Energy Storage Glossary

If you have been in the solar business for more than 15 years, you are likely somewhat familiar with the lexicon of batteries. Before SMA America introduced its high-voltage string inverter to North America in the early 2000s, most PV systems in the US and Canada included an energy storage component, even in grid-interactive applications. Just 5 or 10 years ago, interactive PV systems with energy storage were the exception rather than the rule, but the pendulum is showing signs of swinging back in the other direction. This glossary defines some foundational terms, many of which I use in this article, that point out some important differences between solar and storage ratings and describe some battery failure modes.

Anode: The negatively charged terminal of a battery, under discharge conditions, where chemical energy is stored. Chemicals at this terminal have a surplus of electrons available for donation.

Anolyte: The negatively charged liquid electrolyte on the anode side of a flow battery or an electrochemical cell that is divided into two compartments.

Capacity: The nominal energy rating of an energy storage system as measured in kilowatt-hours. In the context of solar, capacity refers to the power (kilowatt) rating of a PV system under standard test conditions; in the context of batteries, it describes the amount of energy a system can deliver or absorb over the course of an hour, which will vary based on charge rate. Note that a battery’s nameplate capacity rating does not equal its usable capacity, which is a function of other characteristics, such as allowable depth of discharge and system efficiencies, including inverter, cabling and transformer losses.

Cathode: The positively charged terminal of a battery under discharge conditions that establishes an electromotive force. Chemicals at this terminal have a deficit of electrons.

Catholyte: The positively charged liquid electrolyte on the cathode side of a flow battery or an electrochemical cell that is divided into two compartments.

Charge rate (or C-rate): 1. The rate at which a battery can charge; 2. The rate at which a battery discharges relative to its maximum capacity; 3. The maximum safe continuous discharge rate for a battery.

Cycle: One charge and discharge sequence. Note that all cycles are not created equally but vary in intensity based on C-rate and depth of discharge.

Cycle life: The rated number of charge-discharge cycles a battery supports based on a specific depth of discharge.

Dendrite formation: Needlelike metal accumulations on the anode of a battery that, if uncontrolled, can cause a hazardous short-circuit condition.

Depth of discharge: The energy discharged from a battery, expressed as a percentage, relative to the total amount of stored energy. While some batteries support 100% depth of discharge, many do not.

Discharge duration: The length of time that a battery can discharge at its nominal power rating.

Energy density: The amount of energy that a battery stores in relation to its volume. (See also specific energy )

Electrolyte: A chemical medium that allows the flow of electrons between the cathode and anode of a battery.

Oxidation: Chemical reactions that result in an electrode’s release