There are two main battery types: single-use - primary and rechargeable - secondary. Electric vehicles (EV) and most of the other autonomous systems use secondary batteries (except for small toy vehicles and special applications) hence in this chapter the term battery mean secondary battery unless noted otherwise. From an economic perspective, batteries are a serious business approaching 100-billion-euro market size. About a third of all batteries are used as automotive traction batteries (EV, HEV), other third is used in industrial applications and portable applications (consumer electronics) while the last third is used in other applications like power tools and conventional car batteries. From everyday knowledge, it is known that batteries have different voltages. A wall clock typically uses an AA or AAA size 1.5V battery while a car has 12V lead-acid battery under the hood. There are two reasons for different battery voltages: chemistry and series connection. The chemical composition of battery materials determines the voltage in the range of 1.2V to 3.9V. How come a car lead-acid battery has 12V? It actually has multiple smaller batteries inside and they are series-connected (mind the polarity) to sum up their voltages. These individual internal batteries are called cells. Figure 1 shows some multi-cell batteries. It would be technically correct to say that a battery is in fact two or more series-connected cells of the same kind. Hence a battery composed of just a single cell would not be a battery but rather just a cell. However, to not cause confusion it is accustomed in everyday language to use the term battery for any number of cells while a cell means a single element. This notation will be used here as well. One of-the-shelf battery is the car lead-acid battery which has six 2.1V cells inside (the voltage is rounded to 12V for convenience), another multi-cell battery example is the 9-volt battery which is composed of six 1.5V cells (alkaline or carbon-zinc chemistry). When one installs two AA batteries in a TV remote, they are series-connected to form a 3V battery.

In electrical engineering, a battery is recognized as a voltage source. A major difference is that the voltage of this source will gradually decrease when a load is applied (discharge) while connecting a battery to a higher voltage source will cause its voltage to gradually increase (charge up). A more precise definition claims that a battery is in fact an electrochemical device which can provide voltage and release electrical energy stored inside of it in the form of chemical bonds.

Voltage

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 3.9V. 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.

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:

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 C-rate. The value of 1C 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, 2C discharge rate of a 10Ah battery is 20A while 0.5C charge rate of the same battery is 5A

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.

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.