Testing the hardware of our road quality measurement setup

Not only the first bit of hardware arrived but also the second (long USB power cables) so now it’s testing time: Overall power consumption, capacity of the battery packs/power banks and voltage drop for the long USB power cables.

Hardware setup (car)


Overall power consumption

In theory our test setup should consume:

  • each of the 4 RPi-B should consume max. 700mA + 50mA (GPIO) (5V; 3.75W)
  • RPi-2B max. 800mA + 50mA (GPIO) + 80mA (GPS sensor/USB) (5V; 4.65W)
  • Switch (D-Link DGS-1008D) max. 7,5W
  • overall: 4 * 3.75 W + 4.65W + 7.5W = max 27.15W

Testing(1) the overall power consumption:

  • idle: ~19W
  • 100% CPU: ~25W

(1) testing device: SilverCrest model 9149

Battery pack capacity (powering RPis)

Unfortunately our battery packs(2) can not be charged and provide power to consumers at the same time. But as a fully charged pack was able to power two RPi-B(5) for 7h 15min before indicating low battery they are suitable for our test scenario (sensing road quality for a period of a couple of hours).

(2) TP-LINK TL-PB10400 Li-Ion 10400 mAh
(5) both RPis were sensing and transmitting sensor data with 50Hz through websockets continuously

Powering a RPi with a long USB cable (voltage drop)

A possible problem mentioned in my last posting (Choosing a test setup) is the voltage drop over long USB cables for powering our RPis. The RPi specifications require a voltage of 4.75V to 5.25V. Even with short cables (30cm) of bad quality we measured only 4.64V between TP1 and TP2 on our RPis. To avoid an even bigger voltage drop in long cables we ordered high quality 28/21-AWG-cables(3) with a range of 1.8 and 3m. Lets see what the multimeter(4) thinks about our purchase:

cable type voltage (TP1/TP2) RPI model power supply
noname 30cm cables 4.64-4.89V Model B USB charger (A3H10)
3m 21/28AWG 4.98V Model B USB charger (A3H10)
1,8m 21/28AWG 5.00V Model B USB charger (A3H10)
cable type voltage (TP1/TP2) RPI model power supply
noname 30cm cable 4.63V (1A socket)
4.88V (2A socket)
Model B battery pack
3m 21/28AWG 4.85V (1A socket)
4.96V (2A socket)
Model B battery pack
1,8m 21/28AWG 4.88V (1A socket)
5.02V (2A socket)
Model B battery pack

Awesomely, the longer cables provide even higher voltage (due to the smaller cable diameter) than the short ones, mission accomplished!

(3) Anker cables: http://de.ianker.com/product/A7105011
(4) Laudmann LM-800 multimeter

All in all the hardware fits our needs. A first test of our measurement software will follow very soon here on this blog.