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Old 07-07-2018, 04:28   #1
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Join Date: Dec 2009
Location: Quebec (boat currently in Sardinia)
Boat: Bruce Roberts 45 (steel) - Armonid
Posts: 67
PV system design

Old boat was equipped with two large alternators and a 700 Ah house bank. The logic being to have two to three days of reserve that would be replaced by occasionally running the engine.

I've since installed a 3x70W solar array, greatly improved the efficiency of our fridge and recently invested in a decent battery monitor.

Turns out that we now draw a negligible amount of Ah at night. We have not yet spent a night on auto pilot so it is premature to draw any conclusion, but so far the maximum draw between full SoC had been a tiny 16Ah...

I'd be curious to read on PV design issues. For instance, our array delivers between 60-80 Ah/day, whereas our alternators would easily produce that in less than an hour... Is there an impact on the proper bank size? Or should we still start with daily budget (say, 60Ah/day) x number of days (say, three), double that to get the proper bank size (that would be around 400Ah in our case) and then compute the PV array that will match our daily budget without consideration to the speed at which the house bank will be recharged?

To put things a bit differently, I am wondering if the larger the house bank, the better, or if there is a point where additional capacity has a negative impact. Right now we seem to be at equilibrium wrt Ah generated by our array and what we consume, such that if there were unexpected draw (having to drop/raise anchor several times in a row, at night) bringing the bank back to 100% would take several days.
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