Hi Ian and Judy:

I have struggled with a similar situation for years now, since I am only able to be aboard the boat for three weeks at a time, 3 to 4 times a year. I am on my 4th set of house batteries in 12 years, so I only get three years at a time. Here is my story of trial and error heavily influenced by Nigel Calder's Book. (Note that my boat is in the Caribbean and when I leave it closed up it gets very hot below deck).

In 2002 I purchased 3, Uni Solar USF-11 max power 10.3 watt flexible 12 volt solar panels (amorphous crystal panels aren't very efficient) that measured approximately 15 inches by 22inches. Note that the open circuit voltage on these 12 volt panels in bright sunlight is about 23.6 volts. Two of them I wired in series to yield a 24 volt panel (open circuit voltage of 47.2 volts). These I used to trickle charge the eight house batteries, and the single panel I used to trickle charge the starting battery. Don't be worried by the high open circuit voltage because as soon as they are attached to the batteries the voltage falls to 25ish volts/ 12.5ish volts. I installed some weather proof bulkhead connectors in a shielded area in the cockpit that was out of sight. I could plug these panels in when I left the boat, rig the panels on the foredeck with some line such that they were best oriented to the sun and left the boat un-plugged from shore power. Initially I had all Delco group 31 sealed batteries similar to the originals Amel had fitted when new. This system worked quite well for me for about 5 years. The batteries were always at 100% when I returned to the boat but the battery longevity didn't seem at all improved. During this time I read a lot about batteries and I decided when I changed the next set I would go flooded lead acid deep cycle batteries. What was available to me were Trojan SCS225 flooded lead acid batteries. 4 years ago, having just installed the new set of Trojans, and while still aboard, I left the Dolphin 30 am (small) charger on while in a slip one night, to awake the next morning to find my batteries boiling, gassing furiously, bubbling over and using an IR thermometer measured them at very hot temps that I don't recall now. Several of the batteries were bulged and measuring individual cells I found that about half of the batteries were destroyed. Faulty Dophin charger caused this damage. A reset of the charger and careful monitoring it any time I use it has never resulted in a recurrence of that event. So that is a good reason not to leave the boat "plugged in".

I have since had to replace the Uni-Solar panels. They lasted about 7 years and then succumbed to the Caribbean sun and moisture. This failure mode happened while the boat was in hurricane storage for 6 months and I returned to find the batteries severely depleted, a state from which I don't think they ever recovered. While I nursed the batteries along for a season, I finally had to replace them.

Currently I have renewed my setup with some 1 foot by 1 foot square (smaller) panels rigged and wired in the same fashion. They are hard glass panels that are more efficient than the flexible panels and are constructed to have an air-space underneath them. They get too hot to touch in bright sunlight and solar panel efficiency falls off with rising temp. They have worked well for two seasons and are easily stowed below decks when I am aboard. They have kept my new set of Trojans topped up for two seasons now.

On another note however, the Trojans seem to go through battery water at a furious rate. I think their cap system isn't up to the task in the extreme heat of the Caribbean. They will not go a month without having to have a significant amount of distilled water added. I carefully follow each cells SG (specific gravity) and voltage and watch them like a hawk, but I am just amazed at how much water they require. I don't motor a lot and I carefully monitor to make sure they aren't being over charged. The solar panels each have their own controller despite the fact that I was assured that such small panels could never over charge the banks. I have resigned myself to having a caretaker evaluate the batteries at least once a month and topping up the water. So, at least with these Trojans, you aren't completely maintenance free just by having the solar panels.

There doesn't seem to be a perfect answer to leaving the boat. At least that is my experience. Please share if you happen upon a better solution.

All the best,

Gary Silver
s/v Liahona (on the hard in Jolly Harbor, Antigua)
SM 2000 Hull # 335