Batteries for Storing your Solar Power
As your electical power usually needs to be available when the sun is not shining, it is usually neccessary to store electricity.
The normal storage is the Lead-Acid battery.
This is a good point for some warnings:
1. Lead Acid Batteries can contain a large amount of electrical energy which they are capable of discharging very quickly if any form of conductor is placed accros their terminals.
2. Lead acid batteris contain Sulpuric Acid which is corrosive.
3. Lead Acid batteries give off hydrogen when they are being charged, which when mixed with air is explosive, and can be ignited by a small spark.
For the above reasons, I would not do any work on the batteries (ie changing connections) unless I was wearing goggles to protect the eyes. I would also ensure that the batteries have not been charged during the last 2-3 hours to reduce the risk of hydrogen being present.
A Lead Acid cell has a nominal voltage of around 2 volts (depending on it’s state fo charge). A battery will consist of severel cells connected together in series to produce the required valtage, usually either 12, 24, or 48 volts.
For a small system, a 12 volt car or truck battery (consisting of 6 cells as one unit) could be used though they are not designed for the job.
The photograph to the right shows a battery of 12 individual cells for a 24 volt system. These cells will have been designed for deep cycle use.
What is a deep cycle cell? Deep cycle refers to the fact that in a solar power system, it is likely that the battery will become charged during a sunny day, then they may become almost fully discharged with use, before they are again fully charged.
On the other hand, a car or truck battery is not designed for this type of use. Although the starter motor of the vehicle will draw a considderable amount of current from the battery, it will nowhere near fully discharge it (unless the vehicle fails to start and the driver continues using the starter motor for some time). The car or truck battery is therefor designed to be used virtually fully charged all of the time.
Although a car or truck battery may be used for a small solar power system, it may only have a life of 2 to 3 years if used continously, compared to 10 to 15 years for a deep cycle battery.
However in a small solar power system where power is not required every day and the batteries are therefor fully charged most of the time (in a weekend lodge for instance) a car or truck battery will have a more reasonable life expectancy.
The Lead Acid Battery
This is the type of battery you are most likely to be using in a static situation. It is the most cost effective,and is capable of producing high currents.
These have been around for a long time, having been invented in the mid 1800’s with the basic design principally unchanged.
The battery consists of individual cells, each producing approximately 2 volts. Each cell consists essentially of two electrodes of lead, in a 33% solution of sulphuric acid. As the battery is charged however, chemical changes occur in both the electrodes and the electrolyte (the sulphuric acid).
The lead acid battery described above is known as a wet cell lead acid battery due to the electrolyte being liquid.
Deep Cycle Batteries
The type of battery fitted to a car or truck is designed to give a high current for starting the vehicle but this would normally only discharge the battery by a maximum of 10%.
For a solar powered home, however, batteries designed for Deep Cycle use are required. These batteries, while not being able to supply the high current of a Starting Battery, will cope with regular discharging by 40% and occasional discaharging by 80%.
Batteries described as Leisure Batteries are a halfway house between a Starting Battery and a Deep Cycle Battery, and may be suitable for a week-end home where the batteries are normally maintained in a fully charged state.
These are standard flat plate vented batteries as described above, otherwise described as a VLA or Flooded Lead Acid batteries.
These are again of flat plate design but are semi-sealed or VRLA (Valve Regulated Lead Acid). Valve Regulated batteries have the following advantages which may or maynot be important when used in a solar power setup:
- Release of hydrogen during charging is significantly reduced, reducing (though not eliminating) the need for ventilation.
- No topping up of the cells with distilled or demineralised water is required
- There is no chance of acid spillage
- Batteries can be designed to be place horizontally or to be stacked, reducing floor space requirement.
There can however be disadvantages with the VRLA design, including:
- No ability to top up the battery if the electrolite should be low
- May not cope with higher temperatures as drying out may occur.
These batteries are a type VLA vented battery using tubular positive plates
OPzV batteries are a type of VRLA with tubular positive plates
Two other types are also available, both of which may be termed a sealed lead acid battery though they do have a pressure relief valve:
- Absorbed Glass Mat (AGM) – where the electrolyte is absorbed in a fine fibreglass mat between the electrodes
- Gel – where the electrolyte is in the form of a gel.
Both the above types of battery can withstand being turned over without spilling electrolyte and therefore have specific uses.
Due to these batteries being at least semi sealed, care has to be taken to ensure that no excessive gassing occurs (which occurs at higher charging voltages). Therefore these batteries may require a specific charge controller. Charging with the wrong type of controller can cause an explosion.