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K & C Stork Solar Power Consultants

Batteries PDF Print E-mail

Using Batteries
For home-power systems not connected to the grid, batteries are still the best energy storage option (although fuel cells are becoming cheaper and will compete with batteries in years to come). Systems that use batteries as their main storage are called standalone systems.
The batteries used in remote power and solar power systems are specially designed to endure deep discharging, Car batteries are not suitable for these systems as they are designed to provide massive amounts of current for a short period of time, and be kept fully charged at all other times. They also rapidly decrease in capacity as they age and need to be replaced.
The most common battery used in remote area power supply is the lead-acid battery. Storage systems usually consist of a number of 2 volt cells connected in series to produce the voltage required by the system. Most commonly used are 12, 24 or 48 volt assemblies. Smaller individual 2, 4, 6, 8 or 12 volt batteries are available. Nickel-cadmium (ni-cad) batteries have lives up to ten times that of a lead-acid battery and provide more stable discharge voltages. However they are also much more expensive, and require much higher charging voltages, and many system components are not designed for large voltage ranges. Ni-cad batteries are also very difficult to dispose of, as there is no-one in Australia that can fully recycle them, so they must be shipped overseas for final processing.
Batteries can require a lot of maintenance and must be kept charged, clean and safe. They also have a predefined lifetime and will need to be replaced about every 10-15 years or so depending on the brand, model and level of care they experience. Lead-acid batteries have been known to have survived for up to 30 years, but this is rare. A battery is regarded as lived its life if its capacity is less than 80% of rated capacity.

Installing Batteries
There are specific Australian Standards regarding the installation of batteries, but I will cover some more important points here for reference only. Installation of ALL batteries should be seperate from any and all other equipment, and be ventilated to the outside air. Batteries under charging conditions will give off hydrogen sulphide gases (lead-acid batteries) and this is not only very corrosive, but highly flammable. These gases can corrode metal, including electronic equipment and be ignited by some electronic equipment as well, causing in some cases massive explosions. DO NOT UNDERESTIMATE THE POTENTIAL DAMAGE THIS CAN CAUSE. I have personally had the dis-pleasure of being involved in a battery explosion, even though it was only a standard car battery it blew the windows out of a shed, and covered everything inside with corrosive acid including me.

Battery Capacity
The capacity of a battery is usually specified in amp-hours (Ah) for a given discharge rate (eg C100, C20 etc). The capacity, or the batteries ability to deliver energy depends on its discharge rate. Faster rates mean less capacity is available. A battery rated at 100 Ah @ C100 can provide 1 amp of current continuously for 100 hours. Similarly, a C20 = 100 Ah battery would be able to provide 5 amps of current for 20 hours.
To have more of an idea of what this means, if the latter battery was a 12 volt assembly, it would provide (5 amps x 12 volts) 60 W watts of electricity, enough power to run a small stereo, sewing machine or 3x20 W compact fluorescent lights for 20 hours.

Battery Charging
Most batteries should be charged at between 2.4-2.5 volts per cell (14.4-15 volts for a 12 volt battery). At cell voltages over 12.3 volts, a process called gassing occurs. Gassing is the splitting of water into hydrogen and oxygen, and has a beneficial effect of stirring up the electrolyte in the battery. However too much gassing means the charging energy and water in the battery are lost. Areas in which batteries are stored should be properly vented to ensure the escaped gases do not collect and cause an explosion.
The amount of energy stored in a battery at any given time is known as the 'state of charge'. This may be measured in two ways:

  1. Using a hydrometer - A hydrometer can be used to measure the density of the electrolyte. The electrolyte (water-acid mixture in the battery) normally has a specific gravity of 1.15 when less than 90% discharge (flat), and around 1.26 when fully charged.
  2. Battery Voltage - The rate at which a battery needs to be charged changes depending on its state of charge. This property can be used in reverse to determine the energy left in the battery. By measuring the voltage at a constant charge rate, the capacity can be estimated.

Battery Life
When energy is removed from a battery and then recharged it is said to have undergone a 'cycle'. If more than half of the batteries storage capacity is discharged it is called deep cycling.
The life of a lead acid battery depends on the depth of discharge, the deeper the cycle, the shorter the life of the battery will be. Most solar storage batteries will undergo one shallow cycle per day. At around 20% depth, the battery will last anywhere between 200-6000 cycles (and up to 50,000 for nicads). As a rough indication, a well maintained quality battery bank should last 10 to 15 years.
Over discharge will severely affect the battery life. Most batteries specify a maximum depth of discharge, but as a general rule, they should never be drained beyond 80% of their total capacity.

Battery Maintenance
When you first install your new power system it is always best to keep an eye on your batteries frequently until you get a feel for how they work. Especially keep an eye on how much energy is discharged and how frequently. This will help you when maintaining your battery bank.
When left standing by itself, a battery will slowly discharge at a rate of around 1-3% per month. Batteries should never be stored when they are almost flat, and should be recharged once every 1-2 months.
When left for long periods at low state of discharge, batteries undergo sulphation which could result in a permanent loss of capacity and difficulty in recharging.
Flooded cell type batteries are often subject to water loss due to gassing. These batteries need distilled water added now and then.
Batteries should always be kept at around 20-25C. At lower temperatures the capacity of the battery decreases, and at higher temperatures self discharge and corrosion increase.
Batteries should also be kept off the ground, especially from concrete floors since the temperature of the ground is often lower than that in the battery. When the battery is cooler at the base than at the top, thermal stresses develop that could crack the battery casings, and the very real possibility of the internal plates buckling, and shorting out.