I use sometimes my iPad charger to charge my iPhone, it has more amperage so it charges faster.
Probably not true, side by side using the iPad AC charger vs the iPhone AC charger the iPhone should charge at the same rate. The iPad charger does not feed the device, it is designed to allow the device to draw. The iPhone is not going to draw more than 5 watts from the charger, even though the iPad charger can supply 10 watts. So no, the iPad charger is not damaging your iPhone's battery. Pitting either the iPhone or the iPad charger against charging from a USB port on a computer, yes, either charger will likely allow the iPhone to charge faster than the computer.
But I think my iphone battery is lasting less than normal during the day.
Make sure that you are following Apple's recommendations of a minimum of one complete charging cycle per month to keep all of the electrons moving and thus, maintain battery health. Also turn off any services that you may not need to limit how much your iPhone drains its battery. For example, if you do not use Bluetooth, then there is no need to be broadcasting BT, the same goes for Wi-Fi.
http://www.apple.com/batteries/iphone.html