The subject of batteries for field shooters used to be as simple as charging them until the red light went out, slapping them on the camera and shooting until they died. Now, the typical ENG/EFP crew carries a much wider array of battery-operated devices. Notebook computers, cell and satellite phones, PDAs, belt-clipped radios, micro-mixers and even GPS receivers may accompany camcorders and batt-lights. Modern field shooters must know their way around battery systems.
Modern batteries communicate digitally to chargers like the Anton-Bauer Dual 2702 Powerchager shown above while talking to the user through an LCD window.
Batteries are usually defined by the chemistry they use. The three most common types are nickel cadmium (NiCd), nickel metal hydride (NiMH), and lithium ion (Li-ion). Each has its strengths and weaknesses. We’ll compare their performance later in the article. But first, let’s define the specifications we use to judge them.
Regardless of the battery type involved, there are a few fundamental specifications that field crews will frequently encounter, including energy density, fast-charge time, self-discharge time, maintenance requirement and C-rate.
Energy density is a measure of how much power the battery will deliver for its weight, and is usually measured in watt-hours per kilogram (Wh/kg). This is one of the central factors in matching battery type to application.
Fast-charge time is another factor to consider. Usually measured as a fraction of the battery’s rated capacity over time, this parameter has seen dramatic advances with the advent of battery-centric charging using smart batteries and chargers.
Another primary factor in matching batteries to their uses is a spec called “self-discharge time,” usually measured as a percentage of capacity per month. This refers to the rate at which the fully charged battery will lose its charge while at rest. Self-discharge is an important parameter because this decline in voltage is not linear.
This photo shows the two most common camera battery mounts. The camera on the left has the Anton-Bauer Gold Mount. The other has the Sony V-mount.
Most battery types tend to lose a significant portion of their charge within the first 24 hours of storage, followed by a slower but steady discharge. Storage at higher-than-normal room temperatures will degrade internal resistance and accelerate self-discharge on any battery.
A significant specification is the maintenance requirement. This typically refers to how often an equalizing or topping charge should be applied. In the case of nickel-based batteries, the maintenance requirement will include “exercising” the battery by running it down to its end-of-discharge voltage and then fully recharging to combat the infamous memory effect in NiCd batteries.
The C-rate is a measurement of the charge and discharge current of the battery. A discharge of 1C will equal the published current capacity of the battery. A battery rated at 500 mAh (milliamp hours) will discharge at 1C to deliver that current for one hour. If discharged at 2C, the same battery should provide 1000 milliamps for a half hour. Note that the measurement is made from maximum capacity to the end-of-discharge level, not to 0V. On NiCds, for instance, the typical end-of-discharge level is 1V per cell. Li-ions generally discharge to 3V.
While there are many other battery specs, such as load current, cost-per-cycle, overcharge tolerance and cycle life, the specs mentioned above will form the basic stepping stones to a good battery-to-application match. Let’s see how the various battery chemistries compare on these main specs.
Despite the emergence of new battery types, the nickel-cadmium or NiCd batteries maintain a prominent place in powering professional camcorders, batt-lights and portable comm radios. This is due to their exceptional performance in high-current applications. NiCds also accept fast charges quite well compared to the other battery chemistries. Typical fast-charge time on NiCd units is one hour, while NiMH batteries will fast-charge in two to four hours and deliver about one-fourth the load current.
Cameras have shrunk while lenses and batteries have kept their size and weight, allowing each to balance the other. Without rear-mount batteries, smaller cameras would be front-heavy, and on shoulder-mounted cameras, balance rather than weight is the critical factor.
NiCd batteries will self-discharge slightly faster than NiMH and much faster than Li-ion types. The big edge that the NiMH and Li-ion batteries have over NiCd is in energy density. In applications that require a high power-to-weight ratio, the Li-ion is the king of these beasts, with a typical spec of 100Wh/kg to 130Wh/kg. By comparison, NiMHs offer a power-to-weight ratio ranging from 60Wh/kg to 120Wh/kg, while NiCds range from 45Wh/kg to 80Wh/kg.
The Achilles heel of NiCd batteries is their maintenance requirement. They must be regularly exercised (some harried shooters might say exorcised) to avoid the formation of crystals inside the battery and the resulting tendency to discharge only as far as the minimum voltage level to which they have been frequently run. Also, since cadmium is an environmentally toxic metal, NiCd batteries are increasingly seen as a liability. Some countries now severely limit their use due to disposal problems.
Memory or mismatch?
Frequently, what appears to be a memory effect may be a mismatch between the cutoff voltage level of the device and that of the battery. To get the full capacity of the battery, its end-of-discharge voltage must be higher than the cutoff voltage for the camcorder or other device being powered. A mismatch in these values will cause the device to quit while the battery still has power. Mimicking the memory effect, this will cause a nickel-based battery to be repeatedly recharged before reaching its own end-of-discharge voltage and eventually develop a real memory.
Getting simpler again
The latest “smart” batteries, chargers and cameras can communicate digitally. The battery can control the smart charger for the perfect charge cycle and the cameras can display all the needed power parameters right in the viewfinder. Just when the mix of battery chemistries and their characteristics was becoming increasingly complex, the advent of digital communication between the central components promises to make things a good bit easier.
Bennett Liles is a writer and TV production engineer in the Atlanta area.