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The D4 has a max charge current of 750ma/2 slots (or 375ma/4 slots). That is a bit leisurely compared to the VC4 (1000ma 2 slots/ 500ma 4 slots) or OPUS (2000ma available for C or 26650 cells).
The mAh capacity of the cells after charging (as shown on the screens) is the mAh that the charger put into the cell. It is not necessarily the capacity of the cell. For example, you might have put the cell into the charger when only 10% of the capacity was used. A nominal 2600mah cell would show 260mAh on the screen, after charging, in that case. So you would need an analyzing charger to tell you the capacity of the cell, which is the strong suit of the OPUS. That analyzing function has to be set in order to find the cell capacity. Normal charging will just show how many mAh were put into the cell.
I think the OPUS 3100 v2.2 is the latest iteration. GB currently has a sale on the OPUS.
http://www.gearbest.com/chargers/pp_174044.html
Edit: The link above is for EU plug
US plug here:
http://www.gearbest.com/chargers/pp_173012.html
A caveat that may not be important to you is that the VC4 is the only charger of the 3 that has CC charging for lithium cells. The others use simulated CC (pulse charging) for lithium cells. The VC4 will also charge D cells.
I’m not the expert here, just giving my take.
1+
Only a discharge test gives actual capacity.
Make sure to get the US plug.