In previous articles, we discussed types of batteries and different battery packs. Many embedded circuits and devices rely on batteries for power, and many of these devices use primary batteries that may need to be replaced. Other embedded devices are rechargeable and use secondary batteries to stay powered.
It is not difficult to select the battery type, chemical composition or packaging for a particular circuit or application. Pros and cons, as well as specific applications, should be important considerations. Lightweight primary batteries, such as alkaline and zinc-carbon batteries, are widely used as cylindrical cells in non-rechargeable devices. Button cells are generally used as primary batteries in compact devices (such as watches and bracelets) or are used only to power specific sections of a circuit.
Rechargeable embedded devices (i.e., handheld devices, mobile devices, toys, headphones, and other portable consumer devices) typically use cylindrical NiCd or NiMH cells, or rectangular or lithium-ion pouch cells. For extremely compact embedded products such as Bluetooth headsets, lithium-ion batteries remain the preferred choice. Portable devices that require longer battery life use prismatic (rectangular) lithium-ion batteries. NiCd and NiMH cylindrical cells are generally used as secondary batteries in cost-sensitive products. Heavy, bulky lead-acid batteries are used as secondary batteries in non-portable applications such as UPS and backup power.
The following factors primarily influence the choice of battery chemistry for an application:
- Reuse – The first thing you need to determine is whether a given circuit should be rechargeable or whether it should rely on battery replacement. Hence, primary or secondary battery types can be used for the device.
- Life cycle durability – The durability of a battery becomes an even more important consideration when it is the primary battery used in the circuit. Even if it is the secondary battery used in a circuit, it is important to note how long a charge-discharge cycle will last and how many cycles a battery can run without damage (such as swelling in the case of prismatic and bag cells). It is also important to note the length of a secondary battery's charge-discharge cycle to determine the charging procedure and schedule.
- Energy Density – This is the amount of energy that can be stored per unit of mass or volume. Each battery chemistry has a specific energy density that influences the size and weight of the battery. Portable devices essentially require batteries to be lightweight and compact. Non-portable applications may compromise weight, although they may still have some size restrictions.
- Power Density – It is the maximum rate of energy discharge per unit of mass or volume. Power density plays an important role in suiting battery chemistry for a given application. Many applications require a high discharge rate or may be susceptible to a sudden increase in discharge power. This may affect the safety of a battery. Power density is the main influence on the performance of a battery in a circuit.
- Safety – The two most important factors that influence the safety of a battery are thermal stability and power density. A battery must have sufficient power density to meet any possible discharge rates in a circuit. Each battery chemistry also has specific operating temperatures. At high temperatures, battery components can break down and undergo exothermic reactions. Cells must be adequately spaced for better thermal stability. It may be necessary to provide mechanisms such as liquid cooling or air cooling to manage heat in a device. It is also important to note the battery temperature for different discharge rates and current levels.
- Geometry and Size – Different battery chemistries are available in various shapes and sizes. For a given battery chemistry, the optimal battery shape and size must be selected so that it does not compromise the required amp-hour capacity, life cycle length, size or weight restrictions, and safety. Most prismatic and bag batteries are prone to swelling with prolonged use, so there must be enough space in a device to accommodate this.
- Cost – Finally, battery chemistry selection depends on cost. The cost can be adjusted by selecting alternative chemicals or packaging for the battery. However, there should be no compromise on performance or safety for cost reduction.
Once you have defined the battery chemistry and battery (based on performance, safety, portability, rechargeability, and cost considerations), you will need to identify the required battery specifications. The most important battery specifications to note are as follows:
- Terminal Voltage – Any battery is used as a voltage source in a circuit. Therefore, the first specification that must be checked is the required terminal voltage. The voltage of a battery should always be regulated using a transistor circuit or voltage regulator IC to avoid any noise or fluctuations from the battery. The voltage regulator can also reduce the supply voltage to the required value if the battery voltage is higher than that required in the circuit. Sometimes the battery supply may need to be increased using a transistor amplifier if a higher voltage is required.
As the battery discharges, the terminal voltage begins to decrease and the internal resistance begins to increase. The best way to test the condition of a battery is to measure the terminal voltage at no-load or under load conditions.
- Discharge Rate – Discharge rate is particularly important in determining battery performance in a circuit. It is also an important factor in determining the safety of a battery.
- Ampere-hour capacity – The energy capacity of batteries is expressed in ampere-hours. Any battery can transfer a specific number of electrons into a circuit before complete discharge. This is a considerable number – it is the approximate value of direct current that can be supplied to a circuit by the battery for a specific number of hours. If a battery has a capacity of 1 amp-hour, it means that the battery can supply 1 A of direct current for one hour or 2 A of direct current for half an hour and so on. The ampere-hour rating gives an approximate duration of the number of hours the battery can run before fully discharging, providing a certain amount of direct current.
This is always an approximate value and the battery may be completely discharged before the estimated duration. For example, if a 20 amp-hour battery can be connected to a low-resistance load, which can draw a current of 20A, ideally the battery should last one hour. However, it may completely discharge a little before an hour due to heating. Likewise, a battery connected to a low-power load can be estimated to last several years. Still, it may fully discharge before the estimated duration due to life cycle factors such as leakage current, electrolyte evaporation, high temperatures, humidity or electrode deterioration.
The amp-hour capacity of a battery is specified for a specific current. Manufacturers can also provide derating curves for different currents and temperatures. In the case of secondary batteries, the Amp-hour rating helps determine the charging procedure and schedule.
Series and parallel combination of batteries
Batteries may need to be connected to form a larger battery bank to achieve higher voltage or current. When batteries or cells are connected in series, the current remains the same, while the total voltage is the sum of the voltages of all the batteries. Batteries connected in series must have the same amp-hour rating; otherwise, the battery with a lower amp-hour rating will run out before the others and the supply will be interrupted. The total amp-hour rating of the package remains the same as the series connection. It just increases the voltage output of the battery.
When batteries or cells are connected in parallel, the voltage remains the same, while the total current is the sum of the individual battery/cell currents. Batteries connected in parallel must have the same terminal voltage. Batteries in parallel again do not change the overall amp-hour rating and only increase the output current.
Overcurrent protection
Batteries of any type can produce countercurrent. Additionally, there may be fluctuations in battery supply. To protect a circuit against backcurrent and fluctuations, a fuse or circuit breakers must be used. In batteries connected in series, a single fuse is sufficient to protect the charging circuit. For batteries connected in parallel, it is recommended to use a separate fuse for each battery so that neither battery overloads the other. The circuit breaker can be as simple as a protection diode.
Overload protection
In the case of secondary batteries, it is important to avoid overcharging and over-discharging. To do this, initially, adequate production and billing schedules must be maintained. Battery charge indicators can be used to prevent overcharging and overdischarging. Overcharge protection circuits can also be used to protect a battery. In some applications, the power circuit can be used which can power the load directly from the DC source and can switch to battery power in case of power failure. In these applications, a rechargeable battery can be used as a backup power source.
In the next article, we will begin a discussion of semiconductor technology – the basis of modern electronics.