I have noticed it charges different batteries at different rates and peaks.. Some will charge all the way to 4.2v while others are 4.18ish Volts.. The D2 will charge some batteries at less than 500mAh or half amp..
Some of it I believe is the type of battery chemistry limiting the charge rate and the health of the battery it's self.. I'm sure someone on FT forums is a charging wonk and knows how that works.. Doubling the life of your batteries would more than pay for the D2 over a years worth of use so I hope that's true.. My i4 hardly ever gets used now..