WHAT IS A SOLAR BATTERY?

This week we look at solar batteries, an important part of your solar system… especially in this part of the world where back-up power is a priority for most of us :). Well there are several types, makes and brands of batteries out there, and one wonders which battery is the best one for their system??

Well to start with, a battery simply provides us with a way of storing energy for future use e.g when the sun is gone… or when grid electricity (ZESA) is gone :). Every battery needs to be charged from a source, in our case either solar energy or electrical energy. Every battery has a life-span, meaning it will run for some time then will eventually need replacement.

More technically, every battery consists of two electrodes (positive and negative) that are isolated by a separator and soaked in electrolyte to promote the movement of ions [1], this movement of ions produces a current… electricity.

We will start by looking at battery terminology to then better understand the different types of batteries out there, then in later posts, talk about how to make the best selection of battery for your system. If you dive deeper into batteries specifications and data sheets, you will come across the following terms:

1. Battery Capacity:

This is the amount of energy that a battery can store. Usually denoted in KWh (kilo-watt hours) or Ah (Amp- hours)… though technically the 2 terms mean different things. Without taking into account other factors (which we shall talk about later), the kWh term tells you how many kilo-watts your battery can provide in an hour (for example a 2kWh battery will give you 2 kW in 1 hour, similarly it can also give you 1kW for 2 hours, 0.5kW for 4 hours etc). Like-wise the Ah term tells you how many amperes your battery can provide in an hour (for example a 100Ah battery will give you 100A in 1 hour, similarly it can also give you 50A for 2 hours, 25A for 4 hours etc). However, you rarely can get all the energy out of a battery… if you do that, the battery will not last. This is where the next term comes in ↓

2. Depth of discharge (DoD)

This is a measure of how much energy a battery can discharge (use up) because of varying battery chemistries. In simple words the DoD tells you how much of a battery’s capacity you can use e.g. a battery with a DoD of 50% implies that you can only use 50% of it’s capacity before recharging it. If you were to compare the 2 batteries below, with the same capacity↓

(i) Battery 1 has a capacity of 100Ah, DoD of 50%... you can only use 50% x 100= 50Ah before recharging

(ii) Battery 2 has a capacity of 100Ah, DoD of 80%... you can only use 80% x 100= 80Ah before recharging

So, based on DoD alone, it would seem sensible to use Battery 2 over battery 1…. BUT DoD is not enough!!

3. Battery C-rating

This is the rate at which the capacity of the battery is built up (charging) or given up (discharged) depending on different battery make-ups and chemistry. In other words, it is a measure of a battery’s guaranteed capacity at different discharge duration. It is usually denoted with the letter C and a number next to it, for example C20. To illustrate this, look the example below:

-A 150Ah Battery, with a C-rating of C20 will give you a current of 7.5A i.e. 150Ah/20hrs= 7.5A

So, the battery above will discharge for 20 hours. A higher C-number means that the battery will discharge in more hours than a smaller C-number. For example, as illustrated above A C20 rating is the performance when discharging for 20 hours.. similarly a C10 rating is the performance when discharging for 10 hours.

Why is all this important you might ask?? The most important thing to note is that with most batteries, the total capacity falls with a smaller C-number. In simple words, your battery’s capacity FALLS when you discharge it faster!! Take a look at the picture below from Sukam’s Bazooka Battery [2].

The picture above tells us, for example, that though this battery is “rated” at 150Ah, if discharged in 8 hours the capacity falls to 120Ah.. similarly if it discharges in 5 hours the capacity falls to 105Ah and so on… the capacities shown at those different capacities is what the manufacturer guarantees that you will get.

So this means that when you are told your battery is a 100ah Battery, it is only 100Ah at a certain C-rating (discharge time)!! It’s important that one tries to get an understanding or explanation of their preferred battery’s C-rating at their intended hours of use.

More on batteries in future posts… watch the space!!

SOURCES:

1. https://batteryuniversity.com/

2. https://www.youtube.com/watch?v=STxas6Bra4M

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About Author:

Lovewell Chitiyo is a Mechanical Engineer with 15 years experience in the building services Engineering consultancy, renewable energy systems and sustainable design fields. He is currently studying Masters in Renewable Energy at Stellenbosch University in South Africa

Lovewell is an Accredited Professional with the Green Building Council of South Africa for Existing Buildings Performance and New Buildings; Interiors, holding several qualifications and certification in Renewable Energy and Sustainability.

Lovewell also has vast experience in design, supervision, installation and commissioning of several projects locally and regionally in the last 15 years.