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Old 03-05-2011
ext222 ext222 is offline
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Electrical requirements

Electrical requirements

I realize that there have been many threads and articles about this subject, and have tried to familiarize myself with all of them before asking for advice so that I can ask well formed questions. I ask your forgiveness in advance if I have completely misunderstood something. Also, since each part of the system is interdependent on the others, I apologize if I seem to skip back and forth from battery bank size to alternators to solar, etc. It makes sense in my mind.

In October of this year, 5 of us, two adult couples and a three year old boy, plan to set out in a Beneteau 50. We plan to cruise the Caribbean, living aboard and attempting to be as energy independent as possible. As a stock former charter boat, the Beneteau 50 is configured:

Hitatchi 80a alternator
6-55 amp batteries in the house bank
1-55 amp starting battery
Engine driven refrigeration, no DC or AC refrigeration
No Genset
No AC
Standard lighting, no LEDs.
Basic instruments + GPS.
Autopilot
No radar.
Hot water heated by the engine.
No microwave

We intend to add several fans, switch the salon lighting and anchor lights to LED, and add a watermaker and if possible(I know, shame on us) a microwave and a RV/Marine washer/dryer. Our logic on the watermaker is that with 5 of us including a little boy (Do you KNOW how dirty little boys are?) on board, we will use a fair amount of water and would prefer not to have to put in to marinas and buy it. Additionally, the adults are all scuba divers and we want to be comfortable using our fresh water to rinse our gear after diving. The logic on the washing machine is that the wife wants it, which was good enough for me until I started doing the math. Seriously, the ability to wash a few clothes on board might allow us to carry less clothing (and did I mention how dirty little boys are?). Obviously we would need an inverter to add these AC appliances.

All that brings me to my first questions.

My math on the watermaker is 17amps/hour DC to produce 6 gallons of water means that 24 gallons of water would require 68ah to produce. (accepting that there is then some loss to backflush the system.) Am I missing anything on that? That seems reasonable to me for the convenience it provides.

The washing machine (Westland WD2100XC) draws 1300watts/120V at peak, and a cycle last 73 minutes. That seems to indicate that from an inverter I should expect it to use 1300/12=108a/h, or 131ah per cycle, plus maybe 15 percent loss from the inverter, so about 151ah/cycle. That seems to be more than is practical, even with a large house bank. Thoughts?

Without the washing machine, I still come to a daily consumption of about 260ah with 8 hours of daylight sailing or 200ah at anchor all day.

With no DC refrigeration, we assume we will need to run the diesel an hour in the morning and an hour at night to get the cold plates nice and frosty. With no other way to heat water, that will also give us hot water for showering, etc. As much as the engine driven refrigeration lugs down the engine, I am assuming it provides enough load for the diesel to keep from running into glazing issues.

Clearly we will have to enlarge that battery bank and supplement our power needs with solar/wind. How much solar and wind power we will need to get in balance will depend greatly on how much of the charge we can expect to use from the daily diesel run.

Conventional wisdom seems to call for a battery bank of 3 to 4 times the expected daily usage. If we went with a 800amp house bank, using wet cell batteries, can I truly expect the bank to accept close to 80amps from my 80amp alternator for each of the two hours a day I plan to run it, assuming with the proper regulator? Going with the "25 percent of the battery bank for wets" figure, as I understand it, a 800 amp bank will accept 200amps during the initial charging phase, so my only issues are how many amps the alternator produces at the 1500 rpm I need to run the refrigeration and the energy production loss as it heats up, and then topping off. Does that indicate I could get a high output alternator to take advantage of the larger size of the battery bank, and then just use solar or wind for topping off?

Is it possible to have too large of a battery bank?


Thanks in advance for your thoughts.
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