How to Size a Battery Bank - Part 2
Once you know your average daily energy consumption (Wh), number of days of autonomy and the system voltage, you can start estimating the number of batteries you need for your off-grid project.
-continued from "How to Size a Battery Pack - Part 1"
To begin, you need to divide the average daily DC energy consumption by your system’s voltage; this will provide you with the average ampere-hours that your batteries would have to cover in a day.
[(AC Average Load/ Inverter Efficiency) + DC Average Load] / System Voltage = Average Daily Ampere-hours
[(720 Wh/ %90) + 160 Wh] / 12 V = 80 Ah
By multiplying the average daily ampere-hours by the number of days of autonomy, you’ll easily determine the necessary ampere hours that your system has to provide.
Average Daily Ampere-hours x Days of Autonomy = Total Ampere-hours
80 Ah x 5 days = 400 Ah
At this stage it is important to note that, it is not advisable to discharge your batteries below 50% of their capacity. The lower you discharge the batteries, the shorter your batteries will last for.
You have already determined the load capacity for your batteries. You should make a battery selection at this point to see how many batteries you will need in parallel.
Let’s pick the Surrette 6V, 400 Ah batteries
Total Ampere-hours / (Discharge Limit x Selected Battery Capacity) = Batteries in parallel
400 Ah / (50% x 400 Ah) = 2 batteries in parallel
It is advised to round the number up if it’s not an integer (i.e. 2.12 would round up to 3; better than to be safe than sorry).
Calculating the number of batteries in series is easy. You basically divide the system voltage by the battery voltage. Let’s assume your system voltage is 24 Volts.
System Voltage / Selected Battery Voltage = Batteries in series
24 V / 12 V = 2 batteries in series
[image credit: mediaglobal]