# Battery Bank Sizing: Off Grid Solar Power System Design – Step 2

Hi, welcome back. I'm Amy Beaudet from the altE Store. Thank you for watching the second of our video
series on designing an off-grid solar system. Step 2 is to size the battery bank. We'll discuss the different considerations
video, so we know how much power we will use each day. Now let's see what size battery bank we need
to store it.

The answer may surprise you. Just a quick reminder of the components that
make up an off-grid system. We will start at the battery bank. You may recall the loads list we did in our
previous video. We came up with a total usage of 2191.5 watt
hours a day. Note however that 44Wh are DC, not AC, and
therefore not going through the inverter, so we'll need to use that information later. So let's get started. The first thing we need to decide is what
voltage we will make the battery bank. Most off-grid battery banks are either 12,
24, or 48V. So how do you decide which to use? First is what voltage are your loads? Are you just powering a small video surveillance
camera or light that runs off 12V DC? Or is it an AC system that will be using an
inverter to convert from DC to AC? If it is using an inverter, what size does
the inverter need to be? Generally, the higher the output wattage,
the higher the DC input.

For example, if it is a 2000W inverter, it
may be available in 24VDC, but a 6000W inverter will certainly require 48V. Another possible consideration is the distance
between the solar panels and the battery bank. Depending on what type of charge controller
you use, you may need to match the voltage of the solar array with the voltage of the
battery bank. I'll get more into that in a future video. But if you do need to match voltages, keep
in mind that if the panels are far away from the batteries, you can reduce the gauge of
the expensive copper wire needed by using a higher voltage.

Since using a higher voltage array results
in lower current, you can potentially save money by running the system at 48V instead
of 12V. Let's get into this a little deeper. We'll use an 8A4D battery as an example. It is 12V and 200Ah. 12V x 200Ah = 2400Wh for 1 battery. Remember that wiring in parallel increases
the amphours, but keeps the voltage the same, and wiring in series increases the voltage,
but keeps the amp hours the same. If I needed 4800Wh capacity, I can wire 2
of these batteries in parallel . However, if I needed 9600Wh, I would need 4 of these
batteries, 4 x 12V x 200Ah. Since I want to limit the number of parallel
strings I use, I can't wire them all in parallel , but I could wire them in 2 parallel strings
of 2 in series.

In doing so, I made a 24V 400Ah battery bank,
24V x 400Ah = 9600Wh. Or, I can make a 48V system by wiring them
into 1 string of 4 in series. To decide which voltage you use, you can then
refer back to the other considerations of if you have any specific voltage DC loads,
or if the inverter you picked requires a certain voltage. If you did need to have a 9600Wh battery bank,
as in the previous example, but you need a 12V bank for your loads, you can still accomplish
this. You would want to pick a lower voltage but
higher amp hour battery.

9600Wh / 12V = 800Ah. So how can we build that? We can pick a lower voltage, higher amphour
battery, like the Concorde PVX-405 at 6V and 405Ah, and wire them 2 parallel strings of
2 in series. The 2 6V batteries in series makes 12V, and
the 2 405Ah batteries in parallel equals 810Ah, more than enough. When wired together, you get a 12V, 810Ah,
9720Wh battery bank. Now that we've figured out how much power
we use a day, we need to know how many days we plan on running our equipment off the battery
bank if there is no sun to recharge it, or days of autonomy.

This is a delicate balance, because the more
days we select, the bigger, and more expensive the battery bank gets. But we don't want to go too small either,
because the less we drain the batteries, the longer the bank will live. This is where that generator I mentioned can
come in handy. For example, you could pick 3 days of autonomy,
and plan on using the genny to charge up the battery bank if needed on day 4. Depth of discharge, or DoD, is how far down
you can drain the battery. A lead acid deep cycle battery that is made
for renewable energy systems can be drained down pretty low, but the less you drain it,
the longer it will live. You'll often hear people say you can drain
a deep cycle battery down to 50%. That's true, but if you do, it will last half
as long as if you drained it to 20%.

Each battery will have a depth of discharge
chart. you can see here that if you drain this battery
down to 50%, using half it's power, you can get about 1500 cycles, or 1500 days if you
do that every day. That's just over 4 years. But, if you only drain it 20%, you can get
3400 cycles, over 9 years. That sounds great, except you have to remember
that requires a bigger bank to use a smaller percentage. So you have to balance the upfront cost of
the system with how often you have to replace the batteries. You may also hear the term State of Charge
(or SoC). That is the percentage of how full the batteries
are. It is the inverse of DoD. So a battery that is at 30% depth of discharge
is at 70% state of charge. Batteries are rated at 77 degrees Ferenheight,
or 25 degrees Celcius. When the temperature gets colder than 77 degrees,
the amp hour capacity decreases, but the lifespan increases.

When a battery is hotter than 77, the capacity
increases, but the lifespan decreases. To compensate for lower temperatures, we will
need to increase capacity. This chart shows the change in capacity based
on temperature. You see here at 77 degrees, the capacity is
at 100%, what it is rated for. For example, 100Ah. But you see here at 50 degrees, you are at
81% of the rated capacity. So if you still need 100Ah, you would need
to multiply that by 1.19 to get a battery rated at 119Ah, and at 50 degrees, it will
be able to store 100Ah.

The colder the battery is, the larger the
rated battery needs to be to store your power. OK, now that we know the variables, let's
do some math to figure out what size battery bank we need. From our loads list (you remember the loads
list, don't you?), we are using 2192Wh a day, but only 2148Wh was AC. We will divide it by the efficiency of the
inverter we use, let's say 92%, to make up for lost power used by the inverter. Then we enter our DC loads, 44Wh. this gives
us 2379Wh. Next step , Then we multiply the 2379Wh by
days of autonomy and temperature compensation . I'm going to be storing it in a 50 degree
room, so I use 1.19. We'll divide that , by .5 for 50% depth of
discharge.

Now notice that we are using 50% depth of
discharge, but that's after our 3 days of autonomy, so I can run my loads for 3 days
with no solar recharging the batteries, and after 3 days, I'll have used half my rated
capacity. That should give me plenty of stored power,
and a long battery life. This gives me 16986Wh battery bank needed. Then , I divide this number by the voltage
of the battery bank we picked. I'm going to use a 48V battery bank, so I
divide by 48V to get 354Ah. OK, we are almost there. We take our 354 amp hours, and divide it by
the maximum number of strings we want to use. I'll go with 2. That says I need 2 strings of at least 177Ah
batteries.

Let's pick some batteries that will fit this
bill. An MK 8AGC2 battery is rated at 6V 190Ah,
we can use that one. Next , we take the system voltage of 48V,
divided by 6V battery, which tells us we need 8 6V batteries in series. Let's add that all up. 2 parallel strings of 8 in series = 16 batteries
needed. That's it for the second video for designing
an off-grid PV system. Watch the next videos in the series for how
to size the solar array, and charge controller and inverter, using the numbers you came up
with from your loads list. Also watch more of our Video Series on our
web site, and peruse our selection of deep cycle batteries. We've got a team of highly trained Technical