This guide explains:
- What factors to consider when choosing camera batteries.
- How to choose camera batteries and ensure they will deliver enough working hours.
- What constitutes a battery system and why you need one.
- How to choose a battery system to power more than one device on your rig.
IMPORTANT! Tinkering with electricity is dangerous, and can be fatal to you or your gear if you don’t know what you’re doing. Instructions presented here are just broad suggestions and are not to be copied. Please consult a certified electrician or engineer for practical use. I’m not responsible if you follow my suggestions and something bad happens.
How cameras are rated for power
Look at any camera’s specifications, and you’ll see its power rating listed. Here’s a list of some cameras with their average power consumption and rated voltage:
*These cameras run on the first voltage (either 7.2V or 7.4V) when used with batteries. If you’re powering them via the DC outlet, the voltage is the second one (8.4V).
This chart isn’t intended to be accurate. Don’t use it to make your calculations. Please refer to the manufacturer’s documents for correct values.
If you’re stumped by these terms:
- Voltage (V)
- Power (Watts)
- Ampere draw (mAh), and
- Power consumption (Wh)
Don’t worry, we’ll look at them right away.
What is Voltage and what is Current?
Imagine your enemy running in the forest, and you want to set a trap for him. You dig a ditch so he’ll fall into it. The bigger the ditch, the better the chances of him falling.
This ditch is voltage. It has the potential to trap your enemy. The bigger the ditch, the greater the potential, or greater the voltage.
When your enemy falls into it, he generates current. Current flows.
The ditch (voltage) allows the possibility of current (fall). If you want to generate a current, your best bet is to ensure there is a voltage ready somewhere. A wall socket is one such ‘voltage-ready’ point in space, waiting for some device to be plugged in (enemy) so current can flow. An electrical device is one that needs current to work.
Pull out the device (or switch off the socket, in other words, break the circuit) and the current stops flowing. The device stops working. The wall socket is waiting for the next device.
The voltage at any point is measured in Volts (V). Current is measured in Amperes (A).
The most important thing to know about V and A is that they are different for DC (batteries and DC generators) and AC (portable AC generators and mains electricity). Both are measured in Volts and Amperes, but the way they are measured is different, and sometimes finding a one-to-one correspondence is impossible. It’s like men and women. Both are human, but try changing one to the other. The point is, don’t use DC formulas for AC, and vice versa.
In this article, we will only refer to DC power, because that’s what camera batteries run on. If you want to know how to power your cameras, gear and lights with AC, head over to my article: How to Estimate if your Lights and Gear will work on Portable Generators and the Mains Power Supply
What is Power?
If your ditch is large enough, you can trap a bigger enemy, or many enemies of the same size. If you do this, you have power over your enemy. If your ditch is too small, you have no power over your enemy. This is one way of using the word ‘power’.
In the world of mechanical engineering, a horse has power – it has power to pull a cart. Steam engines and fighter jets have the power to carry and deliver payloads. Electricity has the same kind of power. It can move things too – table fans, motorized sliders, trains, computers and even your brain.
For this reason, we equate the power of electricity to the power of mechanical forces. It makes comparison easier. If a horse can pull a carriage 20 feet in 10 seconds, how many seconds will it take for an electrical locomotive to pull the same carriage over the same distance, and so on. This power, the ability to move things, is measured in Watts (W).
Power, voltage and current are related like family:
Power (W) = Voltage (V) x Current (A)
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. 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 (like the Fluke 27B 27 B Analog Digital Multimeter or Fluke-289/FVF FlukeView Forms Kit). 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.
What is the name of your battery?
The difference between a battery and the mains supply is, the battery has limited power, after which it is drained out. This supply is its ‘juice’, which is provided by chemical reactions. Think of it as trying to generate flames by burning logs. If you want more fire, you need more logs. It’s the same with batteries. If you want more power, you need more chemicals, therefore bigger batteries.
The challenge for everyone is finding chemicals that can generate more electricity while being smaller, cheaper and safer.
