How much power does the T400 draw?

[charlespax]

I used a µCurrent and Fluke 289 to measure the current draw from the battery during battery-powered operation.

While powered off the T400 draws 0.16 mA. With a 1020 mAh battery this theoretically gives 265 days of standby time.

While powered on and with no backlight the T400 draws 38 mA. With a 1020 mAh battery this theoretically gives 26.8 hours of operating time. During logging to SD and not connected to serial this figure should be slightly lower. A current draw of 38 mA was measured with spikes during SD write operations.

While powered on and with the backlight enabled the T400 draws 45 mAh. With a 1020 mAh battery this theoretically gives 22.6 hours of operation.

batteryPwr = 1020 mA hour // BL-5C as marked, not measured
pwrOff = 0.16 mA
timeOff = (batteryPwr/pwrOff)/(24 hours/day)
pwrOn = 38 mA
timeOn = (batteryPwr/pwrOn)
pwrOnLog = 38 mA
timeOnLog = (batteryPwr/pwrOnLog)
pwrOnBacklight = 45 mA
timeOnBacklight = (batteryPwr/pwrOnBacklight)

[charlespax]

While powered on and with no LCD connected the T400 draws 38 mA. The LCD does not appear to contribute much to power consumption

[charlespax]

The three 10k resistor divider for the battery voltage measurement circuit draws approximately 0.123 mA.

V=3.7
R=30000
I=V/R
mADraw=I × 1000

[charlespax]

ATmega32U4 datasheet p.379 indicates the processor alone should draw about 10 mA at 3 V, 8 MHz.

[charlespax]

See PaxInstruments/t400-electronics#36 Reduce off state power draw.

[charlespax]

Very little. Closing.