I have some current-draw figures for the PMD-661 (measured by Everett Basham) which will allow anyone to calculate how long their batteries will last. I am also seeking current-draw figures for the Tascam DR-100 and Zoom H4N, so if anyone can help out, please have a look at:
http://taperssection.com/index.php?topic=139694.0In what follows, it is assumed that your batteries have been properly charged and have not lost their capacity through age or numerous discharge cycles. All too often, poor run time is a result of poor battery capacity for a variety of reasons. If you do not achieve the run times indicated, the problem most likely lies with your battery. Stick with batteries made by a respected manufacturer (me: I avoid anything with
Made in China on it and have found that Eneloops made in Japan are among the best) and you should achieve the run times indicated below. And remember, todays best NiMH rechargeables have more capacity than alkalines and last hundreds of cycles.
I will use the nominal voltage for NiMH of 1.2 volts. It actually goes up to about 1.35 volts when fully charged, drops rather quickly to a fairly constant 1.2 volts for most of the discharge time, and then drops rapidly when the voltage reaches 1.0 volts, at which point the battery is nearly exhausted. Variation in voltage directly affects current draw (the higher the voltage, the lower the current). However, as stated, I will use 1.2 volts.
Current DrawEverett's tests reveal that the PMD-661 draws 200-220 mA when recording (OLED display set to 1 on a scale of 1-8 for light output; 2nd VU meter off), and up to 280 mA with the meters fully on. It is unlikely that changes in recording conditions (mp3, various sample rates and bit-depth) will have much effect, but this hasn't been tested as yet. I will choose a figure of 250 mA as representing minimum current draw (meters off or reduced), and 300 mA as a typical current draw.
Inverter DrawThe current drawn by the inverter (the circuitry which converts the battery voltage of 4.8 volts to 48 volts) has also not been measured as yet, but it is straightforward to calculate. The current drawn by the inverter, I(INV), has to be added to the currents mentioned above. I will assume an inverter efficiency of 80%. Let the current drawn by the phantom mikes be I(PH) in milliamps.
I (INV) = 48/4.8 * 1.25 * I(PH) = 12.5 * I(PH)
The factor of 1.25 comes from the assumed 80% inverter efficiency.
Sample Calculation 1• Batteries: Eneloop 2000 mAh
• Unit set to minimum current draw: 250 mA
• No phantom power
Run time = 2000/250 = 8 hours.
In real life, Everett achieved 12 hours with a set of new, freshly charged, 2700 mAh Sanyos, and 8 hours with 2000 mAh Eneloops.
Sample Calculation 2• Batteries: Eneloop 2000 mAh
• Unit set to typical current draw: 300 mA
• Two 48 volt microphones, each drawing 5 mA (10 mA total)
Total current = 300 + 12.5 x 10 = 425 mA.
Run time = 2000/425 = 4.7 hours
The above calculations show why a unit with 4 batteries is probably a better bet than a unit with 2 batteries – all things equal, current draw will be double in the 2-battery units, and run time halved. Even if the 2-battery unit does have very efficient internal circuitry, the current drawn by phantom mikes must cause the inverter current to double; there's no way around that. For example, 10 mA at 48 volts, equates to 250 mA at 2.4 volts (assuming 80% efficiency). But it gets worse if using alkalines because they have the characteristic of providing
less capacity as the current increases. It can be a double-whammy. When using a 2-battery unit the current draw in
increased (which results in less run time), and the capacity of the alkalines is
decreased thereby resulting in even less run time.
I'd like confirmation that the current draw for a 2-battery unit is greater than a 4-battery unit, so if anyone can take current measurements for the H4N or DR-100, please gmail me at gdburns.
