Why is my Canon 5DMIII draining batteries overnight?

How long have you left them on the charger? From completely dead, it can take quite a while to charge the batteries. I would leave the batteries on the charger for 8+ hours and then try them in the camera by itself, without the grip. If they don't work and other batteries do work in the camera, then it sounds like a dead battery.

Another option you could try is to go to the battery info screen on the third page of the gear shaped menu section. It will give you an exact % of battery left. You can then take the battery out and see if it loses charge on it's own or only when in the camera. If it loses power while the camera is off and only in the camera, sounds like a short somewhere. If it loses power while out of the camera and sitting by itself, it's probably a damaged battery, though if a lot of batteries suddenly developed the same problem, there's a good chance something in the grip or camera caused them to become damaged.

The problem with battery life in a lot of Canon cameras is that the batteries can be rated at 750mAh and the camera at 1000mAh. Even though the voltage is the same, your battery won't last as long as one rated at the same as your camera. Usually the rating is on the sticker on the bottom of the camera or in the manual.

I don't know if it's exactly the same thing, but earlier in October, Canon released a product advisory for their LP-E6 batteries and the LC-E6 charger.

Full details can be found here.

Here's another data point. My wife's Canon 7D started draining the batteries. (Around the same time it also started reporting that it couldn't communicate with the battery, so this may be a different situation.) We lived with it for awhile since we didn't want to send it to Canon and just made sure we always had fresh batteries when going out for a shoot.

Eventually, we stopped by the local camera shop where it refused to behave like that, but he upgraded the camera's firmware anyhow. From that moment on it recognized all batteries and stopped draining them.

According to the guys at Magic Lantern, when you open the SD card door and the light flashes while it accesses the SD card, you should wait until several seconds after the light stops flashing as it is still accessing the card (which is silly, because that light has one job!) or you can cause the camera to lock up and drain the battery.

Same thing happened to me after I got my 5D MKIII.

2 things to check here:

1.) Make sure your on the latest firmware.

2.) Camera settings, under the wrench icon, second position: Auto Power off. Set that to 1 min.

I know its a late reply but it could help someone with similar issue as mine.

I had given my camera to a friend for couple of days. After receiving the camera, the battery was drained overnight.

Changed couple of batteries, with and without lens, 3 firmwares. Nothing worked!

After installing Magic Lantern, saw that the Access light was continuously flashing after turning off the camera. This happens when the card is being accessed. Used a torch and noticed that one of the cf card contacts was bent and was touching the other contact. Bending it back fixed the issue for me.

You may want to give it a try before giving it to the service centre.

-Amith

The EOS 5D Mk III drains a bit of power for its viewfinder, even when switched off. As a proof, look through the viewfinder when the 5D is switched off (but with a charged battery inserted) - and then, while looking through it, release the battery: The image will become dark and blurry, until you re-insert the battery.

This, however, should not have such a significant impact on battery life. E.g. if I leave my 5D Mk III with two original batteries in the BG-E11 charged at 100% and wait around a week, the batteries (which are quite old) will be around 85-80%.

Another possible solution, one that worked for me with the Canon 5D and might work with later models; is that the camera has a second back up battery that many people are unaware of. It's very well hidden under the corner of the camera bottom on one end. This button battery keeps current date and time information, etc. But, it also has something to do with how your main battery's life is reported to the camera, i.e. partly charged, completely charged. If this battery dies, then it tends to report all or most of your main batteries as also being at zero and needing a recharge, except charging them doesn't change anything. I wound up buying so many batteries before I figured this out. I had like eight in my bag, a lot of weight. I'd get them all charged up to max and go out the next morning and find maybe one I could work with for an hour if I was lucky. I looked for a long time to find the bit about the second battery on the internet.

The flashing light stopped (and possibly the cause of draining of the battery) when I used a torch to look inside the CF card slot and noticed that one of the cf card contacts was bent and was touching the other contact. Bending it back fixed the issue!