How do I recharge my battery bank?

Question

So I have installed (2) 5250 CCA battery banks on my 22ft boat. One of the banks is strictly for the electric motor (docking and emergency use only). The other bank is for all other electrical appliances and acceressies (12v parallel circuit). I want to use wind, solar and water powered generators to recharge and maintain my battery banks. I have a continuous load of .5 kw with a max load of 1 kw/hr on the acceressies bank and a 1000 kw with a max of 4000 kw/hr load on the e-motor bank (12v). If I divide the power retrieval amongst the 3 charging systems (wind, solar and water) what are the sizes of the charging units needed to recharge and maintain both batter banks? What other components would be need to full fill this system?

Answer 1

CCA spec is used for starting engines; 20 hr capacity is used for deep discharge batteries. I hope you got deep discharge batteries, not starting batteries, as they will last much longer. Marine deep discharge are usually between the two types.

I would start with chargers for use dockside, and maybe a small backup generator with 12V output to run the chargers when underway.

A battery bank with 5kA CCA probably has about 400A*hr (up to 1000A*hr for good deep discharge batteries), you need to check battery capacity to be sure. The best charging rate is usually C/20 to C/10 or 20A to 40A (up to 50 to 90A for deep) or 280 to 1300W. This takes 24 to 12 hours to charge the banks.

You need about 35A (all currents at 12V, would be 1/2 as much for 24V) to do your steady state drain. You should be able to get by with about 1kW of solar for $1000 or so. This theoretically generates some 70A; you will be lucky to get half that on the average, when you have strong sunlight. This will barely keep up with your steady state load when operating, much less charge anything. The panels must be unshaded and stay perpendicular to the sun to charge the batteries. This is difficult to do on a moving sailboat, but if you are tied up at a dock, you should be able to position it  reasonably well if you make it as a moveable panel, though the sun moves during the day also. As Bill mentions, if part of a panel is shaded, it won't generate enough voltage, and won't charge the batteries at all. Thus, better to have a bunch of small (say 50W/4A panels) connected in series. That way, if some are shaded, the unshaded ones will still charge.

Wind is tough to harvest on a sailboat. The windmill needs room for the blades, and needs to stay perpendicular to the apparent wind. It will also slow the boat some.

A 1kW generator runs $3k, has 8.2 feet prop diameter, and requires 25mph to run at capacity. http://www.bergey.com/

Only decent place to mount windmill is on top of mast, assuming mast is sufficiently strong for it. If you could mount a 6 ft dia windmill on top of the mast, it could make about 500W with a 25mph wind, or 60W/4A with an 8mph wind. This should run about $1500 or so, should make about 2.5kW*hr/day, using the manufacturer's probably optimistic numbers.

There may be water turbines you could lower away and tow. Not sure how heavy, bulky or expensive they are, sources are hard to find. They will slow the boat, but can work.

A 2ft dia water turbine to tow would generate about 600W or 40A at 5 mph, requiring about 75 lb of drag. Best position is off the bow, as the water is disturbed off the stern. Towing off stern is problem, because cannot allow generator to spin. Up and down motion of bow will cause problems tho.

I would google different sources, price the systems and compare the power generated with what you need, select the one that is most economical for you, and consider making do with supplementary power from the chargers and generator when needed.

Answer 2

no such thing as a kw/hr, which makes me doubt your numbers. For example, you mention 1000kw, which is a MW, more than many power plants put out! Enough to drain 10 large marine batteries in 30 seconds.

You need the size of the batteries in amp-hours or watt-hours before you can calculate the power needed to charge them.

Solar power you will be limited in the amount of area you can use. Once you have that area, you can calculate the amount of power an array would provide. BUT that is only in bright sunlight, with the entire array in the sun, and the array perpendicular to the sun's rays. Anything else, and the power goes down. You will need a charge controller to control the current into the batteries.

Both wind and water power are not practical for a moving boat, as the energy required to push the wind turbine through the air, or the energy required to push the water wheel through the water are many times larger than you can possibly get from a generator. Otherwise they would be perpetual motion machines.

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