Photos are dark despite settings saying they're ok

If you are sure you are in automatic mode, the green camera logo on the top dial, your pictures have to be correct.
The exposure compensation should not be available in automatic, so that can't be the reason. (if you are in automatic)
The only thing you could look for, is the exposure metering. There you can chose 3 modes, and I do advise to chose Multi-pattern (should be default) because then not only the centre is measured.
If the pictures stay too dark, while in automatic and Multi-pattern, i think your camera is defect. It has nothing to do with the battery. If the battery voltage drops beneath a certain point the camera will stop completely and not work half.

Open the printer driver setup window
Select the manual color adjustment (or if already in manual and it had initially been printing fine, you may opt to select Auto)
On the Main tab, select Manual for Color/Intensity, and click Set....
The Manual Color Adjustment dialog box opens.
Specify the brightness

Select Light, Normal, or Dark for Brightness, and click OK.
The current settings are displayed in the settings preview on the left side of the printer driver.
Complete the setup
Click OK on the Main tab.
When you execute print, the data is printed at the specified brightness.

To autofocus the sensor needs a fair amount of contrast (light and dark eges) to operate. If you're trying to take a photo in a dim room of a dark colored subject for instance the focus won't lock and the camera won't take the picture. Try switching your lens to manual focus and shoot that way.

Some times the cameras memory may become confused, so to speak. Or a user may have set something which was not set back, and so causes this to happen.
Most cameras have a way to reset to factory specs. or default as stated in some.
Then, set the adjustments the way you like them.

l For manual copy density adjustment, press the AUTO/
MANUAL/PHOTO ( / / ) key to select
MANUAL ( ) and adjust with the LIGHT ( ) and
DARK ( ) keys as desired.
l For photographs, select PHOTO ( ) and then adjust with the
LIGHT ( ) and DARK ( ) keys as desired.
l There are 9 exposure steps indicated by 5 indicator lights. (The
indicators light up one or two at a time.)
Successive pushes of the LIGHT ( ) key will change the
indicator lights in the order (3) / (3•2) /(2) / (2•1) / (1).
Successive pushes of the DARK ( ) key will change the
indicator lights in the order (3) / (3•4) / (4) / (4•5)/ (5)
The automatic exposure level can be adjusted to suit your copying
needs.
1 Press the AUTO/MANUAL/PHOTO ( / / ) key
to select the PHOTO ( ) mode.
2 Press and hold the AUTO/MANUAL/PHOTO ( / /
) key for approximately 5 seconds.
l The PHOTO ( ) indicator will go out and the AUTO indicator
will begin to blink.
l One or two exposure indicators corresponding to the automatic
exposure level which has been selected will light up. The factory
default setting is level “3”.
3 Press the LIGHT ( ) or DARK ( ) key to lighten
or darken the automatic exposure level as desired.
4 Press the AUTO/MANUAL/PHOTO ( / / ) key.
l The AUTO indicator will stop blinking and will light up steadily.
l This automatic exposure level will remain in effect until you
change it again by this procedure.
Automatic exposure adjustment
AUTO

I have had "dark pictures" from each of my Nikon DSLR (D70, D80, D200, D300) cameras usually when taking pictures from a tripod and/or the self timer. Situations where you take your eye away from the viewfinder which allows stray light to enter thru the viewfinder and throw the auto exposure way off. Not sure if this is what is happening to you.

Your aperature setting is too close. Go to menu and go to your aperature setting it allows how much light is let in open it up a setting at a time until you have enough light to get great photos.

Check the ASA setting - read up the manual

Hi! It's weird that 1/2 of your photos were too dark. You didn't mention about the flash. Eventhough it is on auto mode, at night you still need to turn on the flash manually. Normally, it is a curve arrow pointing downward, you need to point your mode selector there. What brand of camera are you using though?

Set to ISO 800
Set camera to manual mode
Set Shutter to 125
Set Aperture to 5.6 or under

You're ready to go.

Too much grains, then lower the ISO.

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