BMS Development-Battery Key Parameters and Definitions

This article briefly introduces the commonly used parameters of battery systems and their meanings. I hope it will be helpful for fans to understand and learn battery systems. Most of the definitions of battery parameters in the article come from BMS development course notes, but for some controversial parameters, the terms and definitions in the national standard are used.

Some of the parameters and their uses in this article are covered in the article “What cell data are needed in BMS development”. If you need to know more, you can check it out by yourself.

Terms and Definitions

Battery Rate

It usually refers to the current value required when the battery discharges its rated capacity within a specified period of time. The value is equal to a multiple of the battery’s rated capacity, usually represented by the letter C. The rate is usually used to characterize the battery power capability. Generally, we call a lithium battery discharged at 1C a standard battery a lithium battery, and a lithium battery discharged over 10C a high rate battery.

Internal Resistance

Usually refers to the resistance of the current flowing through the interior of the battery when charging and discharging, which is divided into ohmic internal resistance and polarization internal resistance. Regarding the sources of internal resistance, ohmic internal resistance and polarization internal resistance, I will talk about it in the notes on battery structure.

Internal resistance is an important parameter to measure battery performance. In BMS development, it is often used to estimate the power characteristics and thermal characteristics of the battery.

Rated Capacity

Refers to the capacity released when the battery is fully charged and discharged to the specified end voltage at 1C at room temperature.

Actual Capacity

It refers to the actual amount of electricity released under certain conditions (temperature, current) after the battery is fully charged. For energy management engineers, actual capacity is more valuable than rated capacity.

The actual capacity of the battery is mainly affected by the discharge rate, temperature and aging, so when talking about the actual capacity of the battery, the discharge conditions must be specified.

Open Circuit Voltage

That is, OCV refers to the potential difference between the positive and negative electrodes of the battery when the battery is not working.

In battery management systems, it generally refers to the potential difference between the positive and negative electrodes when there is no current and the battery voltage reaches a sufficiently balanced state. From a strict perspective, it is generally called the steady-state open circuit voltage.

Terminal Voltage

The battery terminal voltage refers to the potential difference between the positive and negative electrodes of the battery under working conditions or unbalanced conditions.

Cut-off Voltage

Cut-off Voltage means that when the battery is discharging (or charging), the voltage drops (or rises) to the lowest (or highest) working voltage value when the battery is no longer suitable for discharging (charging). Simply put, it is when the battery is fully charged and discharged voltage. The cut-off voltage is affected by temperature and charging and discharging time.

Self Discharge

Also known as charge retention capacity, it refers to the ability of the battery to maintain the power stored in the battery under certain conditions (such as a specific temperature, a specific SOC) when the battery is in an open circuit state.

Cycle Life

Cycle Life refers to the number of charges and discharges that the battery has experienced when the capacity (or internal resistance) drops (rises) to a certain specified value under a certain charge and discharge system. It is generally expressed in cycles.

Calendar Life

Calendar Life refers to the time it takes for the battery to stand still under certain conditions and the capacity (or internal resistance) drops (rises) to a certain specified value, usually expressed in months or years.

CC-CV Charging

CC stands for constant current, CV stands for constant voltage, and CC-CV stands for the method of charging the battery with constant current to a certain voltage and then maintaining constant voltage charging.

State of Charge

State of Charge (SOC) is that under certain conditions, the current stoed power of the battery accounts for the percentage of the current available capacity, expressed in %. SOC is one of the most important parameters to characterize the battery state.

Depth of Discharge

Depth of Discharge (DOD) refers to the percentage of power released by the battery to the current available capacity under specific working conditions, generally expressed as %, DOD=100%-SOC.

Health Status

State of Health (SOH) is an indicator that characterizes the current performance of a batty. It indicates the level that the current performance of the battery can achieve compared with a new battery. It refers to the degree of deviation between the current performance of the battery and the normal design indicators. It is generally characterized by changes in capacity or internal resistance.

Energy Density

It refers to the energy stored per unit volume of unit mass of the battery. The former is also called volume density, in Wh/L, and the latter is also called mass density in Wh/kg.

There is a big difference between the energy density of the monomer and that of the PACK, and this difference is also defined as the group efficiency.

Power Density

It refers to the power that the battery can release or absorb per unit volume or unit mass. The unit of the former is W/L, and the unit of the latter is W/kg.

Monolithic Consistency

Indicators that characterize the differences of individual battery cells within a battery pack generally use design evaluation indicators such as differences in battery terminal voltages, differences in battery internal resistance, differences in self-discharge rates, and differences in capacity.