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In our continued journey to understanding GOING SOLAR, in the last weeks we have looked at some of the major components involved in going green. So far we have looked at SOLAR PANELS, and the CHARGE CONTROLLER. This week we take a basic look at the battery. This is a huge topic in itself and the longest I may present yet, I will try to condense it all here in 2 parts for easy consumption.
In order for your standard solar system to be fully useful, you need energy storage. This is where batteries come in! Here we are dealing with mostly lead acid batteries and although you can use Lithium Phosphate batteries (at somewhat of an expense). Modern lead-acid batteries come in flooded (FLA), gelled (GEL) and absorbed glass mat (AGM) types just to name a few. For the purpose of this article let’s assume that they are all the same.
First let’s look at the technical terms that define a Solar battery. These are basically Voltage, Capacity, Cycle Depth and Battery Life. Combined these form the basis for deciding the type of storage required for your power system.
Lead acid batteries are made up of 2 volt blocks or cells. Link 6 of these in series and you get a 12 volt block much like that which we use in starting motor vehicles.
Probably the most important information about a battery, this is measured in amp-hours (Ah) and is the amount of energy stored in the battery. This is measured by discharging a full battery at an industry standard amperage over a standard period of time. This rating is given to us by the manufacturer and serves as a base line for calculating the battery size we need for any use.
Thus in summary battery capacity is the battery’s discharge over time. For example a battery rated 100Ah at C20 would have been rated based on the battery being discharged at 5amps over 20 hours or 100amps / 20 hours = 5 amps per hour.
In the same breath a 200Ah battery at C20 would provide 10amps discharge over 20 hours. Going back to calculations or power and our dear friend Mr Watts here, we can calculate the amount of real power stored in a full battery. Using that formulae Amps x Voltage = Power, a 200Ah battery thus has a theoretical total, 200 x 12 = 2,400watts of power stored. (note:- a full battery would be approximately 2.28 volts per cell, while an “empty” would be around 1.8 volts per cell)
“Real Battery Capacity”
What I like to term “real battery capacity” is what we consider usable power. Consider a bottle of cooking oil. If we were to measure its contents to the absolute millilitre of oil, we would find that what we put into the bottle is not what we can get out. There will always be a bit left behind, that is un-usable. We can then say that the oil which we can get out is the usable volume of oil.
Although for different reasons, this analogy describes the battery’s real capacity in the same way. With a battery though, the reason we cannot take everything out is that amongst other, reasons a battery needs to have a little bit of power left in it to protect it against damage. The recommended maximum discharge level is 50% of the nominal capacity of any given battery size. Beyond this we consider the battery as going into deep discharge.
Looking at the last calculation we did to determine the full real power capacity of a 200Ah battery, if we factor in the recommended maximum discharge level of 50% the available power is now: 200 x 12 = 2,400watts x 50% = 1,200watts
Putting this into context and applying the theory from our article on power. The 1200 watts available would power a 60 watt load for: 1200 watts / 60 watts = 20 hours.
It is important though before going on to note that a solar battery does differ from a car battery. A car battery is designed to provide huge surges of power in short bursts to start your car for example, and then to charge up quickly again. These are called cranking battery’s and are NOT meant for the storage of power. The battery’s we are discussing today are considered deep cycling batteries. These batteries are designed to store power and discharge it at a steady slow rate up to a maximum depth of 80% of their nominal capacity without damage.
Cycle Depth and Battery Life
If you bought a new battery today and put it on your shelf in the shed and somehow managed to keep it charged every day, the battery would last about 10 to 15 years. This rate is determined by the speed at which the internal parts degrade due to the acid they are stored in.
However you are not going to do that, you are going to use it. In this case the factor that determines its life span becomes cycling, or basically how many times you charge and discharge it along with how deep you discharge it before recharging it.
A shallow cycle would be for example 10% discharge, a deeper cycle between 30 & 50%, while a deep cycle could be 80% of the battery’s capacity.
Discharging a battery up to 80% regularly will reduce the battery’s life dramatically. Typical life spans of batteries are rated in cycles vs depth of discharge as follows:
- 4000 cycles to 10%
- 3300 cycles to 30%
- 2500 cycles to 50%
- 1500 cycles to 80%
From the above information we can determine a battery life expectancy: A good AGM or GEL solar system battery discharged on average 10% per day could be expected to have a battery life of around 4000 days or 10.9 years. While discharging the battery to 80% every day, you could expect a battery life of around 4.1 years.
Next week we will look at charging the battery and determining battery capacity/size for your solar system, using the calculations we will find here. So read up and sharpen those pencils, and have a great week ahead.
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