I bought a Canon PowerShot S100 less than two years ago which gets relatively light use, and with it purchased a spare battery. I noticed that one of the batteries will now show as charged in the charger (ie, the LED lights as green) and will show a full bar in the camera, but almost immediately the camera shuts down indicating it is out of power.
Has anyone seen a battery fail like this? Am wondering is there only one type of battery to be purchased for the S100?
It sounds more like a shelf life issue with the battery. Rechargable batteries should only be charged shortly before using them to get the longest usage out of them. They lose power on the shelf much quicker than regular batteries. Make sure the battery is fully discharged and try recharging it again. Make sure the battery contacts are clean on the battery, in the camera, and the battery charger. Hopefully this will take care of the issue.
If you did any updates to the camera, it is also possible that any non Canon batteries might not work properly anymore with the camera.
You can deplete the battery just by leaving it in the camera with the camera on. Disable any automatic camera shut down setting fist so the camera does not turn off until the battery level goes low enought to turn it off.
If you don't have a cover then you can use any other non metallic thing. I've used thick rubber bands before to protect the contacts from shorting out on something.