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Old 30-01-2012, 21:08   #31
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Re: How Do I Know How Many Batteries I Need ?

I picked a large size just for the calculations its an 18.6 i believe, in reality I will be down around 14 maybe as high as 16. Could you explain why I need 300-400 AH of battery capacity? Is that all due to the inefficiency of the inverter? In my mind if the fridge uses 110AH per day then 200 amp hours of battery would be plentty for that one system, please explain where my logic is wrong?
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Old 30-01-2012, 21:18   #32
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Re: How Do I Know How Many Batteries I Need ?

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Originally Posted by jakegator View Post
I picked a large size just for the calculations its an 18.6 i believe, in reality I will be down around 14 maybe as high as 16. Could you explain why I need 300-400 AH of battery capacity? Is that all due to the inefficiency of the inverter? In my mind if the fridge uses 110AH per day then 200 amp hours of battery would be plentty for that one system, please explain where my logic is wrong?
You don't ever want to use more than 50% of your capacity (or the batts will die), and even if the fridge were the *only* draw you'd be pushing that. (on a daily basis, with no margin of safety)
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Old 30-01-2012, 21:20   #33
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Re: How Do I Know How Many Batteries I Need ?

OH! ok thank you, I completely forgot about that! Also, would you recomend have a DC battery bank and then one for AC to reduce the stress on the batteries or does it really not matter?
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Old 30-01-2012, 22:22   #34
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Re: How Do I Know How Many Batteries I Need ?

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OH! ok thank you, I completely forgot about that! Also, would you recomend have a DC battery bank and then one for AC to reduce the stress on the batteries or does it really not matter?
Even 14cf is a big fridge to find space for on a boat. How big is your boat?

You don't want to cycle the batteries below 50% very often (ie once a year or so). On the other hand it's hard to get them to charge higher than 80% or so. That means you get to effectively use 30% of the battery capacity, hence you want a bank 3x as big as the daily load with new batteries. The batteries will lose capacity over time, the bank should really be 4x load in anticipation of this. Since you probably won't be getting them up to 100% most of the time you want to do an equalization process once per month. That's a whole 'nother topic.

I would go with 1 large house bank to run all house loads, and second bank with a single battery strictly for engine starting. My logic is that on a cool day when fridge demand is low, or on a regular day when other demands are low the house bank does not get drawn down as much which is better for the batteries, or on really hot days when the fridge demand is high you can choose to conserve other demands and hopefully keep the bank from going below 50%. If you split the batteries into separate banks each bank is more likely to go below 50% more often.

Optimally the 2 banks would each have their own alternators. The stock alternator and regulator should be fine for the starting battery. I would add a little 5w solar panel to trickle charge this battery constantly.

The house bank optimally would have a large capacity alternator and a smart regulator. I would talk directly with the battery manufacturer about the appropriate charging protocol for their batteries and learn to program the regulator so it follows the correct protocol. The preprogrammed protocol is not necessarily the right one.

The solar panels need a regulator too. There are 2 types:
PWM which costs about $200 and optimizes for longer battery life.
MPPT which costs about $600 and optimizes for more charge put into the batteries for a given amount of solar cells.
The short-term costs seem to be an even tradeoff, less expensive controller but more solar panels needed to put the same amount of energy into the batteries. Then the question becomes which is more of a premium, finding space to mount more panels and using PWM or using the MPPT and replacing your batteries somewhat more often.
Personally I would go with the MPPT.
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Old 30-01-2012, 22:44   #35
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Re: How Do I Know How Many Batteries I Need ?

The boat is a 36 foot centre cock pit with a12ft beam, as I am the only person living aboard I have chosen to change the layout quite a bit, mainly getting rid of an extra head and 6 extra berths, the galley is snug but functional (theoretically). As for the deck I have room room for very easy mounting of two 230 watt panels and as for the wind generator I will probably mount it on the main mast, not very easy to get to but it will get better wind. As for the size of the fridge I am going to go look at sears and homedepot at then in person to make sure I get the right size for my needs, so the size is not by any means set yet.
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Old 31-01-2012, 01:25   #36
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Re: How Do I Know How Many Batteries I Need ?

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Based on the 12v input, of course, since this is what your batts supply.
A 1500 watt tea kettle used for 10 minutes a day would use 1500/13 = 115 amps * say 1.2 for a generous margin for inefficiency of conversion = about 140 amps /6 = about 23 amp/hours.
Another view. Inverter load 1500W/0.9 (efficiency, may vary)= 1666.67/ 11.4 (system voltage-cable voltage drop 5%, can not be lower than 11V, inverter manufacturer recommendation)= 138.89A. This is the current flowing in the cable and A/n (n= the number of batteries in parallel) is the current flowing in the battery or batteries at any time the kettle is turned on. This is as much as a capstan or a small starter motor given that it may take longer to bring a kettle to the boil than to start an engine. Consideration must be taken that a battery capacity varies according to speed of discharge. A battery manufacturer may provide four rate of discharge times, 20, 10, 5, 1hr. I normally do not like to exceed the 10hr rate of discharge.
A small domestic 240Vac freezer is rated 0.8A. Supplied by an 1100W inverter at 24.5Vdc the current drawn is 8A. This is in line with a 10hr rating for the type of battery I use.
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