Calculating total power draw of device (camera) and accessories


I work with cinema cameras and at any given point the camera will be adorned with a number of accessories e.g. Monitor, Follow Focus Motors, Cinetape etc which are being powered by ports on the camera itself.

My knowledge of electricity and power is fairly limited at the moment but I want to learn how I can calculate the total power draw of the accessories and the affect they will have in reducing battery life.

As an example I work with the Arri Alexa frequently and as per the manual this accepts input of 10.4V to 34.0V. If the power drops below the 10.4V it will shut down. the camera is usually powered by 24V or 26V batteries.

When I add a monitor or other accessory how to I calculate it affect? Also, how does wattage and amperage have an affect here. I understand that voltage is akin to pressure but struggling to visualise the equivalent behaviour of Amps and Watts.

Thanks in advance.

Best Answer

Generally, you must determine the current used at your lowest battery voltage - 24 volts in this case. The data sheet states that the Alexa "in base configuration" draws 85 watts. 85/24 is 3.6 amps. You must find the current draw for every accessory you plan to attach, calculate its current, and then add them all together to find a peak total current, call it X amps. Power (watts) is equal to voltage (volts) times current (amps).

Now find the capacity of your battery in amp-hours. Divide this by X and you will have the approximate time, in hours, that your battery will power the camera before it runs out of juice.

Be aware of a few things. Most batteries will perform better at higher temperatures, so if you're planning to operate in a cold environment you will need to derate the battery. Most battery capacities apply to a new battery, and most batteries will gradually lose capacity as they get charged and discharged. Take this into account. Battery capacity is generally specified at a specific current. If you are drawing more current than the specification, the battery will provide less capacity than you expect.