When you put the battery in, the battery should flash a couple of times. Do not attempt to screw it in, simply drop it into the slot and it will charge as soon as the lid is closed.
Always hold onto the lid when opening it, as the spring loading can be quite fierce in action, which over time could cause a problem.
You can use it to charge a battery when its actually on charge itself.
I personally do NOT use it to charge the manual battery that comes with the Titan, although there are conflicting reports on as to whether it can be charged in the PCC or not.
It is a tiny bit longer than the auto battery, so I make sure its safe by only charging it on either the mains charger or USB charger.
Hope this helps.