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I work with movie cameras. Unlike normal cameras, they have many types of batteries, sizes, etc that can power them. Many attachments (wireless video transmitters, follow focus systems, monitors, etc) get attached to them. These cameras will typically give you a read out, on screen, for the current voltage of the battery. My questions are as follows.

  1. How can one calculate the estimated run time of a battery plugged into a camera, powering the camera and accessories all together.

  2. On the screen there's the voltage read out, how can one calculate what voltage the battery will die (power off) at, while powering the camera and accessories.

For example: This battery (all links below) (14.8V, 98Wh), powering camera (min. 43W max. 84W), with monitor (13 W), Motor Driver (input 12-36VDC, max draw 4A at 24VDC), Motor (Max current 2.7A), wireless video transmitter (Nominal 7.3 W). How do I calculate estimated run time on that battery, as well as the voltage everything will die at?

Battery (https://www.bhphotovideo.com/c/product/1505986-REG/core_swx_neo_9ag_98wh_hop_3_stud_mount.html) Camera (https://www.arri.com/en/camera-systems/cameras/alexa-mini#K1.0003873) Monitor https://www.bhphotovideo.com/c/product/1346909-REG/smallhd_mon_703u_limited_time_pre_order.html Motor Driver "MDR4" https://prestoncinema.com/#!/products/product-info/ Motor "DM1X" https://prestoncinema.com/#!/products/product-info/
Wireless video transmitter https://www.bhphotovideo.com/c/product/1403845-REG/teradek_10_1935_bolt_xt_500_sdi_hdmi.html/specs

I'm hoping if I can learn this by this example here, I can figure out how to calculate all this myself later on.

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2 Answers 2

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  1. Estimating remaining time to operate a device purely from battery pack voltage is not possible for many reasons.

Mostly because battery voltage itself tells very little about the state of charge to begin with.

Different devices determine the voltage when to cut power off differently according to their specifications because they have different battery pack specs.

And battery depleted on actual usage, not maximum usage, so it depends if you use motors constantly or not at all.

  1. Look at the on screen voltage until device turns off.
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Battery life is a function of its capacity, voltage, and current draw. When you know all 3, plus the behavior of the battery chemistry and/or regulating circuit (cut off voltages), you can calculate how long it will last. You can estimate based on average draws for a less accurate time left if you don't have the actual numbers.

Lithium batteries like all others have a voltage range. When fully charged, a single cell may be 4.2V and when discharged closer to 3.0V. 3.7V nominal. These values vary between chemistry. Bring them too low below 3V and you will damage the battery.

A 14.8V battery is probably a 4 series cell pack. At 98Wh or (Watts Hours / Voltage = Capacity in Hours) 6.6Ah, it can be any number of parallel cells depending on the cells they use internally.

You can add up your draws in Watts (use Current * Voltage for the rest) and then divide your capacity in Watt Hours by that number. That's how many hours you have. I.e. 98Wh / 30W = 3.26 Hours. But this will highly depend on actual draws not just nominal

These systems will typically use a multitude of ways of figuring out the actual estimated life. Voltage sensing compared to a known voltage curve of the battery, current sensing using a shunt resistor, or even just a commercial IC battery gas guage. This could be combined in a battery charging IC that handles the charging and discharging controls, tracks battery life and can handle estimating remaining life.

Engineers specialize in this. Here's a small article as a rabbit hole into battery state of charge estimating:

https://www.eetimes.com/new-ti-gas-gauge-protection-chip-set-improves-battery-life-and-system-protection-for-multi-cell-battery-packs/

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