Batteries are an energy source that is becoming increasingly important in everyday life. Without batteries, it would not have been possible to listen to music during the bus ride, sit with the computer on the couch or start the car comfortably. But what exactly is a battery?
There is a lot that differentiates between different battery types, including chemistry, size, capacity, and voltage. All these properties are actually interrelated and are a result of each other. The following chapter looks at how batteries and accumulators work.
What is a Battery?
A battery is used to store energy chemically and consists of one or more cells that are interconnected. In the coming chapters, we will equate the concept of battery with cell in many contexts, because it is rarely made a difference between them in everyday speech. For example, AA cells are rarely talked about, but rather AA batteries. We also say battery about accumulators (rechargeable batteries).
A battery consists of three main parts: anode, cathode, and electrolyte. The anode and cathode are the negative and positive poles in a normal battery that are discharged (on the contrary when charging) and the electrolyte is the solution or gel with which the poles contact. An alkaline battery uses zinc (Zn) as the anode (minus pole) and manganese dioxide (MnO2) as the cathode (plus pole). The electrolyte consists of potassium hydroxide (KOH).
The principle of a battery
A battery starts to discharge when the two poles are connected, for example via a lamp. The anode then oxidizes, which means that it releases positive ions and that an excess of negative electrons is formed. This causes the battery to become unbalanced, something that constantly strives to smooth.
The electrons cannot take the shortcut via the electrolyte but instead aim at the cathode by going through the lamp. As the electrons rush through the filament, it begins to glow, causing the lamp to emit light. Once at the cathode, a reduction occurs where the electrons bond to positive ions adjacent to the positive pole. This way, the battery poles are kept balanced at all times. The battery is considered empty when the negative pole is consumed and cannot emit more electrons.
By connecting a DC voltage source to the poles it is possible to make the whole process go backward. Then the positive pole becomes the anode and the negative pole cathode instead. The bidirectional function is limited to rechargeable battery types, such as nickel-metal hydride and lithium-ion.
The difference in voltage, volts (V)
All battery types have different characteristics. Depending on the substances they comprise, they are discharged in different ways and last for a long time.
Devices require a certain “pressure” from the batteries for them to work. If the pressure is too low, the device will not do its job, and if the pressure is too high, it risks breaking. A battery-powered fan spins optimally if operated at the correct voltage, slower if the voltage is too low and too fast for the fan’s best if the voltage is too high.
All alkaline AA batteries have a voltage of 1.5 V. If two AA batteries are in one device, these are connected in series. The appliance is then powered by 3 V. It does not necessarily have to be alkaline batteries, but other battery types work well. The important thing is that they fit and have a voltage of around 1.5 V. This means that lithium-thionyl chloride batteries cannot be used, even though they fit purely physically. They have a voltage of 3.6 V and in series, it would be 7.2 V.
The difference in capacity, ampere-hours (Ah)
Capacity is the value that describes how long a battery can supply a given current. The batteries LR20 and LR03 are different in size but still have the same voltage (1.5 V). What distinguishes them, however, is capacity. The higher the capacity of a battery, the longer it will work in an appliance. The capacity of rechargeable batteries is often stated in thousands of parts of an amperage hour (mAh), for example, 2500 mAh. It is possible to replace a battery with a higher capacity, provided it has the same voltage and fits physically.
The voltage is the same for batteries AAA, AA, C and D but the capacity differs.
The capacity of a battery is affected by the chemical composition of the battery. This is despite the fact that the batteries can have the same size and voltage. GP Batteries manufactures disposable batteries based on four different battery technologies. The diagram shows GP’s disposable AA size batteries. The GP’s website (gpbatteries.com) contains datasheets showing how long a battery lasts when it is connected to a theoretical device with 10 Ω discharge resistance. In this case, the device can be operated at a voltage down to 0.9 V.
The capacity varies between batteries of the same size. It depends on their chemical composition. How long the batteries last depends on the type of device they are in.
Different types of batteries are discharged in different ways
When a battery is discharged, the voltage drops. The old brownstone battery has a very linear discharge curve, which means that the voltage is directly proportional to how much energy is left in the battery. What is being sought is for the voltage to be as high and stable as possible throughout the discharge curve.
The difference in self-discharge
Batteries have an undesirable feature called self-discharge. It is the one that makes disposable batteries have an expiry date and that rechargeable batteries must be maintained to some extent in order not to lose their energy. The self-discharge varies between different types of battery and is stated in what percentage of capacity the battery loses per unit of time. The time unit is month or year depending on relevance in the context.
