The battery bank in your solar system is by far one of the most important elements. It is critical that a number of guidelines are followed in order to ensure that your battery bank provides you with adequate electricity and long life at a reasonable cost.
Batteries used to store energy for your power system need to be robust both in terms of size and capacity. The greater the capacity, the better you can expect your system to function.
There are commonly two types of batteries in use for solar systems – flooded lead acid and gel cells.
Everyone is familiar with a flooded lead acid battery, it is the type used in your vehicle. It is reliable, has long life when cared for, and will produce a huge amount of energy over its’ lifetime. Some maintenance is necessary, usually in the form of adding distilled water periodically. A flooded lead acid battery will use more water when it is cycled faster. Low power draws from your battery will conversely show a lesser consumption of distilled water.
There are several manufacturers who provide extremely high quality batteries which are designed specifically for solar systems. These batteries are often very heavy, which is a result of having much thicker lead plates than a standard type battery. They are made to handle the constant cycling of power in and out. This is in contrast to your vehicle battery, which is designed to provide a big boost of power when you start your engine. Thereafter, the alternator in your vehicle both charges the battery as well as running the accessories.
Should you use a 12 volt system, it will be perfectly safe to charge your battery bank to nearly 15 volts without damage. A 24 volt system can go to nearly 30 volts. During the summer months, it is common to see the solar panels produce sufficient energy to get a battery bank very close to the 15 or 30 volt range, depending on your battery bank configuration.
A bank of gel cells is not so tolerant of such high voltages, and you must ensure that your controller keeps the voltage at a lower range than the lead acid batteries can handle. It is a good idea to maintain a limit of about 14.2 volts with gel cells. An occasional small overcharge may not hurt, but better to be safe than sorry. Gel cells are more costly than most lead acid batteries and any serious overcharge will shorten their life span quickly.
When you set up your system, consideration should be made on whether you will use 12, 24 or 48 volts for the battery bank. You will not get any extra power by using one number over the other, but you may save some money if you opt for the 24 or 48 volt systems compared to 12 volts.
DC power does not travel well over long distances at low voltages. The higher the voltage, the better it will travel.
Let us use the example of your car battery. Notice how heavy the battery cables are? It is that way so they will handle the large amp draw at low voltage when you try to start your engine. Fortunately, the battery is usually located very close to your starter motor which ensures all the battery power arrives safely.
With your solar system, there are two distinct areas of travel. Firstly, the power needs to travel from the solar panels to the battery bank. Secondly, it needs to travel from the batteries to the inverter.
The power from the panels, using a small 500 watt system for example, will be far less than the power going to the inverter. It is possible that a 3000 watt inverter, which can temporarily draw 6 or 7000 watts (this is known as the surge) when something first starts – tablesaw maybe – will need to handle 15 times the amps that the incoming panels will provide.
Any given wire size can handle only so much power. Should you opt for a higher voltage system, 24 or 48 volts, you will require a smaller wire size to handle a similar amount of power. Since smaller wire is less expensive than thicker wire, a simple configuration change from 12 to 24 volts is going to save you some cash!