The ins and outs of Solar Batteries


    In the world of Stand alone solar power system there is usually only one type of battery used... The lead acid battery, as such our entire discussion will be on the lead acid battery.

    If you have been researching solar system you would have come across two terms quite frequently. Gel cell batteries, and Flooded cells. Both types of batteries are of the Lead Acid variety, we will talk about each of them in more detail later.

    The order in which we will discuss things are:

    1. The basics of a Lead Acid battery

    2. Deep Cycle Batteries only

    3. The differences between Flooded Cell and Gel Cell batteries.

    4. Understanding battery terminology, ratings, and energy capacity.

    5. Battery charging



    1. The basics of a Lead Acid battery 

    One of the interesting things I learnt in the early days was a single battery cell (what I am describing below) has a nominal voltage of 2 volts. A 12 volt battery is actually made up of 6 individual cells tied together in series to create the 12volts. I thought you brought batteries 12 volts at a time.. imagine my surprise when I purchased my first 2V battery cell that weighed 68kg.... anyway lets move on.


    a lead acid battery in a fully charged state

    A lead acid battery cell is made up of two posts immersed in sulphuric acid. The Positive post is lead coated in a lead oxide. The negative plate is Lead with no coating (technically the terminals are not pure lead, they are always alloyed with something to improve their electrical and mechanical properties).

    In a fully charged state:

    - the sulphuric acid is strong

    - The negative plate is not coated with any deposits (well almost)

    - The positive plate is coated with lead oxide.


    As the battery cell is discharged, electrons migrate from the positive post to the negative plate, which produces a conventional current flow from the positive to negative. The electron migration occurs as a result of the Lead oxide, sulphuric acid, and lead reacting together.

    the internal structure of a lead acid battery fully discharged

    The final result of a fully discharged battery is lead sulfate deposits on both the positive and negative posts and the sulphuric acid diluted right down.

    The Recharge Process

    I dont think you will be to surprised to hear the recharge process is exactly the opposite to the discharge process. The lead sulfate is removed from the positive and negative post and lead oxide is re deposited back onto the positive post, as well as the sulphuric acid intensifying.

    Some interesting things do occur if you keep charging the battery cell past is fully charged state; basically the electric current starts the split the water into hydrogen and oxygen. The process of continually charging past the point of full charge is called gassing (because hydrogen and oxygen gas is created). When you first hear about gassing, It seems like gassing would be a bad thing? Too much gassing is bad, however gassing of batteries does have some benefits which we will talk about later.


    2. Deep Cycle Batteries only

    Stand alone power systems require a special type of battery known as a deep cycle battery, these batteries are designed to be fully discharged many times.

    A normal lead acid battery (like your car battery) will only allow you to take about 20% of its stored power, after that point it is permanently damaged; it doesnt take long before the battery is rendered useless.


    3. The differences between Flooded Cell and Gel Cell batteries.

    I have chosen to specifically pick out Flooded cell and Gel cell lead acids for discussion, because these are the batteries you will be buying.

    Flooded Cell:

    These are the batteries you are most familiar with.. your car battery is a classic example of a flooded lead acid battery. The battery has a filler cap that allows you to add distilled water to the cells as the acid levels get low.

    Why do the acid levels drop? Gassing.. every time the batteries recharge a certain amount of gassing will occur, and every now and then a solar system will switch into a charging mode that will generate a fair bit of gas. A flooded cell battery needs water levels replenished.


    Gel Cell:

    Gel cells are all the rage in standalone power systems these days, they are a lot more expensive but they do not require any maintenance, you dont need to add water to these batteries.

    Gel cells are so called because the electrolyte is turned into a thick gel. The batteries function exactly the same way as we discussed earlier, except that any hydrogen and oxygen gas created is recombined back into water via internal mechanisms... hence no loss of water.

    Gel Cell batteries are actually one type of Valve Regulated Lead Acid (VRLA) battery.

    There is another battery type that is also a VRLA battery, and it is not suitable for standalone power systems; the battery is called an Absorbed Glass Mat (AGM) battery.

    Many battery retailers call all batteries in the VRLA family, gel cells. Make sure you do not end up buying AGM batteries for your standalone solar system.


    4. Understanding battery terminology, ratings, and energy capacity.

    Batteries capacity is measured in Ampere Hours (Ah), it is the discharge current (in Amperes) multiplied by how long (in hours) the battery can sustain that current.

    For example a battery with a capacity of 1000Ah could sustain a 100A current draw for 10hrs, or it could sustain a 10A current draw for 100 hours.

    If that was all there was to battery rating it would be to simple.

    The tricky bit is that a batteries capacity (in Ah) changes based on the actual current being drawn from the battery. For example: a battery with a nominal rating of 1000Ah may only be able to delivery 800Ah when 90 amps is being drawn, but if the current is only 10A then the battery can support 1000Ah.

    Battery manufacturers usually provide the capacity of a battery across a number of standard hours, those being  100, 20, 10, and 5 hours; these ratings are called the C ratings. When a retailer tells you the battery is a 250Ah battery they are normally quoting the C10 rating of the battery.


    A battery is rated as per the below table:

    C Rating Capacity (Ah)
    C100 1000
    C24 900
    C10 825
    C5 722


    Based on the table above we can say they battery will support 1000/100 = 10A over 100 hours, or 722/5 = 144A over 5 hours.

    Battery Capacity and temperature:

    The capacity of the battery is also affected by temperature. The colder things are the less capacity the battery has. The bad news is the hotter the battery gets the shorter its lifespan. All battery capacity are quoted based on a temperature of 25 deg C.


    So how much energy is in a battery bank?

    So lets say you have a planning to put 6 x 2V, 1000Ah batteries in series to store your energy.

    This means your battery bank voltage will be 12 volts and you have nominally 1000Ah of available capacity. To convert this to kWh is really easy 12 Volts x 1000Ah = 12,000 Wh = 12kWh of available energy storage.

    This does not mean you can actually use all of that 12kWh of stored energy.

    graph showing relationship between depth of discharge and battery life

    The chart above shows the relationship between Depth of Discharge (DOD) and the cycle life of the battery. A batteries life is measure in cycles, its up to you to work out how many cycles a battery will do per day/month/year, which will then let you work out how many years a battery will last.

    Depth of discharge refers to the amount of stored energy you take from the battery.

    The above chart was taken from the Hoppecke OPzV Gel Cell batteries. You can see that if I take 100% of the energy out of the battery I will only be able to do it 1500. If you assume that a battery goes through 1 full cycle each day (which is about right for standalone solar systems) the batteries will last 1500/365 days in a year = 4yrs.

    If I decide to only take 50% of the energy, the batteries will cycle 2500 time, which means they will last 2500/365 = 6.8yrs.

    If I decide to only take 30% of the energy, the batteries will cycle 3500 times, which gives me a life expectancy of 3500/365 = 9.5 yrs

     When designing your Stand alone system you want to try and get at least 9 to 10yrs out of the batteries, because they are very expensive.

    So lets assume we are aiming for DOD of 30%; if we go back to our original example (6 batteries @ 1000Ah). It technically holds 12kWh of energy, but to make sure we get a long life out of the batteries we can only really access about 3.6kWh of that energy on any give day.


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