enough that it is possible to use thin wire.

Step 6: Gluing Everything Together

First, pass the two solar panel lead wires through the small hole drilled earlier. To ensure that the whole system is waterproof, cover the soldered pads on the solar panel with some waterproof glue. Once the wires have been pulled into the walkway light's metal top, seal it with some waterproof glue and put some glue on the original solar panel. Clamp the new solar panel onto the top of the walkway light until it has dried.

Step 7: Soldering the New Solar Panel to the Original Circuit

Now that you have the top done, the new solar panel wires need to be soldered to the charger circuit. Double check the polarity of the wires by hooking them up to a voltmeter with some light on the solar panel. When you have the polarity right, solder the positive solar panel wire to the yellow wire of the charging circuit and the negative wire to the black wire of the charging circuit. I covered the solder joints with heat shrink tubing; electrical tape should work just as well. To test that the solar panel is wired correctly, reinsert the battery. When light is shining on the solar panel, the LED should be off. When the solar panel is covered, the LED should light up. The solar panel needs to be completely covered for the LED to light up; it doesn't take much light for a solar panel this large to trigger the charging circuit.

Step 8: Reassembly

To reassemble the system, follow the steps to take it apart in reverse. Slide the plastic disc with the circuitry back one the metal tab that hasn't been bent, then push it down until is flush with metal top part. The metal tab that was bent outwards earlier must be bent inwards to reassemble the light. Press the light down onto the clear light enclosure meshing the light enclosure's tabs with the corresponding holes on the light. Twist the light clockwise until it clicks to lock it in place. To test the light in its new location, first ensure that the battery is fully drained. Then let it charge one full day in its new location. If it runs for the desired duration at night, congratulations, you're done! If it doesn't work as well as you'd like, repeat this Instructable on another walkway light with a larger solar panel. I found that the solar panel size shown here (150 mA) worked for all but one location in my front yard.