Battery management system (BMS):

Battery cells in a cell stack age at different rates depending on different influences. Cell aging results in capacity differences, deviations in internal resistance and different leakage currents. That affects the charge losses as well as the capacity and thus the cell voltages of each individual cell. Charge states outside the specifications reduce the cell life further, which can soon cause avalanche effects if they are not rectified. Cell monitoring and balancing systems can detect these differences in stacks and correct them continuously via cell balancing. This procedure greatly increases the service life of the individual cells, but does consume some energy, increasing self-discharge in the stack.

Three main methods are used:


Method comparison
Passive Active
(unidirectional)
Aktive
(bidirectional)
method excess
energy means
resistors

distribution of energy
from cell to cell (one direction)

targeted distribution
the energy in
individual cells
(both directions)
losses high middle low
efficiency 0 % 70-91% > 92 %
costs low high high
max.  balancing- 
currents
(without cooling)
to 2 A to 5 A to 5 A
balancing time high middle low
heat generation high low low
Cell protection and security good good
very good

  Active bidirectional balancing


Passive Balancing:

By means of switchable or fixed bypass resistors, cells with an excessively high voltage are discharged. The resulting electrical energy from the different states of charge is converted into heat and is lost.



Active unidirectional balancing (Power Pump or "charge pump" method):

The excess charge of a cell is "pumped" into the next with two transistor switches (MOSFET) and a throttle. This process is also referred to as uni-directional active balancing, since it only allows controlled discharge of a cycle. The charge distribution of the cells is, however, only possible in one direction.



Active bidirectional balancing (TESVOLT):

In contrast to the unidirectional balancing each cell can be loaded from the other cells of a stack, or can be discharged in all other cells. This allows a fast and efficient balancing of the stack. This method is realized via bidirectional flyback converters, which are attached to each cell.

The bidirectional balancing system was developed by TESVOLT. It is based on the Power-pump method, which has been extended by adding an electrically isolated flyback converter to each cell. The BMS monitors the temperature, voltage and charge state of each cell and controls them in a cell assembly (stack). From the individually measured parameters the BMS determines the state of health (SOH), and the State of Charge (SoC) of each cell to allow the early detection of errors and to prevent damage. In addition it allows the charge of each cell to be defined, as well as seperates the charge of the affected cell from the other cells. The BMS is modular and flexible in design. Through a master/slave principle , the use of small stationary lithium storage (10 kWh) for MWh storage plants is ensured.