BMS Definitions & Glossary

The BMS Definitions & Glossary is an A to Z extension to our website that just gives you an alternative way of finding information.

Active Balancing – the idea here is to redistribute the energy across the cells. Give energy from the cells with the highest SoC to the cells with the lowest SoC. This is the ideal cell balancing approach.

Active Mode – the BMS is on, communicating and monitoring all sensors.

Ah – Ampere-hour is the unit of cell capacity.

Balancing – all about the dissipation or movement of energy between cells. The aim being to align them all with respect to state of charge. Aligning the state of charge of all of the cells in a pack will allow the pack to deliver the most energy and power. This becomes more crucial as the pack ages and differences between cells become more significant.

Capacity – cell nominal capacity is defined as the quantity of charge, in ampere hours (Ah) that a cell is rated to hold.

Cell Matching – what level of cell matching do you do prior to assembling a battery pack? Assuming the battery pack will be balanced the first time it is charged and in use. 

Cell Voltage Delta – difference in resting voltage for cells at different points in a series string. A threshold maximum voltage difference will trigger cell balancing.

Centralised BMS – long leads are required to connect the central control unit to every cell in the pack.

C-rate – a measure of the rate at which a battery is charged or discharged relative to its capacity. It is the charge or discharge current in Amps divided by the cell capacity in Ampere-hours. An nC-rate is the constant-current charge or discharge rate that a cell can sustain for 1/n hours. This current (in A) equates to the cell nominal capacity (i.e. C Ah) multiplied by n h−1 i.e. i=nC. For example, a fully charged 20Ah cell should be able to deliver 10A for 1/n = C/i = 20/10 = 2hrs, or the c-rate is n=i/C = 0.5h-1 

Coulomb Counting – SOC Estimation by Coulomb Counting is based on the measurement of the current and integration of that current over time.

Current Derate – a reduced current charge or discharge capability.

DCIR – Direct Current Internal Resistance is the internal resistance of the cell.  This is the resistance in charge and discharge to a direct current demand applied across the terminals.

De-Rate – when the charge or discharge current (power) limits communicated to the vehicle are reduced.

DoD – Depth of Discharge is equal to 1 – SoC

ECM – Equivalent Circuit Model is an electrical circuit designed to represent the dynamic behaviour of a battery cell using electronic components (capacitors, resistors and inductors).

Insulation – use of a poor conducting material to restrict flow of current to almost zero.

Insulation Resistance – value of resistance between any two points.

Isolation – disconnection and separation of electrical equipment from every source in such a way that the disconnection and separation is secure.

Isolation Resistance – the value of resistance, measured at a specified voltage, between a HV bus and ground.

Kalman Filter – an estimator of information of a system from noisy (or uncertain) directly or indirectly related measurements.

Lossless Balancing – this approach switches cells in and out of the circuit during charging. This means we have a lot of switches and that these switches have to be designed to carry the full current.

Master and Slave BMS – a slave will monitor and control a sub-set/module of cells and communicate back to the master.

typical OCV vs SoC
Open Circuit Voltage (OCV) – is the potential difference between the positive and negative terminals when no current flows and the cell is at rest.

Passive Balancing – simple form of balancing that switches a resistor across the cells. In the example shown with the 3 cells the balancing resistor would be switched on for the centre cell. Discharging this cell and losing the energy to heat in the balance resistor (typically 30Ω to 40Ω).

Precharge – when closing battery contactors onto a capacitor load there would be a very high current that could cause damage to the contactors or cells, or result in the fuse blowing. Thus a precharge resistor and contactor allows that maximum current to be controlled.

Remaining Useful Life (RUL) – a key function declared by the battery management system. A prediction of how many cycles the pack has before hitting the minimum requirements for operation.

Runtime Balancing – each cell is connected to an individual low DC-DC power converter, then each converter is connected in series. This then allows the power delivered and received by each cell to be completely controlled based on their capability.

Sleep Mode – the battery pack is isolated, there is no balancing, the BMS will be in a low power mode where it occasionally looks at sensor inputs and listens out for a communications request to wake.

SoC definition for Samsung SDI 94Ah cell
State of Charge (SoC) – abbreviated as SoC and defined as the amount of charge in the cell as a percentage compared to the nominal capacity of the cell in Ah.

State of Health (SOH) – this is the total available charged capacity of the cell as a percentage compared to the nominal capacity in Ah when the cell was new.

Temperature – a critical parameter that you need to know before charging or discharging a cell. A cell is a 3 dimensional structure that is also inhomogeneous and hence you will observe temperature gradients within the cell. The temperature limits, gradients and heat rejection rate will define the overall power capability of the battery.
Temperature Gradient – the maximum temperature differential in a cell is normally specified as ~2°C to minimise the degradation in capacity of the cell. This requirement will drive the cell selection versus application along with the cooling system design.
  • High temperature and the SEI layer on the anode grows faster. If the SEI layer grows fast it tends to be more porous and unstable.
  • At low temperatures we see slower diffusion and intercalation with the possibility of lithium plating. Lithium plating removes lithium from the active cell, reducing cell capacity. Also, lithium plating can subsequently into lithium dendrites that can cause electrical shorts.
cell temperature limits
Temperature Limits – these temperatures will change with chemistry and by cell manufacturer, therefore, it is really important to use the limits as advised by the manufacturer. In addition you will need to test the cell to gain the detailed understanding of how the cell behaves in your application versus temperature.

The limits will also be blurred by the design of the battery and control system. One example is the maximum operating temperature for the cell.

Usable SoC Window – If we want a battery cell to last a lot of cycles, extend the life in a power application or to ensure the available power is consistent then we need to set a usable SoC window that is smaller than 100%.

Increasing the Usable SOC Window – If you are in a battery design role there is always pressure on increasing usable SoC window. Nobody wants to pay for and carry around unused battery capacity. However, there are some really good reasons to restrict that window.

usable SoC window

Wireless BMS – one option for reducing the wiring complexity and weight in a battery pack is to use a wireless connection. Here the modules or cells report voltage and temperature signals back to the central control system in the battery wirelessly.