One Man's Adventures into Renewal Energy
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Solar Panel Stats

The two charts below are quite interesting in how the wattage coming in from the panel affects the battery voltage. You'll notice how when the incoming watts are high, the battery voltage rises aswell.
It's fairly apparent that the battery voltage is being skewed by the incoming watts.
The actual voltage of the battery is lower, but because the charger pushes more voltage into the battery than it currently has stored, the measurement is artificially raised.
The only way to get the actual voltage of the battery would be to temporarily disconnect the charging circuit to take a reading. This would be tricky to build, so we'll just have to make do.
But when the watts drops (cloud going over), you can see the battery voltage drops aswell, but drops to more likely the actual voltage of the battery, as opposed to the charging volts.
The only time you get the actual charge state of the battery is when the sun has gone completely in.
It's an interesting correlation.

You'll also notice the sharp drop that occurs between 5-9pm. This is around the time the Solar Charger senses no power from the panel coming in (sun gone down) and so switches on my garden lighting for 2 hours.

24hr Voltage Chart for - 04/06/2017

24hr Watts Chart for - 04/06/2017

Highest Ever Wattage achieved by my 120 watt panel.
Achieved on 20/03/2016 17:01

Next - How did I build the monitor


Morten - 09/11/2014 17:28:21
Have you tried a MPP tracker for the solar panel, you should be able to get a lot more out of the solar panel then than just relying on the panel voltage alone...

- 04/07/2014 12:50:07
Hi Robert - Yes, I've thought about using a relay to temporarily detach the charge for a minute, but the thing that puts me off is the fact I'll lose the benefit of the Sun for that one minute, and having a slightly skewed battery volts readout doesn't bother me that much in comparison. And yes - the arduino is a great platform...I'm sure you'll have lots of fun tinkering.

Robert - 25/06/2014 05:48:21
also... seeing your projects has gotten me interested in the Arduino. Mine is coming in the mail on Friday!

Robert - 25/06/2014 05:46:19
as to checking the actual voltage on your the start of the program send a signal to a relay to switch off the NC contacts. I think that a 1 min delay before reading will give very close to true voltage before reconnecting the panels for the amperage reading.

Jim(uk) - 08/09/2013 23:04:10


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