Does shooting at high ISO have a practical impact on battery life compared to exposure time?

Question

I am shooting a nightlapse (RAW frames, Noise Reduction off) using my 6D with a few images every minute (I use Magic Lantern's intervalometer). My battery was in good condition and was close to full (yet not 100% full) and it took just over a 1000 pictures before dying in a few hours. I know my camera can take more images, and I'm suspicious about the high ISO (10000) being a battery drainer too in addition to 8 seconds of exposure on each frame.

Now, I know that it's mainly the exposure time that drains the battery, but how much does, if any, using high ISO (therefore higher gain on sensor) impact battery life compared to the exposure?

I was using Canon EF 17-40 f/4L @ f/4 and f/4 is not the best choice for nightlapses. I've also got a Samyang 24mm f/1.4 and I know that I can get 8x more light with that lens (though color quality is not the same) but would it be worth it? How much would it affect the battery life if I lowered down the ISO to 1250 with the same exposure time? (I'd like to reduce noise so I'm planning on lowering the ISO instead of lowering the exposure to 1s) Or would it not matter at all and not worth sacrificing the color quality and sharpness of my 17-40 f/4L?

Answer 1

SImple answer: not to a measurable extent.

Difficult answer: A high ISO-equivalent setting cranks the analog gain up. More gain requires more power per electron (or milliVolt if you prefer), but there's going to be far fewer electrons in each pixel bucket. A low ISO-equiv. setting will apply less gain to more electrons.
That said, if you're in ShutterPriority mode, what I just wrote is no longer true, as you'll collect the same charge regardless of the follower amp gain. However, unless there's something seriously wrong with the analog circuit design, the power draw to support high-gain modes (not the power drawn in applying gain to pixel signals) should not be noticeably different from the power in low-gain modes.
In sum, without knowing the exact circuitry and operational logic in a given camera, it's just about impossible to estimate the actual difference in power draw vs. ISO-equiv setting. But, compared with things like LCD display, shutter/aperture/focus auto-measurements, writing to the SD card, etc. the analog gain power is pretty much insignificant.

Answer 2

If you have noise reduction completely turned off then the effect of using a high ISO setting on battery life should be minimal.

Most of the increased power demand of using high ISO settings is due to the increased processing required to implement noise reduction. Even when saving in raw format, the camera still processes the raw data to produce a jpeg preview image. This would be even further exacerbated if dark frame subtraction were enabled via the High ISO Noise Reduction option. In that case the camera is actually recording two images and then spending additional processing power combining them into a single file.

Because it takes longer to write a larger file to a memory card than it takes to write a smaller file, saving in raw format will use appreciably more battery power than saving in jpeg format. That's true to an extent regardless of the ISO selected. But since high ISO images also tend to be noisier, high ISO raw files tend to be larger than raw files shot at lower ISO with the same camera. Thus, it takes longer to write raw files taken at high ISO to the memory card than it takes less noisy images (with fewer unique colors/gray tones) shot at the same resolution.

For more on battery considerations when doing time lapse photography please see Does shooting RAW vs JPEG have a significant effect on battery life?

I think it boils down to: (also considering the answer of @CarlWitthoft) does high ISO create too much noise that it really starts affecting the RAW processing/writing time to card to become an important factor on battery life?

To accurately answer that would require precise measurements using the particular camera in question. How much additional energy is needed to double the exposure time vs. how much energy is saved by writing a slightly less noisy (and thus slightly smaller) raw file to the memory card? I suspect when shorter shutter times are used (e.g. 1/4 second @ ISO 25600 vs. 1/2 second @ ISO 12800) the advantage might go to the smaller file size but when longer shutter times are used (e.g. 8 seconds @ ISO 800 vs. 4 seconds at ISO 1600) the advantage would go to the higher ISO setting.