IPhone battery decreasing while charging! Why?

I have an IPhone 6+ and the battery decreases while using it even while charging. Why is this happening? My brother has he same issue with his iPhone 6. His charger is a little weird and I used it one time! The second since I used his charger for not even 5 minutes, my phone started doing this! Before that it never did this or had this problem. Why is it doing this?

BATTERY CLOSES TO DEAD SOON!
ALL CHEAP MADE BATTERY WITH FAST CHARGING METHOD USUALLY KILL THE BATTERY IN ONE YEAR.
REPLACE IT!

Wipe out All running apps. Plug it it to charge - and listen for the PING, and look for the *bolt* that shows it really is charging. Then SHUT IT OFF. You are using power faster than the charger is replacing it. You can also get a 2.0 generation charger - works faster. If this does not work, then TOTALLY drain the battery, and charge it again.