Selecting the right battery
Part 1: Selection criteria and charger implementation
As more devices become mobile, efficient battery usage has become essential. Engineers tend to give utmost importance to the current consumption of the device. They devise numerous ways to reduce the power consumption because the longer the battery lasts, the greater perceived market value for the product. One important aspect to consider here is the battery itself.
Selecting an appropriate battery for a particular application is important because the battery determines the number of hours for which the device can work without the need for recharging, the amount of weight it adds to the system, and how much it adds to the BOM (bill of materials) cost. Along with the selection of battery, proper design of charging circuitry is also crucial because improper design can reduce the battery life or can even lead to battery failure (i.e., dangerous leakage or explosion). Failure leading to produce recall can cause massive losses and damage a manufacturer’s reputation.
This article discusses the following topics:
1. Different types of rechargeable batteries
2. Comparison and selection criteria of different types of batteries
3. CC-CV charging method (Part 2)
4. Implementation using a microcontroller (Part 2)
Types of rechargeable batteries:
Rechargeable batteries come in various shapes and sizes, ranging from a coin cell to a battery weighing on the order of tons. These batteries can be classified based on their chemical properties. Some of the most commonly used batteries are:
1. Nickel Cadmium (Ni-Cd) battery
2. Nickel-Metal Hydride battery (NiMH)
3. Lead Acid battery
4. Lithium Ion battery
Nickel Cadmium (Ni-Cd) battery:
Ni-Cd batteries are one of the oldest battery technologies on the market and have distinct advantages like low cost for low power applications, sturdiest for rough environments, and the ability to be recharged many times. These batteries pack nickel hydroxide (Ni(OH)2) electrode as the cathode (positive) and cadmium hydroxide (Cd(OH)2) as the anode (negative) in electrolytic solution comprising of potassium hydroxide (KOH).
Figure 1: Discharge rate of Ni-Cd battery
One Ni-Cd cell gives 1.2V during discharge. These batteries have a flat discharge rate that falls rapidly at the end of the cycle as shown in Figure 1. Thus, it is difficult to estimate the amount of charge left. The advantage of this kind of battery is that it can withstand deep discharges without damaging the cell.
Along with the advantages mentioned, Ni-Cd comes with disadvantages as well, resulting in shrinking market share. Some of the major disadvantages include:
• Memory Effect: If the battery is repeatedly overcharged after partial discharging, it loses its capability to hold the maximum energy. This condition is referred as the memory effect. Thus, it is advised to either go for precisely controlled charging or regularly do a deep discharge such that the battery can recover its capacity. However, care must be taken since excessive deep discharge can permanently damage the cells.
• Cell Reversing: A battery consists of many cells connected together where each cell differs from the others slightly in terms of capacity. Thus, when one cell completely discharges before the other cells, the remaining cells will still force current through the discharged cell. This is known as cell reversing and results in undesirable and irreversible chemical reactions which permanently damage the cell.
• Over-Charging: When fully charged, the positive electrode (nickel) generates oxygen while the negative electrode (cadmium) generates hydrogen gas. These gases must be properly vented out from the system or operating conditions can become hazardous. To address this problem in sealed Ni-Cd batteries, the negative electrode (cadmium) is built with higher capacity. This causes the positive electrode to reach its fully charged state before negative electrode does. Thus, the oxygen released by positive electrode gets absorbed by the negative electrode and oxidizes.
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