For any complete energy-harvesting system designed to provide power to anything but small, short-duration loads, storage batteries represent a necessary but significant portion of the initial expense. The cost of 18650 batteries over the lifetime of the system can have an even larger impact if care is not taken to maximize the useful life of the battery component.

Storage battery specifications such as robustness and projected lifetime depend, to a great extent, upon the chemistry of the cells. Li-ion, LiFePO4 battery and sealed lead acid are widely used battery chemistries. Most commercial Li-ion cells can now be charged to 4.2 V/cell. A LiFePO4 battery allows a much higher charge and discharge rate, but the energy density is lower. The typical cell voltage is 3.6 V. The charge profile of both Li-ion and LiFePO4 is preconditioning, constant current and constant voltage. For maximum cycle life, the end-of-charge voltage threshold could be lowered to 4.1 V/cell. Battery chemistry notwithstanding, there are several design criteria engineers should know that affect the life expectancy of any type of storage cell.

A basic system with a photovoltaic panel for energy input provides either a blocking diode or switch in series with the battery to prevent draining the battery during periods of low light. To avoid overcharging, excess current can be shunted from the solar cell once the battery is fully charged. In practice, even this simplified design requires monitoring and control electronics.

Although those of us living in frigid climates tend to associate battery death with the coldest part of the year when your car won’t start in a blizzard, most battery damage actually can occur in hot weather. Temperature plays an important role in ensuring long storage battery life. All batteries depend on chemical reactions that are thermally activated. With strong temperature dependence, these reactions can get out of control, leading to sudden and serious damage. At low temperatures, Polymer Lithium ion Battery battery electrolytes can also freeze when the charge is depleted. This is especially important in photovoltaic energy harvesting since the lowest temperatures are often coincident with the lowest solar energy levels compounding the detrimental temperature effect.