To keep my service batteries topped up, it would be good to have a supply of about 1 Ah [Ampere-hours] per day. A general rule is that a lead acid battery benefits from a trickle charge of about C/100 per day where C is the capacity in Ah - which is 100 Ah for my boat for each of the two 12v batteries that provide in series the 24v service supply.
A 10 watt solar panel can supply (at maximum) 10 watts: usually at
something like 17 volts and 0.6 Amp. In practice in the UK, the panels will
not be exposed to direct sunlight for many hours a day and the sunlight will strike
them at an angle not perpendicular to the panel. In my experience [at 53°
N] the output per day without a converter/controller will be about 1 Ah at 13
volts, with a significant contribution even on a cloudy day.
To make best use of this solar power, it would be optimum to use a DC-DC converter/controller which could provide maximum current at the best charging voltage [about 13.5 volts]. However, the average daily output (1 Ah which is less than C/100) can be fed safely to the batteries without any converter/controller: so keeping the installation simple.
A mono-crystalline panel in an aluminium frame of size 370mm x 250mm is listed as providing 10 watts and is available for about £25. I would need two of these - one for each of the 12 volt service batteries. I fitted these several years ago in a fixed position above the cabin roof, connecting each directly to one 12 volt battery, and they performed the task of keeping the battery bank charged up. It is important to fit them where no part is in shadow - since they work by having a number of cells in series and if any cell is dark, it cuts the output significantly.
Cabin roof showing solar panel on one side.
After two years or so, I noticed that one panel was not providing any power. I stripped it down to see why. One thin foil (aluminium?) strip joining two cells had corroded and gone open circuit. It was actually quite easy to fix that - so I put it back in service and it has lasted another year so far.
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