Battery is dying suddenly

I've lost my iPad charger and used the original iPhone charger for about a week. The iPad started to discharge unusually fast. I was hoping it will change if I buy a proper iPad charger but it did not. Battery is draining about 5-7 times faster then before.

There is no other event that I could relate to this change.

An original iPhone charger charges an iPad much slower. (its 5W instead of the 12W iPad charger) Could this damage the battery?