Most pros I know do more than carry a spare battery, they carry three or four spares, besides the full size battery pack most already have attached to their camera. Camera power is like money, there’s no such thing as too much.
Perhaps you’re not shooting at a pro level or don’t want to carry three or four spare batteries. There are several tricks for making the batteries you carry last longer.
Don’t Drop Them
This was more critical back in the NIMH days, but very few batteries are improved by repeated impacts with a hard surface. I try to take a knee to change batteries, if I can do so safely. That way if I drop it, it doesn’t have so far to fall.
That also means making sure you hear the locking click of the battery holder and battery door before going back to the shoot.
Keep them in a padded pocket in your vest or camera bag.
Minimize LCD Screen Use
That means cutting down on the image pre-view after the shot and resisting the temptation to scroll through your images every few minutes.
Live view, image viewing, the LCD screen is a big draw on the battery. I review my pictures before leaving a shoot and check the pre-view as I go, but that’s about it. I don’t scroll through each series as I shoot it.
Turn Off The Flash When You Don’t Need It
The problem with automatic shooting settings is the camera doesn’t know when it’s going to need the flash, so in auto modes the camera has to charge the flash capacitors just in case. That charging and flash prep happens every time you turn the camera on and off.
Don’t Push The Shutter Button Half-Way For No Reason
Pushing the shutter button for the heck of it kicks off a whole raft of power-sucking processes inside your camera. It has to run the autofocus motors, fires up the camera’s internal computer and display and, if the flash is on, top off the flash capacitors.
Easy On The Video
Video mode is a huge power vacuum, particularly in cameras like the Canon 5D MKII. The LCD is in live mode continuously and the camera is generating quite a lot of heat.
Cut down on the amount of time you’re in video mode and your batteries will last a lot longer.