# The State-of-the-Art of Sensors and Environmental Monitoring Technologies in Buildings
## 4 Battery and Energy Harvesting Technologies for Sensing and Monitoring in Buildings  
### Characteristics and Limitations of Battery Technologies  
Batteries are an integral part of almost every technological aspect of our daily lives and continue to be one of the leading candidates to provide energy for sensing and monitoring devices used within modern-day buildings. This is attributed to features such as portability, high power densities (in most cases), and the sheer variety of technologies currently available on the market. Out of all the established technologies, Nickel Cadmium (NiCd), Nickel Metal-Hydride (NiMH), Nickel Iron (NiFe), Lead Acid, Lithium-ion (Li-ion), and Reusable Alkaline, have been the fastest growing, and most extensively studied and deployed technologies [106; 107; 108; 109]. Table 4 provides a comparison of each of these battery technologies with data extracted or calculated from the latest published literature, and various manufacturer datasheets and user manuals available online. It is evident from the data presented in Table 4 that each battery technology comes with its own set of distinct benefits, e.g., NiCd batteries have fast charge times, NiMH batteries have high power densities, NiFe batteries have a relatively low impact on the environment, and Li-ion batteries have the highest power densities of all the widely adopted technologies. However, batteries generally have a number of limitations which restrict them from meeting all the energy requirements of today's sensing and monitoring devices deployed within buildings. These limitations include:  
1. Lifespan and Replacement--The lifetime of sensing and monitoring devices is always longer than the lifetime of the battery and, therefore, at some point, battery replacement is required (depending on the lifespan of the selected battery technology). 2. Self-discharging--Batteries self-discharge over time, which is caused by internal chemical reactions. Depletion due to self-discharging leads to reductions in the lifespan of the battery. 3. Size--Battery size usually exceeds the size of most sensing and monitoring devices. 4. Cost--Despite significant advancements in recent years, the cost of batteries continues to be higher than the cost of most sensing and monitoring devices summarized in Tables 2 and 3. This cost increases further when more than one battery is required for a particular application (e.g., connecting multiple batteries in series or parallel for larger voltage requirements). 5. Installation--Most batteries are able to operate in any position and provide good resistance to shocks and vibrations. However, in some cases, batteries have to be positioned on shock-absorbing dampers and in upright positions. 6. Disposal--Common battery technologies such as NiCd and Lead Acid contain environmentally hazardous materials and cannot be disposed with regular household items. Although NiMH and Lithium-based batteries are more environmentally friendly, it is still recommended to recycle all used batteries appropriately. 7. Maintenance--Apart from Li-ion and Alkaline, all other widely used battery technologies require regular maintenance every 30-180 days depending on usage. To address the above restraints, the concept of 'energy harvesting', i.e., the process of converting ambient environmental energy into electrical energy to power small, low-power electronic devices, has become increasingly appealing in recent years. In the following subsections, we highlight some of the main sources of energy harvesting within buildings and discuss the technology's potential viability for sensing and monitoring applications in buildings.