The reigning world champion chemical mix – used in devices ranging from DSLRs to Anton Bauers to laptops to mobile phones – is Lithium-Ion (Li-ion). Here are some features of Li-ion that should tell you why it’s used instead of other batteries like Alkaline (AA, AAA, etc.) or Lead-acid (car batteries):
- High energy density – so they can get smaller in size.
- Very low rate of self-discharge – about 10% at room temperature. This means you should expect your battery to always be 10% less than its rated capacity (more later).
- Unstable terminal voltage, but well within the tolerances of modern electronic equipment.
- Volatile: Chance of explosion if short-circuited, allowed to overheat, or not manufactured with rigorous quality standards – which means these batteries are made under strict conditions and are more robust.
The energy (juice) a Lithium-ion camera battery can deliver is measured in either:
- mAh, or
‘h’ stands for hour. It’s typically saying: How many Watts or Amperes can this battery give me per hour? It works the same way as our home electricity meters, which are rated in KiloWatt Hours (KWh).
A few examples will make it clear. Here’s a chart that shows how some common batteries are rated:
|Battery||Volts||Capacity mAh||Capacity Wh|
|Canon BP-970G (C300/C500)||7.2||7200||–|
|Sony BP-GL65 (F5, F55)||14.4||–||65|
|Anton Bauer Dionic 90||14.4||–||90|
|Switronix PB70 (BMCC)||14.8||4800||70|
You need to know the Wh. If the battery specifies consumption in mAh (like the Canon, for example), the easiest way to figure out the Wh is by using the Power formula:
- Wh for Canon BP-970G = Voltage x Ah = Voltage x mAh / 1000 = 7.2 x 7,200 / 1,000 = 51.84 Wh
- You try the same for the Switrnoix PB70. Wh = 14.8 x 4,800 / 1,000 = 71.04 Wh (It is officially rated at 70 Wh)
If you want to find the power capacity in mAh from the rated Wh and Volts, you simply use the same formula:
- mAh for the Dionic 90 = (Wh * 1000) / V = 90,000 / 14.4 = 6,250 mAh
- mAh for the Sony BP-GL65 = 65 x 1,000 / 14.4 = 4,514 mAh
Take a look at the first chart on this page again. 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.
Back to energy consumption. Using either Wh or mAh, and the camera power rating, you can find the approximate run time for each battery.
Let’s take the Dionic 90 as an example and compare it to the cameras in the first table. Here’s how many hours of operation you can get from one battery:
The formula to find the number of hours of operation is:
Hours = Battery Capacity in Wh / Camera Power Rating in Watts (W)
There are many factors that reduce the rated energy capacity of a battery. There is discharge, charging cycles, battery life, current draw, etc. Therefore, the above formula is not entirely accurate. There are more complex formulas but we don’t need to worry about them for our purposes.
Why not? Simple. Our cameras don’t consume power evenly. E.g., if you switch on your camera you consume X, but when you hit the record button you consume X+Y (more). I think it averages out somewhat, and if there are any mismatches it is too complex for filmmakers to worry about. To accurately predict a battery’s life, you’d have to intimately know the chemistry involved. Most of this information is kept secret by the manufacturers, so why bother?
As a rule of thumb, if you absolutely have to depend on batteries and charging stations are scarce, then deduct the number of hours by 20%. E.g., from values in the above table:
- If the Canon C300 can run for 7.7 hours theoretically, deduct 20% and assume it will only run for 6 hours.
- If the Red Epic will run for 1.5 hours theoretically, deduct 20% and assume it will only run for 1.2 hours.
As you can see, if you have spares and have a sound battery strategy (more later), the differences are negligible. Just to be safe though, I will deduct 20% from all further calculations so we’re never caught out. Our formula is:
Hours of operation =
[Battery Capacity (in Wh) / Camera Power Rating (in W)] x 0.8
Before we move on, you have to know about one more factor that affects battery life.
The effect of temperature
Li-ion discharges (power lost when the battery is sitting idle, twiddling its thumbs) faster at higher temperature. E.g., at 40oC, it discharges at 15%. It charges faster and works better at lower temperatures, typically between 0oC and 45oC (32-113oF).
