I do a lot of iPhone repairs. Typically most iPhones being charged from a portable battery pack using either an Apple lightning cable or a MFI lightning cable will charge from 0.8 amp to 1.00 amp. This is a healthy range from a portable battery pack.
If the iPhone is quite old and I see it charging below 0.7 amps 90 times out of 100 it means a new battery is needed as the old one has lost much of its capacity due to lots of recharging. This is easily confirmed using a Mac program called coconut battery. which shows how much of the original capacity of the battery has been lost.
When I see the amps jumping around and I question the iPhone owner it usually turns out to be a history of cheap non MFI cables and chargers, the battery or the dock. In in some cases more than one of those.
If you replace with good quality battery and dock and still have the issue of battery dropping fast then it could easily be the charging circuit having issues and not charging the battery correctly. One I had about 2 months ago was a U2 Tristar that was going faulty but had not blown. Replaced the Tristar and charging returned to normal. This was with the original battery and dock but also I advised the owner to non longer use cheap non MFI cables and chargers.