Batteries in Solar Systems
Some fairly useless information ...
A century or two ago archeologists working in Egypt discovered curious clay jars sealed with tar and containing a carbon rod. They were baffled as to what these ancient relics could be. Modern science now tells us these items were possibly batteries. It is highly probable that they were used in electroplating, a metallurgical pursuit that goes back to ancient times. This of course throws into doubt who were actually the pioneers of the electrical age! Without pondering this further let's progress onto what we are really about, and that is storing the power we want to use in our motor home, house or whatever.
In order for your solar system to be useful you need storage ...
A battery is just a storage device for electricity. The most common storage battery is the lead acid variety, it is robust, relatively cheap and readily available. Storage is by chemical means. Removing electricity changes the internal chemistry, charging the battery restores it. The capacity of a battery, the charge and discharge rates and the life are all related.
A lead acid cell, regardless of size is a two volt device. Regardless of how large you build a lead acid battery the nominal voltage will always be 2 volts. If you put 6 of these batteries in one box and join them in series you will get a 12 volt device as is used to start your car.
Battery capacity is measured in amp-hours (A/H or amp/hour or AH). This measurement is the amount of amps (energy) the battery will provide for one hour. Battery capacity is also relative to speed of discharge. A slow discharge over 50 hours will produce a higher total energy from the same battery than a rapid discharge. Temperature also affects capacity. A battery bank at 30 degrees Celsius will have a significantly larger capacity than if it were cooled to 0 degrees Celsius.
Battery capacity is stated as discharge over time. The time depends on what the battery manufacturer had in mind when designing the battery. Forklift and electric vehicle batteries usually have a stated capacity over 5 - 10 hours @ 30 degrees Celsius, batteries specifically for solar installations have the capacity stated at 100 or 120 hours @ 25 degrees Celsius.
A quality battery will be provided with a specification sheet detailing the performance over a variety of change/discharge rates and will also indicate a cycle life.
Summary so far:
Battery capacity is measured in amp/hours and provided to us by battery suppliers as capacity over time.
If you get a brand new battery and store it with some means of maintaining its full of charge status (like a trickle charger) and put it on a shelf in the shed its life will be determined by the length of time it takes for the acid to degrade the bits that are immersed in acid (like all the internal bits). Typically this could be 10 - 20 years.
The next thing that determines battery life is cycling (discharging then recharging) it. A discharge followed by a recharge is a cycle. The depth you discharge a battery to before recharging it is the depth of cycle. A small cycle could be a battery discharged a little, say 10%. A deeper cycle could be more like say 30 - 50% and a really heavy cycle could be 80%. If you discharge a deep cycle battery by more than 80% on a regular basis you will quickly ruin it.
Battery manufacturers will state the cycle life for batteries designed to store power. Batteries designed to start things are not storage batteries. They are "cranking" batteries and there capacity is not stated, rather there usefulness as a starting battery is stated in "cold cranking amps". We are not interested cranking batteries here but you should know the difference.
Typical spec for a deep cycle battery for solar system use:
4000 cycles to 10%
From the above information we can determine a life expectancy: A solar system discharged on average 10% per day could be expected to have a battery life of around 4000 days or 10.9 years.
The reality is not quite as good as this. Deep cycle batteries really designed for solar systems are not designed for 80% discharge even though this spec is provided. Most deep cycle batteries are designed to be cycled to 30% or less.
Deep cycle battery life is about 7 years. Most will last longer than this, some will fail in a lesser amount of time. The battery is one of the ongoing parts of a renewable energy system that will cost. The battery is essentially your power bill and if you divide the battery cost by 7 you will have the approximate annual cost of a renewable energy storage system. After 7 years your battery bank is living on borrowed time.
Choosing a battery bank voltage:
No matter how much I stare at my thumb there seem to be no rules written on it! Here is a guide though:
Up to 1000 watts: 12 volt battery bank