Battery Bank Sizing
Determine how many watts of battery backup storage you want or need using our battery backup calculator. For this example we will assume we need 4400 watts.
Determine what hour rating you would like to use. We recommend choosing a 20hr or 24hr rating (depending on manufacturer) because this translates to cyclying your batteries once a day. This is recommended for all off-griders. We will use the AGM PVX-3050T batteries because they are sealed and maintenance free. No need to worry about equalizing them. These batteries are 6V and have a 305Ah rating for a 24hr period.
Determine desired depth of discharge. (There is a case to be made for higher depth of discharge when initial cost outweighs longevity.)
For this example we use 30% discharge.
30% Discharge will last you approximately 2000 life cycles
40% Discharge will last you approximately 1500 life cycles
50% Discharge will last you approximately 1000 life cycles
Pick a battery and convert it to Watts.
This example uses 305Ah, 6V, and 30% discharge
305Ah x 6V =1830 Watts
30% Discharge =1830 x 30% = 549 Watts.
However, if we wanted to discharge the battery 40%, we would get
1830 x 40% = 732 Watts.
Also, at 50% discharge
1830 x 50% = 915 Watts
Our goal was to build a 4400 watt battery bank using 30% discharge cycling the batteries once a day.
This can be accomplished by putting 8 batteries in series (48V total)
8 batteries x 549Watts = 4392 (These 8 batteries will be enough to power our 4400 watt needs)
By comparison, take a look at different watt ouputs for different discharge levels.
(40% discharge) 732 x 8 = 5856 Watts.
(50% discharge) 915 x 8 = 7320 Watts.
Notice how the same 8 batteries can give you more power if needed (* As mentioned above, a deeper discharge will result in a reduced lifetime)
We choose 30% because if cycled daily, this bank will last anywhere between 5-7 years without the need for replacement or maintenance.
Less parallel strings are better. Try not to go above 2
Higher bank voltage and lower individual battery voltage is better when storing large amounts of energy.
Useful Solar Battery documents for download or viewing.
Battery wiring is simple once you under stand how it works.