What is the best way to charge boat batteries at sea?
I recently bought a 47 feet sailboat from 93. The boat have a windmill and a battery pack which can deliver ca 350Ah without the emergency batteries. The windmill is not enough to power up the batteries so I was thinking of solar panels as well. And an increase in battery power. I want to use a refrigerator box and 2-3 laptops for 2-3 hours every day. with some lights etc. Does anyone know how much battery power and charging I need then? What type of batteries is best for this kind of use? And what type of solar panels can deliver so much power?
I guess a laptop aproximately use 90W. The rest of the equipement i have no idea about.
If you have a house battery bank of 350 A/hr capacity you can safely use 50% of this from fully charged. If the batteries are of industrial traction type or possibly AGM (absorbent glass matt) construction then you can drag out as much as 80 or even 90% with little harm. Lead acid - deep cycle type are unable to be discharged beyond 50% DOD (depth of discharge) without causing permanent damage, reducing capacity each time they are over-drawn until within a few dozen cycles they are not capable of even a fraction of the original capacity.
If the wind generator is unable to cope with your power needs then you have to either replace with a larger output turbine, or use the main engine - or a dedicated direct charge unit to bring the batteries up to a fully charged state at least once a day. Solar panels are useful if not shaded, indirect light with even amorphous type panels reduces output to negligible levels, but even then you can't expect more than about four hours a day average of anywhere near the rated output of the panel - and bugger all on overcast and rainy days. Solar panels work best in totally unshaded situations, facing the sun and on a tracking frame to follow the sun from early until late - otherwise the investment is very high for a very small and unreliable charger.
Without knowing the brand and capacity of your turbine I don't actually have an opinion on that - other than any turbine under 300W output is not providing enough for your house battery - even if producing the full 300 Watts into 12V, say 25 amp constant it will take at least 20 hrs of that level to fully charge your house battery assuming no other draw on the system. Should you be running a lot of stuff - fridge/freezer, sounder, GPS plotter, radar, radios, incandescent lighting, computer, pumps, TV, inverter, mobile phone charger, auto-pilot etc. you will find that you have to run the main engine or charge engine at least a few hours every day just to keep pace with that. Your only other choice is to reduce your power requirements by using LED lighting (a tenth the power draw), minimising all electronic equipment use and specifying very economical fridge units and so on.
The simple fact is that there are a lot of ways of improving the situation - but it takes a lot of self education and discipline. I cannot give you the entire certificate 4 in renewable energy technology in six easy paragraphs, a basic rule with batteries is that you cannot combine different types - so if you choose high quality gel type industrial batteries (best value in the long term) for the house - the same type of battery must be used throughout. House, starting and reserve. Capacity can be different, but never the type. The internal resistances are too different, attempting to charge multiple types at once will result in severe damage to one or all types, and possibly the entire charging system. You will either learn the power requirements of your equipment - or you cannot design a system to suit it. I attach a couple of links below, and you could also find at least a copy of Miner Brotherton's 'The 12 Volt Bible for Boats' before you go too much further. Use only VSR (voltage sensitive relays) and design a series of battery connections that keep at least one starting battery separate from house drain to start main engine and some power to run bilge pumps and other critical equipment. This is not a difficult task, but rather a specialised skilled trade, so you have to get your head around the facts rather than the advertising spin when it comes to actual practical experience. This you are finding out I think - it is quite do-able. I took our family to a remote 100 acres to live for two years with no power or anything else - it was interesting to get my teenage kids to cope with a max. of 5kW/day. On a boat I aim for 1 - 2.5kW/day of power. Sometimes much less. The average Aussie city family needs 22kW/days at least for comparison. Good luck.
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