Led Acid Batteries

All batteries have a shelf life, and for a common number we will say about ten years. They also have a manufactures spec sheet that tells you how many times they can be drawn down to 50% from full charge. An inexpensive battery can be drawn down about 1200 times, a better battery can be drawn down 1700 times. So when sizing your battery bank you need to include both of these factors.

The Sizing of your Battery Bank

How long it takes to draw down your battery bank is called days of autonomy. To figure out what you need you have to find your nominal battery voltage. To do this you take the number of Kwh of daily load required (as we figured out earlier) and divide it by the battery voltage chosen for your battery bank, this will give us your nominal battery voltage in Ah. We will assume the depth of discharge at 50% and multiply the nominal battery voltage by two. There are losses in all systems, so we add on another 15%. This will give us the number of Ah needed in the battery bank to give us one day of autonomy.

Example: normal house 15.750 Kwh with a 48V battery bank.
15750/48 = 328 Ah Nominal voltage
328 X 2 = 656 ah + 15% Losses = 754.4 Ah
So you require a bank with 754.4 Ah for one day of autonomy.

Now if you choose the inexpensive battery with 1200 cycles at 50%, you take 1200 divide it by 365 and your battery bank will last 3.3 years. So now you may increase the number of batteries to extend your days to autonomy to get the shelf life from the battery you select.

In conclusion you can see it is important not to under size your battery bank, and there is no need to over size it either.