Care of Batteries

Batteries discharge by creating a flow of electrons from one material
to another. As one material oxidises it releases electrons, while the
second material, covered in an electrolyte, gains the electrons. When
plugged into the mains, a rechargeable battery is able to reverse this
process. The electricity from the mains causes the material that gained
electrons during discharge to release them again as it oxidises. The
free electrons then alter the electrolyte, so the material that lost its
electrons in discharge gains them back again and the battery is
recharged ready for use. 

Prolonging Battery Life

Most batteries in film cameras are disposable and quite expensive, so
there is good reason to keep battery consumption to a minimum. And
although all EOS digital cameras use rechargeable batteries, there might
be times when you need to conserve power – for example, when your
battery power is getting dangerously low during a shoot and you don’t
have a spare available. 
EOS cameras use battery power for a whole variety of functions – from
moving the shutter itself, to metering and autofocus (see table). This
means that in order to prolong the battery’s life you need to address
the way you actually use your camera; the more economical you can be
with the battery, the more pictures you will be able to shoot per
There are a few practical things you can do to maximise the number of pictures you can shoot from one battery/charge:

  • Try not to activate the metering until you are ready to shoot. If
    you are photographing a moving subject, this technique is not practical,
    but when you are shooting a stationary subject – for landscapes, for
    example, it is possible to leave the camera turned off until you have
    composed and manually focused the shot, and are ready to shoot.
  • When you have finished shooting, turn your camera off; when you are
    walking around with the camera hanging from your neck it is possible to
    knock a button and activate the camera. Turning the camera off will
    avoid this risk.
  • If you are using an image stabilised (IS) lens, turn it off whenever
    your shutter speed is fast enough to avoid camera shake (a speed of
    1/60sec is approximately the minimum shutter speed necessary when
    hand-holding your camera, but this varies from person to person, and
    depends on the focal length of the lens you are using). The image
    stabiliser draws power from the camera and leaving IS switched on will
    reduce your battery life considerably, particularly if you are using a
    super-telephoto lens.
  • When using autofocus in low light or low contrast situations, the
    camera may struggle to lock on to the subject. In these circumstances,
    the lens might start to ‘hunt’ through the focusing range as it tries to
    find focus. The camera is consuming battery power all the time it is
    trying to focus. To conserve battery power, turn off autofocus and focus
    manually instead.


Battery Check

All EOS cameras allow you to check the charge status of the loaded battery. 
On early film models (EOS 600, 620, 650), pressing the battery check
button inside the flap on the back of the camera brings up a display on
the LCD panel. Three bars indicates full battery power; two bars shows
the battery at half charge; one bar means that the battery is almost
On cameras without an LCD (EOS 750, 850), turning the command dial to
the battery symbol provides sound signal of up to eight beeps a second
from a fresh battery. When this slows to about two beeps a second, the
battery is almost exhausted. 
On later film cameras and all digital models, a symbol of a battery
is displayed on the active LCD. If the symbol is filled with black, the
battery is fully charged. When only half the symbol is black, the
battery is half-charged. When the symbol is open (no black in-fill), the
battery is exhausted. 

Battery Cover

Do you still have the plastic cover for your battery? You should use
it whenever the battery is not being recharged or in the camera. It
keeps the contacts clean, and also removes the risk of the battery
shorting against other metal when it is packed in your camera bag. 
Take a look at the cover. Does it have a small cut-out a few
millimetres in from one edge? This is not just decoration. It is
designed so that you can tell at a glance which of your batteries are
fully charged and which are not. The batteries that come with this cover
have a blue stripe down one side of the back. When you remove a charged
battery from the charger, you can attach the cover so that the blue is
visible (above left). When you remove a discharged battery from the
camera, you can attach the cover so that the blue patch is not showing
(above right). This simple feature is very useful if you have more than
one battery. Only more recent covers and batteries have the cut-out and
blue stripe.

Back-up Battery

You might not be aware of this, but there are actually two batteries
in your EOS digital camera. The second – a back-up battery – is used to
maintain the camera’s date and time settings when you remove the main
battery for recharging. The quoted life of the back-up battery is around
five years, but it can last much longer than this. 
When the battery runs down, you will need to replace it and then
reset the date and time. If when you remove the main battery, the
correct date and time is lost, you know it is also time to replace the
back-up battery. 
The back-up battery is a small, flat, watch-type cell, either CR2016
or CR2025, depending on the camera (see earlier table for details). Turn
the camera off before changing the back-up battery. 

EOS Film Cameras

Back-up batteries are not exclusive to digital EOS cameras. Many EOS
film cameras use them, as well. This is because there are quite a few
film models available in QD or Date versions. These cameras can imprint
the date in the bottom right corner of the film. As with the digital
cameras, a back-up battery is needed to retain the date information when
the main camera battery is removed. 

Not Just the Date

It is not just the date that a camera needs to remember when the main
battery is removed. On film cameras, it is important that you don’t
lose the frame number and ISO speed – and there are other settings that
are is useful for the camera to remember. 
Film cameras can do this, even if they are not QD or Date models and
do not have a back-up battery. Instead, they use EEPROM. This is short
for electrically erasable programmable read-only memory. The camera
continually writes data to the EEPROM. Power is not needed to retain
this data (in the same way that there is no power source in a
CompactFlash card to retain image file data). So if the battery is
removed, the camera reads the EEPROM to restore the settings as soon as
the battery is replaced. 
EEPROM cannot be used for date and time data, as this information
will have changed when a replacement battery is inserted into the
camera. You need a back-up battery to keep track of date and time. 

Changing the Back-up Battery


On the EOS-1D and 1Ds series cameras the back-up battery is on the
ceiling of the battery compartment. Use a small screwdriver to loosen
the screw and remove the cover. Remove the back-up battery. Install a
new back-up battery (the plus side of the battery must face up).
Re-attach the cover. Because both batteries are out of the camera at the
same time, date and time data will have been lost and must be reset on
the camera.

On the EOS 10D, D30, and D60 cameras you access the back-up battery
via a round cover in the base of the camera. Use a coin to unscrew the
cover, replace the battery, making sure that the + sign is uppermost,
and then screw the cover back. If you leave a charged main battery in
the camera while doing this, the existing date and time data will be
retained and you will not have to re-set anything.


For the EOS 5D, and 5D Mark II cameras use a small Philips
screwdriver to unscrew the battery holder screw under the lift-up rubber
covers on the side of the camera. Take off the battery holder. Pull out
the battery. Replace the battery in the battery holder. Make sure the
battery is in the marked +/– orientation. Return the backup battery
holder to the camera and tighten the battery holder screw. If the main
battery remains in the camera, date and time data will be retained.

On the EOS 20D, 30D, 40D, 50D, 300D, 350D and 400D cameras the
back-up battery is located in the main battery compartment. Open the
battery cover and remove the main battery. Slide out the back-up battery
holder. Replace the back-up battery. Slide the back-up battery holder
back into the compartment. Place the main battery back into the
compartment. Because both batteries are out of the camera at the same
time the date and time data must be reset.