In the previous article, we discussed different types of power supplies. For circuit prototyping and testing, linear regulated power supply is the most economical and viable do-it-yourself solution. The power output of a linear regulated source can also be adjusted with the help of a variable resistor. Electronics hobbyists and beginners who don't want to worry about designing their own power supply can purchase an adjustable bench power supply for regular circuit testing and prototyping. Most embedded systems are battery-based, so sooner or later you will need to add a battery-based power supply to your designs (circuits). Even circuits that are not entirely battery-based can use batteries as a power reserve. Therefore, it is essential to know these electrochemical components first and foremost, as they will make your circuits work in most situations.
What are batteries
Batteries are electrochemical devices that store electrical energy in the form of chemical energy. When they are connected in a circuit, current begins to flow through their terminals as the chemical energy stored in them is discharged back as electrical energy. Supercapacitors are different from batteries because they store electrical energy as an electrostatic field, while batteries store energy electrochemically.
Batteries are two-terminal devices and always have a positive terminal (anode) and a negative terminal (cathode). They are made up of three components – anode, cathode and electrolyte. When a battery is connected to a circuit, electrons flow from the cathode to the anode due to chemical reactions with the electrolyte. Obviously, the conventional current direction is from the anode to the cathode.
Types of batteries
There are four main types of batteries:
- Primary Batteries
- Secondary Batteries
- Reserve cells
- Fuel Cells
Primary Batteries
Primary cells and batteries are non-rechargeable, disposable batteries. The electrochemical reactions in these batteries cannot be reversed, so they cannot be recharged once they are depleted and can even discharge if left unused for a long period of time. These batteries are intended for single use and need to be discarded when depleted. These batteries are best suited for battery-based applications, which generally require low power requirements and must run entirely on battery power for a long period of time. Primary batteries are also used in many military-grade applications (radio and communications) as it is more convenient to replace batteries than to recharge them in such applications. Commonly used primary batteries include coin cells, AA and AAA batteries.
RJD series lithium-ion coin cell batteries feature the high power density required in wearable IoT applications. (Image: Illinois Capacitor)
These small, low-cost batteries are generally dry, have good charge retention, and are intended for long-term use. They are not designed or suitable for heavy load applications where a high discharge rate is required, nor are they suitable for power backup or load leveling.
There are many types of primary batteries available. Some of the popular primary batteries include the following.
Alkaline Batteries : These are the most popular primary batteries and are made up of 1.5V cells. They have high specific energy, long service life, are environmentally friendly, safe to use, leak-proof and low in cost. They are ideal for low current applications and are used in many consumer devices such as TV remotes, wall clocks and toys. Their main disadvantage is their weight, as they are heavy.
Zinc-Carbon Batteries : These are heavy-duty batteries and are a low-cost alternative to alkaline batteries. However, they have lower specific energy (energy density) and poor discharge rate performance. They are generally used as an economical replacement for alkaline batteries.
Lithium Primary Batteries : These small, lightweight batteries are intended for use in low current applications. These are used as an alternative to alkaline batteries where size and weight are the main concern in relation to cost.
Mercury Batteries : These are not dry batteries like other primary battery chemistries. These small size batteries have longer lifespan and stable voltage. They are generally used in size constraint applications.
Secondary batteries
Secondary batteries are rechargeable batteries. Their electrochemical reactions can be reversed by recharging them. In the recharging process, the charging current passes through its terminals opposite to the direction of the discharging current. Once recharged, these batteries are restored to their pre-discharge state.
Secondary batteries have a high initial cost; however, because they can be reused many times, they have lower life cycle costs. In many applications, it is more economical to use low-power secondary batteries than primary batteries for extended use. This is why low-power secondary batteries are commonly used in mobile devices and gadgets. These batteries are also useful in heavy-duty applications and those applications where high discharge rate performance is required. It's the reason why secondary batteries are used for applications like load leveling, electric vehicles, and power backup. Some of the popular secondary batteries include the following:
Lithium-ion batteries : These are the most popular secondary batteries. They come in a variety of chemicals such as lithium cobalt oxide, lithium sulfur, and lithium iron phosphate. These lightweight batteries have the highest energy density and excellent discharge rate performance and are widely used in smartphones, wearable devices and battery-based consumer devices. Recently, the 2019 Nobel Prize in Chemistry was awarded to three scientists – John B Goodenough, M Stanley Whittingham and Akira Yoshino – for their work in developing lithium-ion batteries.
A pin-type lithium-ion battery suitable for powering small applications such as wearable devices. (Image: Panasonic Industrial Battery)
Lead-acid batteries : These are the first types of secondary batteries. These are heavy, large batteries used for heavy-duty applications at a lower cost than other batteries, making them useful in non-portable applications such as automobiles and for power backup. Despite the low specific energy and heavy weight, these batteries have a high power-to-weight ratio. They are available as sealed lead-acid (SLA) batteries.
Nickel-cadmium batteries : These durable secondary batteries use cadmium metal and nickel oxide hydroxide as electrodes and are known to deliver their full rated voltage at high discharge rates. These have low self-discharge and long service life. A major disadvantage of Ni-Cd batteries is that they have memory – if they are not recharged properly, their lifespan can be shortened.
Nickel Metal Hydride : These batteries are similar to Ni-Cd batteries except they use a hydrogen-absorbing alloy instead of cadmium. They have greater energy density and do not require a specific recharging procedure. However, these batteries have a short lifespan compared to Ni-Cd batteries and may need to be discarded before a thousand cycles.
Reserve cells
Like primary batteries, backup cells are also primarily for single use, but feature an electrolyte isolated from the electrodes or electrodes isolated from the electrolyte. Most reserve cells have isolated electrolytes. The electrolyte is activated only when the battery is used. Because the electrolyte and electrodes remain isolated, these batteries do not self-discharge (like primary batteries) and have a longer lifespan. Some of the popular backup cells include electrolyte-activated batteries, heat-activated batteries (thermal batteries), water-activated batteries, and gas-activated batteries. These batteries are generally used in military-grade applications.
fuel cells
In fuel cells, the active material is fed into the battery from outside. The battery only remains operational until the active material is powered. Fuel cells are generally used in space applications, but also in applications such as electric vehicles, load leveling and power backup.
How a basic fuel cell works. (Image: FCHEA)
Most battery-based embedded devices use primary batteries. Many embedded devices can use secondary batteries, such as in industrial applications where the use of secondary batteries can be very economical. Spare cells and fuel cells have limited applications and may not be seen in widely used electronics. Just like inductors, batteries are application specific. Therefore, it is not a difficult task to choose a battery type for a particular circuit or application. But you need to consider other requirements such as terminal voltage, safety, compliance, size and geometry when selecting a battery for a given application.
We have already discussed different types of batteries based on reusability and their chemistry. In the next article we will talk about the geometry and size of batteries.
