I have tested my bot's battery life a few times in the last days to find the safe operating limit to avoid memory card corruption.
RWPi Battery Life Tests
EBL 5000mAh Ni-MH C Batteries: $25 15Dec2014
Raspberry Pi B+ 512MB
Mikronauts.com Pi Droid Alpha interface card (MCP3008 Analog Digital Conversion)
Pololu S18V20F5 5v 2A Step-Up/Step-Down voltage regulator
Pololu Pushbutton power switch
Fair Top 5A Current Sensor Module ACS712
(Batteries are 18 months old)
- Safety Shutdown Voltage: (10 readings) of 6.7v leaves 10 minutes remaining charge.
- Voltage knee starts at 7v, 10.25 hours run after removal of trickle charge
- Starting at removal from the charger, robot will sit quietly
10 hours 45 minutes (6.7v) and then should shutdown for safety.
- 0.9v per cell reached at 10.9h,
less than one minute before voltage plummeted precipitously
- The sometimes recommended 0.9v per cell discharge limit ( 5.4v ) occurred at less than one minute of juice left before shutdown.
- Voltage Knee began at 7v, 10.25 hours
- Precipitous voltage drop at 1v per cell, 6v was reached at 10.9 hours.
- Recharge, took about 4 hours to replenish the roughly 3000 mAh run
- Battery was charged fully overnight before the test.
- Test was started promptly after removal from trickle charge.
- center.py was executed to initialize tilt/pan servos holding at centers.
- Test began with the command: sudo python battery_life.py
- battery_life.py measured the 7v2 unregulated power through a 2:1 15k Ohm resister divider to ADC6, roughly once per second, printing to a WiFi connected remote ssh shell, until 10 readings of 5.3v were seen, then the system executed sudo shutdown -h now.