A battery energy storage system (BESS) is a set of batteries in container-sized boxes that charges when electricity is cheap and discharges when it is scarce. Software decides when to charge and discharge, and offers the stored power for sale in electricity markets.
It works in three layers. The batteries are built from cells grouped into modules and packs, as described in cells, modules and packs. Each pack has a battery management system (BMS) that watches the temperature and voltage of every cell. On top is the control software, which follows forecasts of prices and demand. Power electronics convert between the battery's DC and the grid's AC; see AC and DC.
Two numbers describe a battery. Its power, in megawatts (MW), says how fast it can give out electricity. Its energy, in megawatt-hours (MWh), says how much it holds in all. A battery with 100 MW and 400 MWh can give 100 MW for four hours.
Consider a 100 MW battery with 400 MWh at a solar park in Pavagada, Karnataka. At noon, solar power is plentiful and cheap, so the battery charges. At 7 pm, demand peaks and solar output is zero, so the battery discharges at 100 MW for four hours. The company earns the difference between the low price and the high price. Software for energy trading and risk management (ETRM) places the bids.
Who pays for this? In India, state and central power companies invite bids for storage. The winner builds the battery, and the buyer pays a fixed amount for each MW available each month. The buyer decides when the battery charges and discharges. A battery that stays idle still earns that amount, as long as it is ready to run when asked.
The grid operator's energy management system can also order the battery to add or absorb power within a fraction of a second. That helps grid balancing. Many small batteries in homes can be joined into a virtual power plant and behave like one large battery. For storage over days rather than hours, an electrolyser that makes hydrogen is an alternative. Variants of the battery include sodium-ion batteries and mobile battery storage.