Tech Papers and Guides
Key battery parameters
It is important to understand all critical battery parameters and apply them to the product usage and requirements.
Most battery cells are made of two electrodes and an electrolyte. The combination of materials used to make these components determines the chemistry of the battery. The battery chemistry defines properties such as voltage, load current, capacity, operating temperature, self-discharge, and physical size.
Primary batteries are not rechargeable. These batteries are made of electrochemical cells which produce a chemical reaction that is not reversible. Therefore, you cannot recharge these batteries; you must replace them when they are depleted. Primary batteries often have specific energy, and the devices that use them are designed to consume low power, allowing the battery to last as long as possible. The most popular types of primary battery chemistry are Lithium, Alkaline, and Zinc-carbon. These batteries are also ecofriendly and sustainable. However, the low load current in these batteries limits their application to devices with low current requirements, such as remote controls and smoke alarms.
These batteries represent the standard power source for many products, especially for portable devices such as digital cameras, laptop computers, tablets, and cell phones. Because they can be recharged, using these batteries significantly reduces the amount of waste sent to landfills in the form of primary batteries. To recharge a battery, reverse the electrochemical reactions by applying a voltage to the battery in the opposite direction.
The initial cost of rechargeable batteries is more than non-rechargeable batteries. However, since they can be recharged multiple times, they can be more economical in the end.
You can classify rechargeable batteries into subtypes, based on their chemistry. This is important because the chemistry determines some of the battery’s attributes, including its specific energy, cycle life, shelf life, and price. The most popular types of rechargeable batteries based on chemistry:
- Lithium-ion (Li-ion)
- Nickel Cadmium (Ni-Cd)
- Nickel-Metal Hydride (Ni-MH)
The super-capacitor, or ultra-capacitor, is different from a battery. Super-capacitors charge in seconds, with very little capacity degradation. They can withstand virtually unlimited charging cycles. Traditionally, super-capacitors are used for applications that experience sudden energy spikes or that use energy in bursts. To handle high peak currents, use a super-capacitor to offload the battery. During high-current periods, the super-capacitor acts as the primary power source. During low-current periods, the battery is the primary power source and recharges the capacitor.
Ideally, use super-capacitors when you need a short-term, quick charge. The combination of a super capacitor and a battery into a hybrid battery satisfies both short term and long-term power needs and reduces battery stress, resulting in a longer service life. A disadvantage of super-capacitors is the need for voltage balancing when several are placed in a series to achieve higher voltages. Also, depending on how the voltage of the capacitor-battery is measured, the charged super-capacitor can mask a battery with low voltage that is near to the end of life. Additionally, there is a power penalty on the order of 1-10 uA to keep super-capacitors charged.
Capacity and discharge characteristics
Battery capacity is measured and represented in ampere-hours (Ah). Ampere-hours are the discharge current a battery can deliver over time. All batteries are impacted by a process called self-discharge. The degree of self-discharge energy depends on the technology and chemicals being used in the battery. Often, batteries are specified with a discharge rate in terms of C, where C is the capacity of the battery divided by hours. For example, C for a 1200mAh battery is 1.2A discharge rate for an hour, C/2 for the same battery is 600 mA, and C/10 is 120mA.
Peak load current
Peak load current is the maximum current at which the battery can be discharged continuously. This limit is usually specified by the battery manufacturer to prevent excessive discharge rates that would damage the battery or reduce its capacity. Battery voltage varies depending on the discharge current applied (load). As illustrated in Figure 1, when the discharge current increases, the operating voltage and the battery capacity will decrease. For example, with a 5Ah battery, the capacity (C) is 5A with a discharge rate of one hour. If the battery has a maximum discharge rate of 5C, then the peak load current is 25A. It is important to remember that increasing the discharge rate to 5C will proportionally reduce the battery life to 1/5th of an hour.
Figure 1: Discharge characteristics of NCR18650B energy cell by Panasonic
Cell voltage varies with the type of battery chemistry you choose. Based on the application, choosing the correct battery with the right cell voltage can impact the efficiency, complexity, and cost of the power circuitry in the device. Batteries are marked with nominal voltage. However, the open circuit voltage (OCV) on a fully-charged battery can be 5% to 7% higher. Figure 2 is a sample graph for cell voltage vs. discharge capacity for different battery chemistry.
By combining Figure 1 and Figure 2 we get a glimpse into the cell voltage over time and under varying loads. If a device consumes 0.2C (blue line of figure 1) of the Panasonic NCR18650B during radio reception, at around 20% discharge (i.e. 600mAh on the x-axis), we find a voltage of about 3.9V. However, if our device starts to consume 1C of current (green line of figure 1), the voltage will drop to 3.7V. This change in voltage becomes even more pronounced as the battery is consumed. The device developer must consider these variations when they design a device.
Figure 2: Cell voltage vs. discharge capacity for different battery chemistry
Often, IoT sensors are placed in harsh environments with significant temperature variations or extreme temperatures. In a high-temperature environment, the self-discharge rate is higher than the nominal rate due to the battery chemistry. Similarly, in low temperature environments a battery’s chemical reactions may be slowed, limiting the ability of the battery to sustain a useful voltage for the required load. For instance, we do not recommend using Zinc-Carbon and Alkaline batteries below 0°C. Lithium batteries can operate at -40°C, but usually experience a significant drop in performance. In rechargeable applications, Lithium-ion batteries can be charged at a maximum rate in a narrow temperature window, about 20°C to 45°C. Outside these temperatures, lower currents or voltages are required and result in longer charging times.
Figure 3: Discharge characteristics by temperature of NCR18650B energy cell by Panasonic
Most batteries, especially those with an aqueous electrolyte, undergo slow self-discharge, even when they are left unconnected. The rate of self-discharge depends on the cell chemistry, its construction, and especially the temperature of storage. Usually, the rate of self-discharge increases with temperature. Shelf-life is more of a concern with primary batteries than secondary batteries. You can recharge secondary batteries before use. Modern alkaline-based primary batteries have a storage life of several years at 20°C. However, storing alkaline-based primary batteries at high ambient temperatures can degrade them.
Finally, note that a shipment of rechargeable batteries is often regulated. Therefore, most Lithium rechargeable batteries are shipped with no more than a 30% charge. The device developer should let you know that you must recharge the device before you use it for best results. (See packing instruction 965: Lithium ion cells and batteries must be transported at a state of charge not exceeding 30% of their rated capacity.)