Thanks to Niko's help, I managed to measure the current drawn by the Yepp while
playing an MP3. The trick was to use a voltage source set to 1.2 V, and then
read the current from the power supply's display. The first method I tried, using
a simple amperemeter, doesn't work, unless the meter is set to measure 10 A
currents. According to Niko, this reduces the amperemeter's internal
resistance and minimizes the voltage drop it provokes, allowing the Yepp to
get enough power to boot, but it also reduced the precision of the measurement
to 10 mA steps. Here are the measurements:
- 60 mA with no headphones or volume set to 0.
- Between 60 and 70 mA with 13 Ω impedance headphones and volume to
maximum.
- 20 mA when paused.
- slightly over 100 mA with the LCD backlight on.
From these values, one can easily calculate that while running, the Yepp
consumes about 72-84 mW, and only 24 mW while paused. Moreover, with a
2500 mAh NiMH battery, it should run 35 hours.
However, NiMH batteries have a high self-discharge rate
of 5-10% the first day and 0.5-1% the following days, so my 2300 mAh battery
used over approximately 6 weeks (charge it, store it charged for 3 weeks, then
use it to replace the old battery and use it for 3 weeks) has an actual
capacity equivalent to 1400 – 1800 mAh, which in turn should give about 20 – 25
hours or usage.