Batteries used in for energy storage applications, such as renewable energy systems and electric vehicles come in many shapes and sizes and can be made up of various chemical combinations. In the past lead-acid batteries were the most common battery type used in off-grid and hybrid energy storage systems, however more recently lithium-ion (Li-ion) based battery systems have become widely available for this purpose. Electric vehicles have long used lead-acid batteries as well, in particular for vehicles such as electric forklifts and golf carts, as these vehicle types do not require a long range and the high weight of lead-acid batteries is not a major concern. In contrast the rapidly growing passenger electric vehicle market has quickly advanced Lithium battery technology largely due to the much lower weight of Lithium battery packs as well as other advantages discussed later.
There are also many new advanced battery technologies becoming available such as lead-carbon, Sodium-Ion, Ultra-batteries (which combine capacitor and lead-acid battery technologies), and flow batteries.
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Lead-acid batteries (Flooded, AGM & Gel)
Lead-acid batteries are the oldest and most common type of storage battery. They have long been considered the battery of choice for off-grid power systems due to their relatively low cost, reliability and service life.
A deep cycle battery is a rechargeable lead-acid battery with very thick active plates and separators made from high quality and high-density alloys. This robust construction enables the battery to be regularly and deeply discharged up to 70% of its total capacity. The deep discharge, charge process can be repeated hundreds of times, although if the battery is only discharged to 20-30% of its capacity it can be repeated several thousand times. Thus the life of a deep cycle battery is directly proportional to the amount it is regularly discharged. There are many other factors that also affect the life of a deep cycle lead-acid battery which are explained further in our lead-acid deep cycle battery article.
Deep cycle batteries are not to be confused with starter batteries. Starter batteries, used in automotive applications, are designed to start a combustion engine by delivering short, high bursts of current and are not suitable for energy storage.
Lithium-ion (Li-ion) based batteries have only been commercially available for 10-15 years and have quickly become very popular in consumer electronics due to their lightweight and high power density. These advantages led them to become the battery of choice for electric vehicles and are becoming very popular for home energy storage systems.
There are many different types of Li-ion batteries available with the most common being lithium-ion phosphate (LiFePO4 or LFP), Lithium Nickel Manganese Cobalt Oxide (NMC) and Lithium polymer. The Tesla Powerwall and Tesla Electric vehicles use the NMC variety in a cylinder cell design while the popular LG chem RESU home batteries use flat rectangular Polymer cells.
There are also many other different lithium chemistries available such cobolt oxide (LiCoO2), Lithium-manganese oxide (LiMn2O4). LFP and NMC batteries are generally considered the most cost effective and reliable lithium battery and at present are the most common type used in off-grid and hybrid solar power systems.
Li-ion batteries have a number of advantages over lead-acid batteries, as mentioned they are much smaller and lighter for the same capacity but they are also able to be deeply discharged 80-90% of total capacity without compromising the life of the battery. Furthermore, they can be recharged faster and are less likely to degrade during deep discharging so have a potential to last much longer. The fast charging capability is a huge advantage for use in electric vehicles as the slow charging time of lead-acid has been a major drawback to consumer uptake.
For hybrid and off-grid solar power systems the high power density of Li-ion means batteries require much less space in a home and the battery system can be pre wired which reduces installation cost. Furthermore Li-ion battery systems do not expel any volatile gases during charging & discharging unlike some lead-acid batteries and (depending on local regulations) are safe to be installed inside an building without the need for complex venting systems.
Individual lithium cells have a very high energy density and can heat up during use, thus they require sophisticated control systems to monitor cell temperatures and voltage.
Cost of lithium batteries
Until as recently as early 2016 lithium based batteries were considerably higher cost when compared to lead-acid batteries. The added cost is mostly due to the complexity of lithium battery modules which are made up of hundreds of individual cells linked together with complex electronic systems. Individual lithium cells have a very high energy density and can heat up during use, thus they require sophisticated control systems to monitor cell temperatures and voltage. This control system known as a battery management system (BMS) protects the battery from such things as over-current, over-voltage and over-temperature conditions, which can permanently damage the battery and in a worst case scenario lead to a fire.
Due to the rapidly expanding electric vehicle market the cost of lithium battery systems for home use has plummeted. Tesla Motors has now almost completed construction of what has been referred to as the “Giga factory” to supply huge volumes of Li-ion batteries into the market with the aim of driving down the cost of the technology. This together with mainstream car manufacturers producing increasingly more EVs, is predicted to reduce the cost of Li-ion by up to 30% over the next few years. The cost reduction along with the growing number of solar inverter manufacturers producing inverters with chargers compatible with lithium BMS's will see lithium energy storage systems widespread in homes with businesses in the near future.
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