Hi Vincent,
I think that calculator is acting a bit funny. I know James is working on it.
Here is the logic I use. I’m not sure if the calculator uses the same (or something similar)
1. Identify the total number of watt hours per day you use
2. Identify the number of days back up you need.
3. Multiply watt-hours per day by back up days (step 1 by step 2)
4. Identify you planned depth of discharge. 50% is as low as I ever plan on. If you want to seriously live on the system (and not just use it for backup) I would go with a 25% dod or better. Convert this number into a decimal (.5 for 50% dod, .25 for 25% dod, .8 of 80% dod). Divide this number by the value in step 3.
5. Derate your battery bank for the lowest average temperature the batteries will be exposed too. Multiple a number bellow by the figure in step 4. This is the minimum watt hour capacity of your battery bank.
Temp in F Factor
80+ 1.00
70 1.04
60 1.11
50 1.19
40 1.30
30 1.40
20 1.59
6. Determine your system voltage. Divide the answer to 5 by your voltage. This is the minimum amp hour capacity of each parallel string of batteries (4500 watt hours / 12v = 375 AH)
7. Select a battery that looks close to your amp hour capacity you calculated in 6. If you can’t find a battery that is close to the capacity, look for one that is as close to half the capacity you need, or close to a quarter the capacity you need.
8. Once you find a battery divide your system voltage by the battery voltage. This will give you the number of batteries you need in series.
9. Divide the number of from question 6 by the amp hour of the battery. Round up the nearest whole number. This is the number of series strings in parallel.
10. Multiply the answer from question 8 to the answer from question 9. This is the number of batteries you need total.
For Example:
1) I use 6000 watt-hours per day
2) I want three days back up
3) (6000 Wh/day) x (3 days) = 18,000 Wh
4) I plan to discharge the batteries no more the 40%. (18,000 Wh) /( .4) = 45,000 Wh
5) My batteries will never get bellow 60 degrees. (45,000Wh) x (1.11) = 49,950 Wh
6) I am planning a 48v system. 49,950 Wh / 48v = 1040 AH.
7) The price per watt-hour in the MKL16 looks good, I want to see if it will work for me. It’s 370 AH at 6v
48v / 6v = 8 batteries in series
9) 1040 AH / 370AH = 2.8… I’ll round up to 3
10) 8 batteries in series x 3 strings in parallel = 24 batteries.
At this point I would look at another battery option to see if it worked better. I’ll start again at step 7:
7) I want to reduce the number of battery interconnections (especially parallel connections) so I find a Surrette battery that is 4v and 1104 AH.
48v / 4v = 12 batteries in series
9) 1040 AH / 1104 AH = .94… I’ll round up to 1 (this actually means I have 1 string in series, so no parallel connections at all).
10) 12 batteries in series x 1 string in parallel = 12 batteries.
At this point I would email my AES sales rep and ask for a quote on 24 MK L16’s and one for 12 Surrette 4-KS-21PS (with shipping) to compare prices. I would break down each battery bank into total watt-hour (battery voltage x battery amp-hour capacity x number of batteries) capacity and then find the price per watt-hour of energy storied (total price of batteries delivered / total watt-hour capacity). I would take to my friends (and fellow forum members) about what they think of the brands, compare warranties, and then buy the batteries I felt best about.
Almost as much fun as a 1040!
