60w charger only delivering 5.5w to battery?
Is it correct that the charging with value = 5.5w, shouldn't it be c.
60w?

This is expected behavior.
The 60W rating on the power adapter is the maximum amount of power that the adapter will supply.  Note that it's not the amount that will be &quot;pushed,&quot; but the amount that can be drawn from it.
Slow Charging Speed

... for my 15&quot; late 2012 MBP (non retina)...I'm currently charging on a 60W original Apple charger (as I
understand it, the 85W is optimal, but not necessary).

This is an incorrect assumption on your part.  The 2012 15&quot; MacBook Pro requires the 85W charger - see Apple:  Find the right power adapter and cable for your Mac notebook
This is why you're seeing the very long charging times - your MacBook Pro is &quot;asking&quot; for more than your adapter can deliver and since the adapter can't supply any more, it lowers the power to the battery charger in favor of the compute requirements.
Your MacBook has two operations going on: powering the system and charging the battery.  In this scenario, priority is given to powering the system.  If you were to shut it down, the priority would naturally go to charging the battery.
Seeing only 5 to 6W of power being allocated to a running computer is perfectly fine (for the 60W charger); the power is being divided to account for your current use.  Use the correct charger and it will operate and charge correctly.
Further Reading
There's additional questions/answers on this very topic.  Please see the following links for further reading:

Danger of damaging 13&quot; macbook pro by charging it with 85w charger?

MacBook 2007 charger

Other than speed, is there a reason that a 10W iPad charger shouldn&#39;t charge a new MacBook Pro 2016?


TL;DR
You're using the wrong charger.  Get the 85W charging adapter and your charge times will decrease to normal levels.