Calculating rechargeable battery charging rate curve
Different chemistries charge in different ways.  In your example, Lithium charges firstly with a constant-current until the cell voltage reaches ~4.2V.  Most lithium cells are about 75% or more charged by this stage.  The remainder charging time is at with a constant-voltage until the current tapers off to the 0.1C rate (in your example, 200mA) at which point the cell is considered to be full.

So, let's say it takes 2 hours to reach 100% charge.  It will be over 3/4 full within 1 hour.  The 2nd hour is required to take it up to 100%.

But if you were to integrate current over time &amp; find out at what time interval an equal amount of energy has been delivered into the battery (for 0-50% as for 50-100%), then you could interrupt the constant-current phase at, say, 40 minutes, and the cell would be, approximately, 50% charged (I'm just plucking some approximate figures out of the air here).

This also doesn't account for inefficiencies, namely I^2.R losses in the wires.