69 lines
3.2 KiB
Markdown
69 lines
3.2 KiB
Markdown
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---
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title: Adding voltage and current measurements to the sunpowered Pi
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date: 2014-04-21 14:39:13
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tags: [hardware, raspberry pi, sunpowered]
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bigimg:
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[
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{
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src: "/images/adding-voltage-current-measurements-sunpowered-pi/voltages.png",
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},
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]
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series: sunpowered
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---
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Remember my [sunpowered Pi](/posts/2014/02/16/sunpowered-server-setup/)
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installation on the balcony ? The only indication I had, since recently,
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on how well the system is doing was an LED blinking red when battery was
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low. Not really helpful in making predictions or making measurements
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over the time. I've searched for simple ADC solution to hook up into Pi
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and allow for some voltage and power measurements. I've decided on [ADC Pi](https://www.abelectronics.co.uk/p/69/adc-pi-raspberry-pi-analogue-to-digital-converter),
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as it seemed very straightforward in its design and also comes with
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example python code. Did I say Python ?! It turns
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out that Python has an excellent [graph plotting library](http://matplotlib.org/) as well as it is very well equipped
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with general purpose functions and hardware specific stuff support, like
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i2c. But back to the system design. The solar panel itself can generate up to
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about 30V under no load. Battery is 12V nominal but can get to 14.5V
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when charging, the specs say. ADC Pi however, can measure up to 5V. You
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have some resistors laying around you say ? Voltage divider it is. For
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current measurement I've used [hall effect sensor module](http://www.pololu.com/product/1185) from Pololu. Summarising,
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I have 8 measurement lines, hooked up as follows:
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1. unused :D
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2. voltage of the power line from Pi to current sensor
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3. panel voltage
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4. battery voltage
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5. load voltage
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6. regulated 12V rail voltage
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7. regulated 5V rail voltage
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8. current sensor reading voltage
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It turned out that for the current measurement to be accurate I can't
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trust the voltage coming from the Pi to be stable. I actually measure
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that and use that to calculate the real reading. The docs round the hall
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sensor are not that obvious at the first sight, so here's the formula I
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came up with, for the greater good:
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`current = (measured_sensor_voltage - (voltage_powering_sensor/2))/0.185`
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Finally, sprinkle all of the above with [some python](https://github.com/cyplo/sunpowered) and get the following:
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{{< figure src="/images/adding-voltage-current-measurements-sunpowered-pi/voltages.png" position="center" >}}
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Looking at this you can spot 2 things. One - April the 5th
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was supercloudy superrainy and the battery did not get much charge. And
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the second being that on the average day the battery gets to the full
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capacity near the half of the sunny period. This is the data I was after
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! I think this means that I can add more batteries to the system and on
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a sunny day they will get filled up and this should suffice for couple
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of rainy days in a row. Here, get some photos as the bonus, also:
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TODOs, for me, and everyone else, to remember that nothing is ever finished in the
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realm of hobby projects:
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- add graph for power usage
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- would be super cool to add the weather data to the graphs
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- clean up python stuff, add more unit tests there
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- write next post on the above ;)
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{{< gallery dir="galleries/adding-voltage-current-measurements-sunpowered-pi/" />}}
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