Many sustainable energy sources produce according to natural rhythms such as sunlight, wind, and precipitation. These cycles don't necessarily coincide with the times that humans need energy. Therefore, energy storage is an essential part of most sustainable energy systems.
Electricity is among the most common sustainable energy transfer and storage mediums. However, unlike combustible fuels it is very challenging to efficiently store.
Conventional lead-acid batteries tend to be the most readily available and inexpensive electricity storage option. However, they have significant issues that make them unsustainable and unsuitable in many cases including:
Toxicity Lead (and to a lesser extent the acid used in lead acid batteries) can be extremely toxic for both humans and other lifeforms.
Short useful lifespan Most lead acid batteries have a useful lifespan of less then 15 years with a cycle durability of less then 350 cycles.
Inefficiency The charge to discharge ratio ranges from 50–95% with a self discharge rate of up to 20% per month
Weight Lead acid batteries have a low energy-to-weight ratio of approximately 180 W/kg
Recycling Although lead acid batteries are 99% recyclable, lead exposure can still take place during the recycling steps, and is a source of environmental contamination where the recycling process is poorly regulated.
Lithium batteries are generally far more efficient and long lasting then lead-acid batteries. However, they are approximately four times the price-per-watt of lead acid and also have significant ecological impacts.