Another option for powering IoT devices is to rely on batteries. Batteries store electrical energy in the form of a chemical potential, and can release this energy under the form of DC electricity. The section below discusses the most important battery characteristics, which are relevant regardless of battery type (the battery chemistry).
General battery characteristics
- Cell Voltage — Battery cells have a nominal voltage level. Cell voltage typically ranges from 1.2V for NiMH batteries up to 3.7V for LiPo batteries. Cell voltage is dependent on the state of charge of the battery, and gradually decreases as the battery is drained.
- Number of cells — Batteries are composed of one or more cells. For instance, a standard 9V battery is internally composed of six 1.5V cells. Battery datasheets typically give an “S” number; this is the number of battery cells wired in series. For instance, a 3S battery is a composed of three cells.
- Capacity — The energy capacity of a battery is expressed in milliamp-hours (mAh). A battery with a capacity of 500 mAh can supply 500mA for one hour before it is drained. Note: this number can be deceptive because the total energy of a battery also depends on the battery voltage: a 7.4V 500mAh battery can store twice as much energy as a 3.7V 500mAh battery.
- Series/parallel — The way the cells of a battery are connected together affects the electrical characteristics of the battery. Cells connected in series results in a higher voltage. Cells connected in parallel results in a higher maximum current. Certain multi-cell battery packs must be balanced during charging.
- Internal resistance — Batteries have a small but non-negligible internal resistance. This is intrinsic to the physical construction of the battery. At higher discharge currents, the internal resistance will cause the battery voltage to drop and the battery will heat up.
- Discharge rate — Batteries have an optimal and a maximum discharge rate, which is related to the battery’s internal resistance. LiPo battery packs can have discharge rates up to 100A, whereas small coin cell batteries are limited to about 15mA. The discharge rate affects the capacity of a battery: higher discharge rates lead to less available energy. Max discharge rate is usually expressed as a C-rating: a 2C 500mAh battery has a maximum safe discharge rate of 2*500mA = 1000mA.
- Lifespan — Batteries have a limited lifespan, measured in number of charge/discharge cycles. As a battery gets older, the capacity drops.
- Self-discharge rate — Batteries slowly lose charge as they sit unused.
- Shallow/deep cycle — Most batteries last longer when they are not discharged/recharged completely, but charged at the halfway mark. This is called shallow cycling. Specialized batteries exist that are optimized for deep cycling.
Common Battery Types
- Alkaline batteries — The most common non-rechargeable battery type. Has a nominal cell voltage of 1.5V. Common form-factors are the cylindrical AA and AAA shapes. Most devices that are powered by alkaline batteries use four (or more) batteries in series, resulting in a total voltage of 6V.
- Coin-cell batteries — Very small, non-rechargeable batteries. Used in low-current applications, such as watches, RTC backup batteries
- NiMH batteries — Rechargeable batteries with a nominal cell voltage of 1.2V. Available in AA/AAA form factor or in custom multi-cell battery packs. Can be used as a rechargeable alternative for standard alkaline AA batteries, though the lower voltage may influence the functioning of the device.
- LiPo batteries — Lithium-polymer batteries are the most energy-dense batteries currently available on the market. For this reason, they are used in most portable consumer electronics devices, such as smartphones, cameras, or fitness trackers. LiPo batteries offer the best performance, but are more complex and delicate to use than other types of batteries. They require specialized protection circuitry as overcharge, over-discharge, over-temperature, and short circuit can all result in catastrophic failure. Careful charging and balancing procedures need to be followed, though this is typically handled by a dedicated chip. LiPo batteries are available in a wide variety of form factors, such as different rectangular pouches and cylindrical “18650” cells. Some LiPo batteries have integrated protection circuitry, though this is not always the case.