What about sub-zero temperatures? Li-ion batteries start losing their energy capability below 0oC. From Nikon:
One of the main issues that occur under sub-zero conditions is that battery life is greatly reduced. The cold conditions affect the electrochemical processes within the battery as the temperature drops. The chemical reaction within the battery which produces the power slows down and results in the battery being exhausted much quicker than if it was warm. Under sub-zero conditions keeping one or two batteries in an inside pocket of your coat etc so that the battery is warmed by your body heat and swapped regularly with the battery in the camera will assist in keeping the temperature of the battery in the camera up to a good working condition.
Keeping aside sub-zero temperatures as a special case, Li-ion batteries are okay for all other conditions between 0oC and 45oC (32-113oF). Some people even refrigerate the batteries to improve performance! Read the manufacturer’s instructions to see how to take care of your batteries.
What’s up with 7.2 V and 14.4 V?
Li-ion batteries are designed in standard voltage-sized cells. The most typical is 3.6 V/3.7 V. This is why you have these weird voltage ranges:
- 7.2 V (3.6 V x 2)
- 7.4 V (3.7 x 2)
- 14.4 V (3.6 x 4)
- 14.8 V (3.7 x 4)
Why do cameras have a DC port at all?
If cameras have a battery compartment and batteries made by the same manufacturer, why do they include a DC port?
Simple. The DC port exists to deliver current from a wall unit (converted to DC, of course). However, it also serves an additional purpose, which is to allow users to use larger batteries to power more than one device on a rig.
E.g., we saw earlier that one Dionic 90 battery has the potential to power a Sony FS700 for more than 13 hours. But if you only wanted to work for 8 hours, the additional energy is wasted. But, what if you wanted to power an external monitor, an audio recorder and an external recorder at the same time? You could, with just one battery. Since this battery is larger (it has 4 Li-ion cells), it can’t fit into the battery compartment, so it needs to be connected via the DC port.
To power devices with the same battery, their operating voltages should be within the same ‘zone’ as the camera’s. Is it so? E.g. check out voltage ratings of some devices:
- Convergent Design Odyssey7Q – 6.5-34V
- Alphatron EVF-035W-3G electronic viewfinder – 6.8-12V
- SmallHD DP7 PRO OLED external monitor – 10-28V
Compare this with the voltage ranges of cameras listed in the first table. Peripheral devices are always designed to operate in the same region as cameras. Usually they average around the 12 V mark, but that’s oversimplifying things.
Anyway, this is how one battery can charge all the devices on your rig, should you wish so. Whether or not you should, is what we’ll look at in Part Two.
What is the ideal battery size?
I’ll give you my methodology, and you can decide whether it works for you.
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?
On slower film shoots with larger cameras, you could get by with just 2 hours. That’s because the set up time is usually longer and actual shooting time is less. On most other productions – indie movies, corporate videos, short films, commercials, etc. – I’d stick to the 4 hour mark.
Obviously, you’ll also need backup. Here’s my rule of thumb for maximum reliability:
Always have 24 hours worth of backup in hand.
This is for maximum reliability. Assuming I have one battery that supplies 4 hours, I’ll have maximum reliability with 6 batteries (6 x 4 = 24 hours worth of batteries).
You’ll always carry a charger, hopefully a dual charger that can charge two batteries at a speed of at least 1:1 (4 hour battery will charge in 4 hours or less). In this case, you might be okay with 4 batteries.
Do you think this is too much? Here’s the system spelled out:
- The first two batteries will give you an 8-12 hour shooting day.
- The third battery is for when shooting is pushed to 12-16 hours.
- The fourth battery is a backup in case one battery fails or if someone forgot to charge a battery overnight.
- Shooting for more than 12-16 hours? Charge the first two batteries. If you can’t find a charging point for some reason, then you need 6 batteries.
- The two extra batteries will also save your life if you can’t charge for one day, for some reason.
See how the 24-hour rule makes sense? You might feel bad about having a battery or two extra in your bag, but it’ll save your ass one day. Even if I had one battery that can last a whole day, I’d still have a minimum of two batteries (probably three).
We know all we need to about the basics of camera batteries. In Part Two we’ll learn how to put this information to good use.