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Batteries: Care of batteries

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 battery/charge.

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 exhausted.

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.