Batteries, Electricity, Solar

Chapter 5: Sizing The Battery Bank

The Battery Bank is your primary source of power when off-grid. While we will also be adding solar panels to recharge our system, the bulk of the power will come from the battery bank.

Because of this we need to seriously consider how much energy we, not only consume in an average day, but how much energy we can realistically harness with our solar panels in that same day.

“Load” Planning

Because each of our needs are going to be slightly different from one another, I’ll show a couple examples of how you can start calculating what kind of loads you can expect to place on your battery bank.

Our main energy needs come from: lighting in the morning and evenings; powering and charging our laptops, cell phones, and tablets; and watching a movie on the tv before bed.

The batteries are also going to supply power to water pumps, fans, and other components that require 12v DC electricity. 

Because we want our skoolie to be as self-sufficient as possible, we want to oversize our solar array and battery bank, at least as far as our needs are concerned.

With such a high amount of energy flowing through these wire connections, it is important to use the correct gauge of wire. For example, we will be using 2 Gauge Wire in Red, and Black, for our positive and negative lines, respectfully.

Capacity Needs

Deep Cycle batteries are rated in AH, for example, an 80AH battery would be able to supply 1 Amp for 80 Hours without charge. Or, 4 Amps for 20 Hours, 4 x 20 = 80.

The conciseness is that you should never use more than 50% of your battery banks capacity, therefore your battery banks total capacity should be 2x your desired capacity. If you need 100 Amp Hours (AH), than at minimum, your battery bank should be 200AH.

Additionally, the battery bank should be sized with some reserve battery power, it’s not a guarantee that we’ll be able to fully the recharge our battery bank every day. Periods of inclement weather should be considered when planning your battery bank capacity.

Connecting Batteries in Series vs Parallel

First, it important to explain the purpose of wiring batteries in Series and Parallel configurations. As we discussed in the previous chapter, batteries are connected in series to increase the voltage of the circuit; while batteries that are connected in parallel, are done so to increase the total capacity of the battery bank and to distribute the load equally among the batteries in the battery bank.

wiring

For a standard 12v DC system, if you use something like a 6v Golf Cart Battery, it is important to remember to wire them in Series to obtain 12v (or the desired voltage), NOT Parallel, as you would to increase capacity.

On the other hand, if you wanted to increase the capacity of your battery bank, you would connect the batteries in parallel.

Series / Parallel Battery Wiring

As discussed in the previous chapter, ”Battery Basics”, we covered how battery cells could be configured into modules and packs. In Chapter 9: Power Inverters, we will discuss why someone may want a 12, 24, or 48v battery bank. All that is important now, is that you know these types of setups are possible.

wiring1

12v Battery Banks

In this example, we are using (4) 6v 200AH Golf Cart Batteries. Because we want a 12v system, we need to start by taking (2) of the batteries and connecting them in series to achieve a 12v battery bank. Then we will do the same with the remaining (2) batteries.
At this point, you should have (2) separate 12v battery banks of (2) 6v batteries each. For the final step we will connect the (2) battery banks in parallel. The final battery bank can be seen below.

It’s not possible to connect 12v batteries in series/parallel without increasing the voltage of the battery bank.

24v Battery Banks

48v Battery Banks

Next: Chapter 6: Solar Panel Basics

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