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Casio Pathfinder watches recharge while you wear them with the help of a small solar cell on the face of the watch. This means that your watch can charge anywhere where there is natural or artificial light. Despite the constant charging, it is possible for your rechargeable watch battery to wear down over time. You can spend $30 to send your Pathfinder to Casio for battery replacement or you can perform the repair yourself for the cost of the battery. Most Casio Pathfinder solar-powered watches use a CTL1616 battery that can be purchased from your local jewelry or retail store.
You have to be very careful when doing this yourself because of the very small parts that will fall out if you just open the watch off casually. If you don't know this ahead of time you will almost definitely lose some of the parts so make sure you have enough space to work.
Set your Casio Pathfinder watch face-down on the soft cloth or craft felt. Use an optical-sized Philips screwdriver to remove the four screws that secure the watch back to the body of the watch. Set the screws aside, but don't lift the watch back.
Slide the watch back off of the body of the watch very slowly. There are three important springs held underneath the watch back; it is essential to remove the watch back slowly to avoid losing them.
Lift the three springs out of your Casio Pathfinder watch using a small pair of metal tweezers. Take note of the length and placement of the springs as you remove them. Set the springs aside with the other disassembled watch components.
Take the black rubber protective disk out of your Casio Pathfinder to reveal an identically-shaped white plastic disk. Remove the white plastic disk and set it and the black rubber disk aside. Use your metal tweezers, if necessary, to lift out the rubber and plastic disks.
Release the battery strap that holds the battery in place in your Casio Pathfinder watch by pushing the battery strap latch to the left. Remove the old battery from your Casio Pathfinder using a pair of small plastic tweezers.
Pick up your replacement watch battery with the small plastic tweezers and insert it into the battery compartment of your Casio Pathfinder watch. Replace the battery strap and push it gently with the tip of your plastic tweezers to snap the latch in place.
Touch the "-" and "AC" terminals on your Casio Pathfinder watch with the tips of your metal tweezers for at least two seconds. This helps ensure that your watch will work properly once reassembled.
Put your Casio Pathfinder back together to complete the new battery installation. Start by replacing the white plastic disk, followed by the protective black rubber disk. Use your small metal tweezers to replace the small watch springs in their correct locations. Replace the back cover of your Casio Pathfinder, insert the four screws and tighten them with your optical-sized Philips screwdriver.
The most likely reason for your problem is that the battery was allowed to go completely flat. When this happens, the battery cannot be charged again because there is no voltage in it at all. It is very common with rechargeable batteries, and the older the battery is the more quickly this will happen if it's not charged.