Some questions about battery and charging

I have an iPhone 4 and an ipad 3,both on 6.1.2 and I have some questions that are killing me
1. If my ipad or iPhone has battery less the 10% or less then 20% ,if I charge the ipad and let it charge until 7 in the morning,that means about 8 or 9 hours, is my battery going to be affected?

2. If I use the ipad while it is charging,is the battery going to be affected or get worse?

3. Do I need to use an app from App Store,like battery doctor? And if yes,will it get my battery better?

These are my 3 questions,both for ipad and iPhone,please don't ignore me and answer if you know right,not if you asume and you are not sure,this is really important to me.

 

Can I charge my iPhone,it is 30% ,and let it charging 10 hours? Will my iPhone burn or explode? I heard on tv and it was iPhone. Sorry for so manny questions.