Technical parameters

The chemical composition of electrodes defines the voltage of a single cell. All types of battery cells have a certain nominal voltage Unom. As previously noted, the nominal voltage of different chemistries is in the range of 1.2 V to 3.9 V. The nominal voltage is somewhere between maximal voltage Umax (charging voltage) and minimal voltage Umin (discharge cut-off voltage, end-of-discharge). The nominal voltage is used for calculations to determine the voltage of the battery pack if cells are series-connected. Discharge cut-off voltage is the voltage beyond which discharge should be terminated to prevent damage to the cell. A battery discharge voltage curve is given in the figure below. For primary batteries, it is desirable to have a flat curve which translates to the stable supply voltage.

Figure 23. Discharge voltage curve of a single Li-ion cell: voltage decreases as the DoD increases Capacity and energy

The second most important quantitative battery parameter is capacity Qbat. Capacity determines how much charge a battery can store. It is measured in amp hours (Ah). Higher Ah rating means the battery will be able to run longer before requiring a recharge. If the load current Iload is known then the runtime t can be calculated as follows:

Figure 24. Runtime equation Current and C-rate

The next electrical parameter is current. A good battery datasheet will provide at least a few current values at different conditions. Common parameters are standard charge current, rapid charge current, max. continuous discharge current and standard discharge current. Often the charging current ratings are significantly lower than discharge ratings. In engineering and battery datasheets there is another battery-specific parameter which is directly related to Ah rating: the rate. The value of 1 C is a number same as the nominal capacity of the battery. The C-rate itself has no unit of measurement but when it is converted to current it is expressed in amps A. C-rate is used to determine current for both charge and discharge. It comes handy when comparing current capabilities of different batteries and simply estimating how large the current is with respect to the capacity of the battery. For example, 2 C discharge rate of a 10Ah battery is 20 A while 0.5 C charge rate of the same battery is 5 A.

Cycle life and ageing

Battery lifetime is a critical parameter of secondary batteries. Depending on the chemistry battery lifetime is affected by ageing mechanisms: cyclic ageing and calendar ageing. As the name suggests calendar ageing is related to the absolute age of the battery: as battery ages, its performance will deteriorate – capacity will decrease and internal impedance will increase leading to decreased current capability. The other ageing mechanism – cyclic ageing, is related to the intensity of battery usage. A full battery cycle is a full charge followed by a full discharge.

Battery manufacturers in battery datasheets give an estimated cycle life – typically few to several hundreds of cycles. For this cycle number to be true it is of importance to follow a specific charge and discharge test pattern: the manufacturer will specify exact charging and discharging current, exact charging and discharging cut-off criteria and exact rest periods between each charge and discharge as well as the ambient temperature (typically 25 °C) at which the battery should be cycled. A key fact is that batteries degrade with each cycle even if the cycle is not full.

However, this degradation rate and linearity are not the same for all models.

Battery pack

As previously described, a battery pack consists of cells and a set of auxiliary components. Both in literature and practice, the word “pack” is often omitted as is here as well. For stationary applications, there is a term “battery energy storage system”, which basically is a battery pack with additional interface converter which takes care of voltage conversion, charging and SoC (System on Chip) control. Each battery module or a small battery pack consists of individual cells. All cells are of the same model and are preferably parameter-matched to provide maximum performance utilization. There are two types of connections which can be used to combine individual cells: series connection and parallel connection. In a series, connection cells are connected in a string so that the positive pole of one cell is connected to the negative pole of the next cell. The voltage of a string is the sum of individual cells. n is the number of series-connected cells.

Figure 25. Voltage of battery string

In a series connection, the capacity rating (Ah rating) stays the same as for a single cell. In a parallel connection, all positive poles of all cells are connected together, and all negative poles are connected together as well. The correct polarity is of utmost importance as the incorrect polarity of a single cell will cause an immediate short circuit which in the worst case can result in fire and/

or explosion. The total voltage of a parallel connection is equal to that of a single cell. Parallel connection affects the total capacity which can be calculated as the sum of combined cells.

Figure 26. Voltage of batteries in parallel

As the capacity rating is increased, the C-rate is increased proportionally as well, resulting in higher permissible

One of a battery pack’s description parameters is the cell configuration: how much cells are connected in series and how much in parallel. A thirty cell series connection is described as the 30S while ten cell parallel combination is described as 10P. Both parts are typically combined:

30S10P – the battery pack consists of 30 series-connected cells and each “cell” is made of 10 actual cells in parallel. This pack contains 300 cells in total.

Figure 27. A: series first 3P12S battery; b:parallel first 9S4P battery, c:a mixed connection battery pack where each dashed box could be an 6S1P (12 V) lead-acid battery hence total

configuration could be labeled as 2S2P.

Battery management system (BMS)

Sometimes even a single cell requires an obligatory BMS, which can have a variety of functions.

The main task of a BMS is to maintain a safe operation of the battery – the safety of Lithium-based ell has always been an issue, which requires special care. The functions can be divided into four groups: protection, monitoring, estimation balancing. Safety essentially is protection.

Some cells have some inherent safety features, such as overpressure, short circuit and thermal protection. The thermal management system can be a part of the overall BMS. Battery packs can be actively cooled (or heated) – the temperature of coolant medium and its flow must be monitored as well. BMS monitoring functions might include data logging of all mentioned measured parameters and additional ones like total cycles, max and min discharge levels, total delivered energy and other time and charge related variables. BMS and its functions can extend further. As mentioned, thermal management can be a part of BMS, especially if active temperature control is used: forced-air or liquid cooling/heating. In EVs charging is controlled by the BMS as it has information about charging voltage, current and can provide temperature and safety control. BMS has a communication interface to the main vehicle control system. Some sort of charger is usually implemented in an EV and in some cases, the charger is a part of the battery pack. EV batteries are equipped with fuses and set of contactors: for work current and precharge.

Smaller batteries can have some human-machine interface (HMI), for example, a set of LEDs, to indicate remaining charge. All of these features are controlled by the BMS.