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 a 100-billion-euro market size. About a third of all batteries are used as automotive traction batteries (EV, HEV), another 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 1.5 V battery while a car has a 12 V 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 1.2 V to 3.9 V. How come a car lead-acid battery has 12 V? 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 2.1 V cells inside (the voltage is rounded to 12 V for convenience), another multi-cell battery example is the 9-volt battery which is composed of six 1.5 V cells (alkaline or carbon-zinc chemistry). When one installs two AA batteries in a TV remote, they are series-connected to form a 3 V 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 that 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 3.9 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 a 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). A 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 that is directly related to Ah rating: the C-rate. The value of 1 C is the 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 in handy when comparing the current capabilities of different batteries and simply estimating how large the current is with respect to the capacity of the battery. For example, the 2 C discharge rate of a 10Ah battery is 20A while the 0.5 C 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 the 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 important 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 the term “battery energy storage system”, which basically is a battery pack with an additional interface converter that 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 that 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.

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 many 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.

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 the 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 a 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 the remaining charge. All of these features are controlled by the BMS.

Fuel cells (FC) are devices somewhat like batteries. Their purpose is to provide electrical energy by converting chemical energy. Same as batteries they are electrochemical devices. The main difference is that fuel cells use some sort of chemical compound (fuel), which is supplied to the cell to produce electricity by a controlled electrochemical redox reaction. The fuel and oxidizing material (oxygen from the air) is consumed during the process. On the other hand, the battery already had all components embedded in a closed package and no material was consumed during charging/discharging. Fuel cell technology has a long history as it was invented in the 19th century. Since then various configurations have been developed and implemented in stationary or portable applications ranging from few Watts of power to mega-watt systems.

The most notable fuel cell technology is the proton exchange membrane fuel cell (PEM FC). The basic elements of a fuel cell are anode, cathode and electrolyte. A PEM layer contains an electrolyte and separates the anode from the cathode. Fuel is delivered to the anode side while oxygen is delivered to the cathode side. Popular fuels are hydrogen and methanol. As an electrochemical reaction takes place, protons from fuel are transferred through the PEM to the cathode side where waste is produced: water in case of hydrogen fuel and CO2 if methanol FC is used. As usual, the electrical load is connected to anode and cathode to deliver electrical energy. Common efficiencies are in the 50% to 60% range which means that a significant amount of heat will be generated during power production – a cooling system like one of the common ICE vehicles is required as the temperature operating range of FCs is limited.

A key issue for hydrogen FC adoption is the lack of refuelling infrastructure. Hydrogen gas is extremely flammable, it can diffuse in and through metals and it can cause metal embrittlement hence manufacturing, handling and storing hydrogen requires additional care. The final problem is the source of hydrogen. The amount of hydrogen in the atmosphere is negligible hence it must be manufactured. It can be produced by electrolysis of water however this process is inefficient even further decreasing the total FC technology full-cycle efficiency. Currently, the majority of hydrogen is produced by reforming fossil fuel – not a sustainable solution. Despite these drawbacks, hydrogen FC technology has been and is used in some commercial EV products. There are a few available automobile models and several public transport buses. The latter has shown good performance as buses have stable cyclic usage and plenty of space for FC and hydrogen storage tank installation.

In simplistic terms, supercapacitors (SC) are capacitors with extremely high capacity. In fact, they use special physical effects (electrochemical pseudocapacitance and/or electrostatic double-layer capacitance) to provide capacity. Depending on brand names and physical effects, supercapacitors are also called boost capacitors, ultracapacitors, pseudocapacitors and electrostatic double-layer capacitors (EDLC). One must not confuse SCs with common high capacity aluminium electrolytic capacitors which are made with rated voltages from a few to hundreds of volts. The rated voltage of a single SC cell is in the range of 2.1 V to 3 V. The capacity of single SCs ranges from hundreds of millifarads to a few kilofarads – they extend the capacitor capacity range as the largest electrolytic capacitors are just around 1F incapacity. However, they have not replaced batteries due to relatively minuscule specific energy (7.4Wh/kg for 3400F capacitor), which makes them inappropriate for bulk energy storage. SC technology is evolving to improve the overall performance. Hybrid capacitors have been developed – they use both SC and Li-ion technology. The result is a so-called lithium-ion capacitor – as the name suggests, it is more like a capacitor with some features of the Li-ion battery.

To conclude this chapter, see the figure below - a Ragone plot, which is an effective tool to graphically compare gravimetric energy density (specific energy) and gravimetric power density (specific power) of various energy storage elements. The lowest performance is at the bottom left corner while the highest performance is at the top right corner - a spot to be taken by future technologies. As can be seen, fuel cell technology can provide the highest specific energy while capacitors can provide the highest specific power. However, Li-ion technology with its high specific energy and good specific power is the right choice for most mobile/portable applications.