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I work in the film industry and my question relates to powering our cameras.

Our cameras are generally powered by block batteries that provide either 24v or 14v power - an example here (VCLX). Some cameras only accept 14v whereas others accept 14v and 24v power. The Arri Alexa Mini is a common camera that accepts both (Alexa Mini)

My questions is this; when powering from 14v a full battery will generally show on the cameras as 16V and the common cut off point for changing the battery is about 12.5v (varies depending on exact camera and configuration).

However when power the same camera with the same battery from the 24v output the general reading from a fresh battery is approximately 26v, the common cut off point is about 21.5v. Why is this?

Why for example can you not run the battery down to 12.5v when powering from 24v?

As is probably evident, my understanding of electronics and power is basic and I'm trying to improve it.

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closed as off-topic by Andy aka, Finbarr, Voltage Spike, PeterJ, Michel Keijzers Mar 8 '18 at 17:20

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "Questions on the use of electronic devices are off-topic as this site is intended specifically for questions on electronics design." – Andy aka, Finbarr, Voltage Spike, PeterJ, Michel Keijzers
If this question can be reworded to fit the rules in the help center, please edit the question.

  • \$\begingroup\$ Have you got a schematic of the thing? \$\endgroup\$ – Andy aka Mar 1 '18 at 15:56
  • \$\begingroup\$ The 24V battery is basically depleted when its voltage drops to 21.5V. Trying to use it beyond that will over discharge the battery and you will barely get any more run time from it as the voltage will drop rapidly when loaded. \$\endgroup\$ – EE_socal Mar 1 '18 at 16:37
  • \$\begingroup\$ @EE_socal thanks. I understand that it is because it's depleted. But why is it depleted at 21.5v rather than 12.5v? \$\endgroup\$ – James Mar 1 '18 at 16:51
  • \$\begingroup\$ @James The camera might well be capable of using 12.5V, but if you run a 24V battery all the way down to 12.5V it will damage the battery. Presumably the camera is trying not to damage your battery. \$\endgroup\$ – immibis Mar 2 '18 at 1:57
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Consider a typical NiMH single cell battery discharge curve (http://data.energizer.com/pdfs/nickelmetalhydride_appman.pdf)

enter image description here

Lets assume your 14V battery is 12 cells and your 28V battery is 24 cells in series. With a nominal 1.2V/cell for a NiMH battery, that gives 14.4V and 28.8V as the nominal battery voltages.

In the 14V case, your cutoff is 12.5V, or 1.04V per cell. Referring to the graph, that occurs at maybe 95-98 % discharged.

In the 28V case, your cutoff is 21.5V, or 0.9V per cell. According to the graph, that's probably 98-99% discharged.

What if you discharged the 28V battery all the way to 12.5V, or 0.52V per cell? Well, first, notice that the voltage drops off almost instantly between 0.9V and 0.5V - you won't get any significant extra battery life. But also, keep in mind that each cell will discharge slightly differently. If one cell has discharged to 0V while the other cells are still around 0.7V, and you keep discharging all the way down to an average of 0.5V per cell, you will start to reverse charge the weak cell, which can damage it.

TL;DR - discharge limits are typically set on a per cell basis to prevent the batteries from over-discharging, which can damage them.

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