Here's an experiment you can do, if you have one a multimeter. Take some regular batteries in your house (Energizer or Duracell AA, AAA, or whatever). Set it to detect voltage on a fresh battery, do the same on a half dead battery, and a dead battery and what voltage will it tell you the battery is? Now what does it say on the packaging?
By definition voltage is the potential difference in something (also the amount of "push" it can push current through a wire). Therefore when it's full of juice there is more potential difference hence more voltage, if there is less juice there will be less voltage and hence will not be able to push current fast enough to power your motor which is why at the end of the day (or middle of the day if you shoot a lot) you'll start to notice when your battery is starting to die.
FYI my 9.6V 2800 mAh NiMh batteries will say something like 11.5V after a full charge.
Also it should "trickle" charge when the battery is done fully charging. I don't know how much it is but I think it was something like it will push 50 mAh into the battery to keep it fully topped up when it's done fully charging.
ಠ_ಠLess QQ more Pew Pew
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