27
Dec
Power Depletion in T minus…
by Trew Audio
/ 0 Comment
Revised February 1, 2008
Battery distribution for an ENG bag or cart is not a new idea. Many of you have been doing it for the better part of a decade. The batteries have changed, the gear has changed, the chargers have improved, but the basic idea is the same; get rid of these dumpster filling alkaline weights and lead acid environment bombs.
Using a battery distribution is simple. You need a power source, typically a rechargeable battery, then a splitter, and finally cables to each unit in your kit. Most of these systems provide master switches, power on LEDs, and circuit overload protection. However, there is really little difference between them and your typical home surge protected power supply. We carry three such systems by Remote Audio; The BDS, the Hotbox, and the Meon. The BDS (Battery Distribution System) is primarily designed for ENG use, but I’ve seen them on production carts as well. This box powers 6 devices from a single supply (more with Y cables), has an internal circuit breaker, and provides 6 amps of current. The Hotbox is a bit larger with two 13 amp breakers The MEON provides 12 outputs with an internal Nickel Metal Hydride battery, charger, and A/C power supply.
Whether you are using Nickel Cadmium (NiCd), Nickel Metal Hydride (NiMh) or Lithium batteries it is important to know how long they’re going to last you on set. The most difficult portion of the equation is current draw or how much power is being pulled by the gear. This is expressed in Amperes, or amps. It is also commonly shown in milliamps or thousandths of an amp (500m = 0.5A). Finding current draw is difficult because many manufacturers fail to publish this information and many that do provide what appears to be extremely elevated ratings. Many units peak current draw on startup, and these “elevated” numbers are used to accommodate needed startup juice. Sound Devices does not publish current draws of their products. The AC/DC power supply they recommend is good for up to 2.5 amps. In a recent test we found that the 744T with two phantom-powered mics, was only drawing 700 milliamps, or .7 amps. As you can see this a far cry from the capabilities of the power supply, but 700 milliamps is the figure needed to calculate expected battery life.
So if you want to calculate your bag/cart’s battery life, you’ll need to collect some information. The most accurate way to test your system is to connect all of your gear to the battery distribution and measure the current when all of your gear is at full tilt. Unfortunately, (unless you have a MEON), this requires two things: a multimeter that measures current draws and a short jumper power cable to access the positive lead for measurement. The meter actually becomes a part of the circuit, so safety of the meter is a factor. Most volt-ohm meters are capable of current readings, but few have more than a 1 amp fuse, and even fewer tolerate 10 amps or more. For our purposes the higher tolerance the better. As for the cable, tell us what your system is and we’ll gladly make you the cable. If you feel real adventurous or confident, the meter interrupts the positive power feed between the battery and what it is powering.
Once you have the current draw, the math is fairly simple. Here are a few equations to help you.
Watts = Amps (Volts)
Watt(Hours) = AmpHours (Volts)
Batteries are commonly rated in either AmpHours or WattHours, which is simply Amps or Watts used over 1 Hour referencing a given voltage. WattHours can be more meaningful because they are dependent on both variables.So here’s an example…
The total of your current draw is 3 Amps.
You are using an IDX NPL7S battery, rated at an average output of 14.8 Volts and 71 Watthours.
In order to find the run time you need to solve for hours…
14.8 Volts (3 Amps) =44.4 Watts
71WattHour Battery/44.4 Watts used =1.6 Hours of runtime.
Remember, this is full power, constant, non-stop shooting. Every time you power down, stop recording, take a lunch break, you are using less power.
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