Batteries simply serve the role of storing electricity for later use, they do not generate electricity. As we know, solar PV modules only produce electricity when the sun is shining, which tends to be lacking at night. Batteries bridge that gap, by storing excess energy that is generated on sunny days, for use at night, and on stormy days. in systems that do not have an alternate power source, such as the Utility grid, batteries are essential.
The most common type of battery is a lead acid cell, much like your car battery. It consists of multiple metal plates, dipped in an electrolyte, and mounted within a special container. You can read more about how batteries work here, or even more-so by coming to class, but this article is going to focus on the different properties of batteries as they apply to PV product selection.
Both Renewable Energy batteries, and your standard car battery, are both lead-acid batteries; however, renewable energy batteries have much thicker plates, and for that reason they are heavier, larger, and last longer. The most relevant quantities, when selecting a battery for your PV system are: Cycle Life, Energy Storage Capacity, and always Price.
Cycle Life describes the number of times which the battery can be discharged and recharged before it will no longer hold a charge. Cycle life is generally greater for batteries which have thicker plates, as they tend to reform more completely. The most dramatic determinate of cycle life is depth of discharge; the more deeply a battery is discharged, the less likely it is to reform completely. For example, a battery that is regularly discharged no more than 20% (80% full) may have a lifetime of 4000 cycles; whereas a battery that is regularly discharged 80% (20% full) may only have a lifetime of 1000 cycles. In a solar PV system, a battery is typically discharged each night, and then recharged the next day, so it’s safe to regard a cycle as the same thing as 1 day. Following this logic, we can determine that our example battery would last 4000 days (nearly 11 years) if we only use 20% of its capacity each day, however it can only be expected to last 1000 cycles (less than 3 years) if we use 80% of its capacity each day!
Energy Storage Capacity is always an important characteristic for a energy storage device! Batteries are typically rated in ampere hour capacity, which is a product of the number of amperes the battery is supplying, and the duration for which it is supplying that level of current. For example, a battery with a 100AH capacity, is capable of supplying 1 Ampere for the duration of 100 hours, before becoming fully discharged. It is important to note that a batteries storage capacity varies depending on the rate of discharge. As an example, please see the chart below:
For the example shown above, if the load draws electricity at the rate of 27.3 Amperes, this battery has 546 AH of storage. If a larger load, say 102 Amperes, was drawing from the same battery, it would only be rated for 306 AH of storage. As a note, a system designed for 3 days of absolute autonomy will often operate at the 72 hour rate; for this battery it would correspond to a capacity of 726 AH.
However, the quantity we are most interested in is energy, which is counted with the unit Watt-hours (we pay our electricity bills in terms of kWh, which is equal to 1000Wh). This factor is often listed on battery specification sheets, however it can be easily be calculated by multiplying the AH rating of the battery by its nominal voltage (Watts = V x A). The battery in the example given above is a 6V battery, which means that it has 3276 Wh of storage at the 20 hour rate (6V x 546 AH = 3276 Wh).
Another important, yet less technical, factor to always consider when purchasing a battery is the manufacturers warranty, and the integrity of the company backing that warranty (A bankrupt company cannot refund you for a defective product).