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BESS (battery energy storage system) is a system that use connected batteries to store energy and provides power delivery to consumers’ power grid in case of emergency blackout (as UPS) or when the power grid is supplied by temporarily available sources like a solar, wind or tidal power stations.

Classification
BESS makes possible to save on the difference in electricity prices at high and low loads on the power grid , load the grid at low load with charging and reduce peak loads on the power supply source. It is possible to design BESS for use as power source for a long time when power station is not supply the power grid.

BESS can supply a wide range of consumers from discrete devices to city quarters, companies and even cities.

BESS can use different battery types: The cell type determines voltage and electrical current of a battery, Lead-acid and Lithium cells can give high electrical current, but first are heavy and have relative low capacity, the second are expensive and dangerous in case of improper use; Nickel batteries have average capacity, but have memory effect or can not give high electrical current.
 * Lead-acid
 * NiCd and NiMh
 * Li-ion and LiFePO4

BESS does not restrict the voltage type of the outer power grid, the power inverting system can be designed to supply different types of consumers’ devices. There are different standards on the power grid voltage:
 * 220-240V AC, 50 / 60 Hz
 * 100-127V AC, 50 / 60 Hz

BESS can be used as proxy like an UPS: the system connects to power grid by input and to consumers’ systems by output; consumers’ devices are not connected to the power grid directly; but frequently used as a support : the system connects to the power grid by input and output and monitors power grid voltage, in case of voltage decrease the power management system activates the inverter to power up the power grid to aid or to displace power plants.

Structure
BESS consists of a energy management system (EMS), battery system, that contains batteries and battery management system (BMS), and a power conversion system.

EMS
The energy management system is needed to monitor the operation of the external power grid and, if necessary, connect the battery system for charging or supplying power. In case of stable operation of the external network, low load or low electricity tariff, the system takes energy to charge the batteries. When the load or tariff increases, charging stops. In the event of a voltage drop or loss of external power in the grid, the voltage generation is started to reduce peak loads or to provide consumers with power. One EMS can operate with a large number of individual inverters and battery packs and can operate different power systems as needed.

BMS
Battery management system is needed for the safe and long-term use of battery packs, designed to balance cells connected in series, which is expressed in identifying cells with reduced capacity and recharging the remaining cells. If necessary, BMS can be created with a variable block switching system, that prolongs battery life time. When using lead-acid or nickel batteries, a specific charging system is not required due to the low probability of a fire hazard. In the case of using lithium batteries, BMS are obliged to determine safe modes of charging and discharging cells to reduce the probability of fire hazard to almost zero.

PCS
Power control system uses power inverters and electric rectifiers to charge and discharge connected batteries. It is not enough just to connect battery voltage to the consumers power grid, because the voltage standard for transmission without significant losses over a long distance prescribes transmission at high voltages with alternating current. To convert the level and type of voltage from one to another, a special device is used that generates in some way an alternating voltage with the desired frequency and amplitude. There can be several types of generation, depending on them, an alternating voltage is obtained that is suitable for use by different consumers: if you make a generator based on active elements (transistors or lamps) in an active (amplifying) mode, you can get a real alternating voltage with a timbre pure sine wave and a minimum noise, this type is suitable for any consumers, but the power grid itself receives significant interference from the air, so such a signal will not reach consumers, and the conversion efficiency is very low, the effective efficiency will not exceed 0.5. You can make an electromechanical generator with a DC motor rotating at the desired frequency and an asynchronous generator connected by a common shaft, in this case the received signal will also be very close to sinusoidal, but this takes up a lot of space, has friction parts that require periodic maintenance, and conversion efficiency will also not be very high. In addition, alternating voltage can be of different types: a real sinusoidal shaped signal obtained on a generator in active mode, a sinusoidal signal obtained after filtering a PWM signal and a modified sinusoid -- a signal with a sampling frequency several times higher than the frequency of the power signal, and supply DC voltage switchable on several levels. To some extent, even a rectangle with a duty cycle of 2 is a modified sinusoid, but there is too much energy in the upper modes and equipment damage is possible. Due to their simplicity, the most widespread are generators of a modified sinusoid, the voltage given to it is suitable for long-term power supply of electronics through switching power supplies, but causes interference in analog equipment.

Battery blocks
The rechargeable battery has some characteristics: operating mode voltage range, maximum recommended current, charge-discharge curve (voltage dependence on battery charge). Characteristics such as the charging curve and voltage range are determined by the chemical type of the cell, while the maximum current depends on the cell capacity – on the physical size. The maximum current is determined by the mode of operation in which the cell does not significantly heat up, since heating reduces the overall battery life. To provide power, you need to use a certain number of batteries. It is possible to use separate battery management systems for each battery cell, but this is wasteful. The cells can be connected to each other in series and in parallel. With a serial connection, the battery voltage rises, with a parallel connection, the capacity and the maximum possible current. When using cells with different capacities, the capacity of the series battery is determined by the smallest cell from the battery, which reduces the overall energy consumption of the battery, with parallel connection this problem does not exist, but the cross-section of the battery wire increases and the transformation of voltage into mains voltage becomes more difficult. It makes sense to use battery blocks that contain series connection of equalized capacity parallel battery sets.