This is a simple project that is fun to make.

Supplies

Lead acid battery 6v
2 switches (1 can have 1 line and the other has to have 2 lines)
A lot of wire
3 solar panels 2 small 1 big
plastic container (for housing) with lid
Battery holder
Rechargeable battery (for battery holder)
Connectors (for lead acid battery)

Step 1: Other Tools and Supplies You Need

Glue gun
Glue sticks
Soldering iron
Solder wire
Screw driver/drivers
Scissors

Step 2: Solar Panel Assembly

Get the cardboard you have and cut 2 right angle triangles which diagonal height is that of the bigger solar panel, then cut a rectangle/square (depending upon the shape of the biggest solar panel) the length of the solar panel and height of the triangles you cut earlier. Then hot glue the cardboard rectangle or square to the triangles, making sure to glue them on the edge of the cardboard triangles. Now put any needed terminal holes so the solar panel fits on directly (no gaps). Next add a positive wire on the positive output of the solar panel. Repeat on the negative.

Now cut more cardboard with the length of the larger solar panel and the largest width of the biggest of the two smaller solar panels again adding any terminal holes if needed. Add a negative wire to the negative output on one small solar panel and same with the positive terminal. Do the same on the other solar panel. Wire 1 positive wire of a small solar panel to a negative of the other small solar panel. It’s positive you connect through the terminal hole (if needed) to the big solar panel’s negative through the terminal hole (if needed). The wires left untouched are our main solar panel outputs.

Step 3: Solar Panel Array Mounting and One More Thing

Glue the solar array near the back of the lid on the triangle, putting the big panel outwards. Now, using your soldering iron to melt through the plastic lid creating a hole, push the output wires through the hole. Finally, fill in the hole where the wires go through using hot glue ( for rain protection).

Step 4: Switches and More

Now feed the positive wire to a 1 line switch and connect its output to the common of the 2 line switch. Put 2 wires on its output lines (one on each). One of the wires is the positive for the spade connector for the lead acid battery.
Then solder the other to the positive of the single battery battery-holder, and also glue the battery holder to the underside of the lid. Now connect its negative to the negative output. Then solder another wire to the negative wire: its a spade connector, who would've thought?

Step 5: Last Step

The 1 line switch is your on/off switch.
The 2 line switch is your selector for internal battery (lead acid) or external battery (AA battery). To charge batteries, turn on the on switch and select the correct battery (the one you want to charge). Now, that’s really it!