Solar PV and the DataDuino

As I have detailed in a previous blog post, I have set up a DataDuino on to my off-grid solar power system. This measures the battery voltage, along with the outdoor and indoor temperatures.

The voltage data will help me understand how much energy is going into and being drawn from the battery. My off-grid PV system is outdoors so the temperature data is interesting with regards to battery voltage changes with temperature.

This blog post gives details an update and some graphs of data from the past three months.

Hardware

I used a DataDuino to locally store data. The DataDuino code needs no changes, but the inputs do need voltage level conversion so they are in the correct range. We also need a 5V power supply from the 12V battery. An overview of the wiring is given here:

Voltages are measured with potential dividers, which are protected by 5v1 zener diodes. The resistors used are 10k and 47k, which allows up to 30V to be measured. They are not precision resistors, which will mean a slight error with temperature. The temperatures are measured using two DS18B20 1-wire temperature sensors. Here is one mounted in parallel on a two core cable, one inside and one outside, but close to the wall of my house.

This my small solar PV system, measuring just the 50Wp larger panel:

Data

Here are the graphs for three different days, 1st Feb, 1st March and 25th March 2013. They show the indoor temperature (C), the outdoor temperature (C) and the battery voltage (V DC).

We can see that it has been very cold on the 25th March (colder that either of the other two, earlier, days). The fast variation in outdoor temperature happens as the temperature sensor is exposed to the sun.

I am using a PWM5 from www.256.co.uk as a solar charge regulator. It is interesting to see the battery voltage as we can see that every day the battery voltage goes up to around 14V DC for around 1 hour, then the regulator drops the battery voltage. This is performing an equalisation charge and then dropping into float charge mode. There is virtually no load on this PV system at the moment, so that battery stays full most of the time.

Data is always interesting and I will analyse this some more. Check out the DataDuino in the shop for build details.

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