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SV-Zanshin
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One of the more challenging aspects of living aboard a sailboat is energy management. Unlike at home where a flick of the switch brings immediate power all the time and the only downside of using too much energy is a hefty electrical bill at the end of the month, on a boat one needs to be careful in ensuring that there are always sufficient energy reserves present in the battery bank(s). The easiest method is to look at the voltage in the battery bank and that can be used as a very rough guide to the state-of-charge but the instantaneous value depends upon many factors (drain at time of measurement, heavy use just prior to measurement, trickle charging by solar, and a myriad of other possible items) and is not very accurate. If one measures the total number of Amp-Hours running into and out of the battery bank and combines that with a bit of programming logic then one gets a basic battery monitor. There are a couple of mathematical formulas that need to be applied, since when charging batteries not all of the Amp-Hours actually get “recharged”. Battery capacity is rated as a certain constant rate of energy use (common are 10 and 20 hours), if energy is consumed faster than that rate the capacity of the battery is less than rated. There's a Peukert law in there as well and then there are line and heat losses to account for. All in all the battery monitors do a pretty good job of calculating in all those factors and most of them also “learn” about the battery bank over time and become more accurate.
I use a Xantrex LinkPro aboard Zanshin and it provides very useful information, although at the moment I am still working on fine-tuning it. I don't know how I would manage my power consumption without the battery monitor, since my energy use varies quite a bit from day to day and just running the generator willy-nilly depending on the battery bank voltage isn't a very efficient use of energy.
The Xantrex LinkPro has a documented output serial interface which I've tapped into in order to store detailed battery state information over time. The serial output is at a TTL level (which means one can't just plug it into a PC serial port or Serial-USB converter) but that voltage is exactly what the Arduino system can use. So I put together a breadboard with a real-time-clock chip and an SD card output and wrote a program which continuously collects the data once a second and writes 30-seconds averages to the SD card. The breadboard has a little Bluetooth add-on which I can query directly from the PC and get the collected data for use in an Excel chart, so I don't have to unplug the SD card and can retrieve data on the fly at any time.
While the Xantrex might be a bit optimistic about the state-of-charge, mainly because the solar panels taper off the charge quite early and trigger the Xantrex to assume that a “float” condition has been reached prematurely, it is fairly accurate overall and I'm quite pleased with the solar panel's ability to keep up with my daily power requirements. On average I run either the engine or generator about 70 minutes a day, which is more than I thought; but most of that runtime consists of the engine being on while I lift the anchor and motor out of an anchorage and when I motor into an anchorage and set the anchor. If I take that into account then my actual generator runtime is quite minimal, mainly when I run it for over an hour to do things like run the washing machine and/or dishwasher or heat up items in the oven.
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