Thursday, 31 January 2013

First Graph

I now have the first graph of the battery onitor output as captured to the SD card!
Plot of voltage over time

The graph is a plot of the voltage of my battery bank from just after midnight until just before 8am in the morning.

Two important facts:

  1. My battery bank appears to be dying. The internal resistance of the batteries seems much higher than it should be. This means a load on the battery drops the voltage more than it should
  2. The voltage measurements are not accurate. According to my digital multimeter the tops of those peaks should be closer to 12.8v. So I think we are reading approx 0.1v too low. I believe this is due to a voltage drop over the earth connection in my test setup.
I had the compressor fridge on throughout the night and the cyclic nature of the plot is the fridge compressor kicking in. It was about 15 degrees C in the van and the fridge was kept closed and was fairly empty.
I had the Webasto diesel heater on at the start of the plot so you can it brought the voltage down low as the glow plug was on. I ran the heater twice more: around 1:45am for about half an hour, and again at around 7:20am for about half an hour.

Zoom in of one section
Zooming in you can see that when the fridge kicks in the voltage drop is around 0.24V which if the fridge is drawing around 3A equates to an internal resistance on the battery of around 80 milliohms. Which whilst better than previous measurements still seems pretty high.

The time is missing from the X-axis, but the fridge is kicking in about every 5 minutes.

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