Overall, it’s generally not a good idea to continually run your battery completely down (below 5%), as you risk a “deep discharge” cycle if you do and don’t immediately get the battery onto the charger. You can take batteries down until exhaustion if you have to, but you shouldn’t do this regularly, and you should immediately put a battery discharged this way on the charger, if possible.
One other comment: if your VR seems to act sporadically and sometimes appear to jump to a correction point instead of move gracefully, I’ll bet your battery is low (and probably down near that 5% mark). VR has a high spike in its draw when it activates. When the battery is very low, the VR system doesn’t get the power it expects and stutters. However, if your VR system seems to stutter with a full battery, that’s usually a sign that something is wrong with the VR mechanism.
• Flash use definitely increases battery consumption. Every time you use the flash, the battery in the camera is used to recharge the capacitor that the flash just drained. It’s difficult to predict just how much increase there is in battery consumption with flash, because how much flash you’re producing makes a difference. If the flash is firing at full power, the capacitor needs to be completely recharged. If you are using just a touch of fill flash or a low manual flash power level, the camera cuts off the flash early and the capacitor only needs to be topped off, not filled. A rule of thumb is this: the further you are from your subject when you use flash, the more you’re discharging the battery and the fewer shots per charge you’ll get.
Overall, the battery performance on the D750 is very good. I can’t think of an occasion where I’ve really needed more than one battery a day during normal shooting (this changes if I’m using a lot of Live View, a GPS, or a lot of internal flash). If you start the day with two fully charged EN-EL15 batteries (one in the camera, one in the MB- D16), you’re not likely to run out of power.
That said, EN-EL15 batteries are small and light, so it really doesn’t hurt to carry an extra one or two with you.
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