This trend was first remarked upon in an email conversation between myself and Matt Previte of YSI on January 7th and 8th, 2015. We had had occasion to examine the voltage levels at the Little Cayman (CCMI2) buoy because on December 29th, 2014 it had suffered a complete loss of power. Subsequent to discovering that power failure I posted an analysis of 2014 voltage levels for CCMI2 with particular attention to the final month of data. In this post I remarked:
Note the unexplained, slow downward trend of low voltages throughout the year. This is not obviously related to the final loss of power but it is still curious.Matt's email to me on January 7th touched upon that subject very briefly:
I'm also surprised by the gradual, overall decline in min/max of the daily battery voltage. I'll ask around to see if anyone else has thoughts on that. It wasn't below operational levels and batteries due wear, but seemed a little odd.My own January 8th reply to this remark included the following:
I'm pretty sure I've seen similar patterns at (some of?) the other buoys, but I will have to let you know next week if I can back up that statement with real data. [...] I agree that the gradual low-voltages decline is mildly worrying without being hugely alarming.In fact I did not follow up on this subject as promised until now, since I've just spent several weeks looking at trends in all of the CREWS/CCCCC data, and indeed the gradually-declining trend of voltage minima appears in the data from all three operational buoys.
For this post, we examine the voltage trends at Buccoo Reef, Tobago (BUTO1). Voltages are sampled every five seconds and then at 10-minute intervals the minimum voltage from the last ten minutes is reported. This graph shows voltage minima reported by the Met datalogger (green) and the Main datalogger (red) as well as their difference (in blue, equal to Met - Main). The first two parameters are graphed on the left axis and the third on the right, with both axes sharing the same scale but offset from one another by 11V.
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| Please click on this image to see it in larger form. |
The most notable feature of this graph is the station's failure from May 15th to August 11th, 2014. There is also a short period of initially charging the batteries following the first deployment on November 27th, 2013. Data for this analysis were last refreshed on June 9th, 2015.
At this station the Main voltages were slightly lower than the Met voltages (by 0.108V on average) so my subsequent analysis of battery minima focuses on the Main voltages.
My informal analysis looked for the 'lower edges' of the minima to try to quantify how much they were decreasing over time and how quickly. This is a largely subjective evaluation. For BUTO1, this 'lower edge' was about 12.75V at deployment time once batteries were fully charged (after six days). This edge crept lower still by about 0.1V every 4-9 months until at present I estimate it to lie at about 12.42V, for a loss of about 0.33V overall. The downward trend at this station slowed in the later part of the dataset (i.e. the trend may have decelerated slightly).
Similar analyses were carried out for this buoy's sister stations at Speyside / Angel's Reef, Tobago (ARTO1) and at Little Cayman, Cayman Islands (CCMI2). Two of the stations (BUTO1, ARTO1) reported lower Main voltages on average and one (CCMI2) reported lower Met voltages. All three stations exhibited a gradual downward trend in voltage minima, losing on average 0.1V every 4-8 months, with some slight changes in pace noted (decelerating at BUTO1, accelerating at ARTO1, constant at CCMI2). There was also one reversal of this trend noted at CCMI2 following that station's power loss and redeployment in early 2015.
The complete analyses for the other voltage minima, including graphs, may be found at this link for ARTO1 and at this link for CCMI2.
(signed)
Mike Jankulak