Marine Products 

HOW TO SIZE AND USE YOUR BATTERY BANK 
Introduction. We encourage you to ignore some "conventional wisdom" on energy management, gain your own clear understanding of DC storage and charging systems and apply that knowledge to your boat. By doing so, you can reduce the engine run time required by your refrigeration system to 50% to 80% less than would be needed with a direct enginedriven system. Also remember, the money and effort you put into your battery and charging system benefit the entire boat, not just the refrigeration system. What size battery bank should you
have? Energy Analysis  the conventional
wisdom.

Assumption  Calculation  Usage per 24 hrs 
Three reading lights drawing 1
amp each will be used an average of 3.5 hours per day. 
(3 x 1) x 3.5 = 10.5  10.5 amphrs 
The refrigerator draws 6 amps 
6 x 18 = 96  96 amphrs 
Making accurate estimates for some items is
relatively easy, for others such as electric autopilots it is nearly
impossible. Nevertheless, to complete the analysis, the total estimated
energy consumption of all such devices is added together to provide an
estimated average total daily energy consumption in amphrs. As you can
imagine, the totals vary tremendously from boat to boat. However, for
illustration purposes, let's assume that our particular cruiser calculates
a total estimated use of 140 amphrs per 24 hours.
In the next step of the analysis it is assumed that our cruiser will not want to discharge their batteries more that 50% since it is commonly believed that doing so will disproportionately shorten their life span. The cruiser must now decide how often they wish to run their engine or generator recharge their batteries. This, in combination with their estimated daily energy consumption then forms the basis for calculating the battery bank size required. For example;

Recharge Frequency Desired  Bank Size Required 
twice per day  140 amphr 
once per day  280 amphr 
once every two days  560 amphr 
once every three days  840 amphr 
So it is seen that, by using the energy
analysis method, the calculation of appropriate battery bank size is made
purely on the basis of energy usage and the frequency of recharge cycles.
Sounds good. So what's the problem?
Why energy consumption is irrelevant
when sizing your battery bank. The proper size alternator for the
battery bank. Capacity  Type  Calculating your battery bank's
acceptance rate. For the sake of this exercise, we will estimate at the average acceptance rate when cycling the batteries between 50% and 80% of full charge. Under these conditions leadacid batteries have been shown to have an acceptance rate equal to 25% of their total 20 hour amphr rating. Stated another way, a leadacid battery bank consisting of three 8D size 12 volt batteries @ 220 amphrs each (660 amphrs total) would have an acceptance rate of 165 amps. One advantage in gel cell type batteries is that they have a higher acceptance rate than do the common lead acid type. Acceptance rate calculations made with gel cell batteries should be based on 40% of their 20 hour amphr rating rather than the 25% figure used with lead acid. Because of their lower 20 hour rating, the bank of three 8D batteries described in the example above would have a total capacity of only 600 amphrs (rather than 660 with lead acid). However, they would have an acceptance rate of 240 amps instead of 165 amps. The highest acceptance rate is obtained with absorbed glass mat batteries (AGM). Acceptance rate calculations made with AGM batteries should be based on 100% of their 20 hour amphr rating rather than the 25% figure used with lead acid or 40% used with gel cells. Our bank of three 8D batteries (as described in the other examples) would have a total capacity of 600 amphrs just as would the gel cells. However, they would have an incredible acceptance rate of 600 amps instead of 165 amps (lead acid) or 240 amps (gel cell). As you can see, once you know the capacity and type of your battery bank you can calculate it's acceptance rate. Simply multiply the total capacity by 25% for lead acid batteries, 40% for gel cells or 100% for AGMs. What about the
alternator? Given this, how is it possible to get 200 to 300 amps of real charging capability? Sometimes it isn't, but don't give up too quickly. Very large alternators with outputs 200+ amps are now quite common. Additionally, it is often very practical to use two or more alternators to charge a single bank. Some boats are already set up to have one alternator charge the engine start battery and a second to charge the house bank. Usually the engine start battery needs little if any charging. An automatic battery bank combiner can be used to allow both alternators to charge the house bank. How much difference does this approach
to battery sizing make? Now, using your knowledge of acceptance rate calculation, you know that the maximum rate at which this bank can be recharged 70 amps (bank capacity x 25%). Therefore, it will take two hours of engine run time per day to replace the electricity that the cruiser is using (70 amps x 2 hours = 140 amphrs). If, on the other hand, the cruiser wants to minimize their engine run time, they could increase their battery bank to far in excess of that recommended by the energy analysis approach to, say 600 amphrs. With this bank the acceptance rate would now be 150 amps, making it possible to replace the same 140 amphrs in less than one hour, or only ½ the time required by the smaller bank. If all that sounds great but you don't have room for that many batteries, use gel cells or AGMs. You'll be able to get that high charge acceptance rate in a much smaller bank. By gaining a good understanding of battery acceptance rates, it is clear how large house battery banks can be used to reduce the engine run time. It is equally easy to see why the popular practice of separating house batteries into multiple banks is not the most efficient use of energy. Battery life. However, the number of cycles is only one measure of a battery's life and probably not the best. Another way to look at battery life is by looking at the total number of amphrs which a battery will store before failure. When viewed in this way, quite a different picture emerges as can be seen in the chart below.

Typical Cycle
Life (100 amp/hr Trojan Deep Cycle Battery)  
Depth of discharge  Number of cycles  Total amphrs
provided during service life 
10%  6,200  62,000 
20%  5,200  104,000 
30%  4,400  132,000 
40%  3,700  148,000 
50%  2,900  145,000 
60%  2,400  144,000 
70%  2,000  140,000 
80%  1,700  136,000 
From our previous look at acceptance rate we
know that batteries can be recharged much faster when they are permitted
to cycle down to 50% and below. From this chart it is obvious that doing
so extends the useful life of the batteries as well.
Conclusion.

This information is provided free of charge as an educational service to the cruising community. No aspect of this data may reproduced, published, distributed or referenced by any organization whether profit or nonprofit without crediting Glacier Bay, Inc. as its source. Additionally, no profit making organization may do so without the written permission of Glacier Bay, Inc.To obtain such permission contact Glacier Bay, Inc., 2930 Faber St., Union City, CA 94587. 
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