For DC systems, the formula for power is given by:
Power (Watts) = Voltage (Volts) x Current (Amperes)
Most cameras specify the rated power clearly. Some include the power that additional devices (like the LCD, lens, etc.) draw, besides the basic camera body. When in doubt, take the highest rating specified just to be on the safe side.
Just because one power rating is specified doesn’t mean the camera will constantly draw that. As we have seen, current varies according to how you’re using the camera. The minimum current rating will be during ‘idle’ or ‘standby’ mode, and the maximum current will be drawn when all the circuits in the camera are running simultaneously.
The only way to know how much power your camera is actually drawing is to test it with a multi-meter over time. But for general calculations, the maximum rating specified by the manufacturer is fine. The most precise values are usually found in the operating manuals of each device.
Capacity – Watt hours (Wh) and Ampere hours (Ah)
Your monthly electrical utility bill is calculated according to how much current your devices have drawn over the last 30 days. Batteries are rated similarly.
To calculate the maximum ampere draw (Capacity) of your device over time, use these formulas:
Capacity in Ampere hours (Ah) = Maximum Ampere rating (Amperes) x hours of use (hours)
Capacity in Watt hours (Wh) = Maximum power rating (Watts) x hours of use (hours)
For example, if you’re using the Arri Alexa for one hour, you’ll need a power source/battery that can supply 85 Watts x 1 hour = 85 Wh. If you need to run it for 4 hours, you’ll need 85 x 4 = 340 Wh.
Some battery manufacturers specify capacity in mAh instead of Wh. mAh stands for milli Ampere hours or Ah/1000.
Let’s say you are using a 12V battery to power the Arri Alexa, and you need that battery to supply 4 hours of operation. As we have calculated above, you’ll need a 340 Wh battery.
Using the formula given above for power, the amperes capacity Ah
= Wh/Voltage (in Volts)
= 28.33 Ah, or 28,333 mAh.
That’s a lot. On the other hand, the BMCC, with the same 12V battery, will run for about 16 hours!
It is great to see modern Sony and Canon cameras aiming to draw less than 20 W. The FS100/FS700 only draws about 6.7W, which is in DSLR territory. That’s a phenomenal achievement, often overlooked. One 50 Wh battery will provide 8 hours of operation!
As a comparison, the new iPad 4 has a 42.5 Wh battery which supplies 9 hours of operation under ideal conditions. This gives a power draw of about 42.5/9 = 5 Watts.
A camera with 5W of power draw and 10 hours of operation on a single small battery? We’re getting there.
The ideal battery size
I’ll give you my system, and you can decide whether that works for you, or use it to find your own method.
I always plan for each battery to run for 4 hours, including intermittent recording and standby time.
Why 4? Is it an astrological thing? Nope, just an old-fashioned idea that advocates meal times to be spread four hours apart (8am breakfast – noon lunch – 4pm tea – 8pm dinner). In a production environment, each battery will last for the entire session between meals or breaks. Make sense?
Obviously, you’ll also need backup. All things considered, here’s my rule of thumb for maximum reliability:
Always have 24 hours worth of backup in hand.
This is for maximum reliability. So, if I have one battery that supplies 4 hours, then I’ll be covered with 6 batteries. However, you’ll always carry a charger, hopefully a dual charger that can charge your battery at a speed of at least 1:1 (4 hour battery will charge in 4 hours or less). In this case, you can be okay with 4 batteries.
Here’s the system spelled out:
- Two batteries – 8 hour shooting day
- Third battery – when shooting is pushed to 12 hours
- Fourth battery – Backup in case one battery fails
- More than 12 hours – charge the first two batteries
Considerations and philosophy for a battery system:
Ideally, the battery system should also:
- Power other devices on the rig
- Balance the front-heavy load of the camera and lens
- Provide a constant voltage for as long as possible
- Be rugged enough to withstand the production environment (temperature, pressure)
- Come with a fast robust charger
Don’t take the ‘power other devices on the rig’ too literally. It’s not always possible or practical. Different devices are made to run at different voltages, and having additional voltage regulators and adapters for each device is a real pain.
The common thinking is: if I have one battery to power them all, isn’t that simple? Yes, only if they are all one voltage, and have similar electrical characteristics. In reality, it’s simpler to keep track of different batteries than troubleshooting electrical problems on set.
Remember, a set is a place to tackle creative problems, not technical ones – create a system that is easy to fix and control, and you’ll be most productive. Doing things in a convoluted way might seem cheaper and smarter but you’re really shooting yourself in the foot. Having said that, if you are in control and can make it work, go for it.
I also am a firm believer of the battery being a device that balances load on a shoulder rig. If your spine isn’t that valuable, no worries. If you are smart, you can spend a few days designing the perfect rig for your body type and save yourself years of pain and medical bills later. I’ll cover this aspect later.
In a nutshell – simplicity, safety, convenience and weight over cost. A good battery system is a long-term investment.
How to find the ideal battery system for your gear
- Make a list of all the items in your rig that need external power.
- Prepare a table of power draws, voltages and ampere draws, as I have shown in the last chapter. Sort it from high to low or whatever.
- Against each device note down the kind of connector it uses to draw power – is it a DC jack, XLR input, USB or custom port?
- Look at the battery systems you are interested in (coming up next) and figure out how many devices it can support.
- Imagine scenarios where you might have to add or remove devices on the fly – is the system capable of being adapted conveniently? Don’t forget to factor in special setups and circumstances.
- Is everything portable? Is this the simplest setup possible?
- After filtering down your list you will be left with a few options, or at least one option. If you don’t have any options you’re expecting too much. At this point, you might want to rethink your ideal battery size (as I’ve shown above) and find the systems that provide the exact Wh or mAh that you need for your rig. You can always have two systems or more if the production really warrants it.
- Stick to the manufacturer that offers the best value for money and service. Consider the manufacturer’s pedigree in video. When in doubt, choose the more conservative option. Price should be your last consideration.
If you do everything right, you’ll be blessed with a flexible battery system that will last you through many cameras and productions.
You’ll be itching to know what a battery system is. That’s what I’ll be covering next.