I have issues charging my iPhone 6s plus when I charge it on a battery case 4,000mah it takes 3hrs to charge full when i charge it via lighting cable via ipad wall charge tends to charge full.Why does it charge slower on a battery case ?
Because the battery pack doesn't deliver power as quickly as the iPad wall charger. Look at the amperage of the battery pack. It's likely 500 milliamps, or .5A or maybe 1A. The iPad charger is probably 2.1A.
That is because like someone already stated, the iPad wall charger will charge quicker. It supplies 2.1A where as your charging case supplies less than that, which will take longer to charge. I don't know of any rapid charging charge cases out there. If you want the convenience of the charging case you will have to deal with the slow charging.