Lithium-ion battery

Lead acid batteries continue to be the standard for energy storage systems globally due to their low cost, reliability, and availability. Lithium-ion batteries are beginning to be used more frequently in energy storage systems as they offer several appealing characteristics for stand-alone PV systems. There are a variety of different lithium-ion chemistries, but there are two in particular that have emerged as the most popular technologies for stand-alone PV systems: lithium iron phosphate (LFP) and lithium nickel manganese cobalt (NMC).

Lithium-ion batteries are composed of many small cells that are wired in series and parallel to achieve a desired nominal voltage. These cells are very sensitive to over-charging and therefore require a battery management system (BMS) to ensure proper battery charging. Improper charging can not only irreparably damage the batteries but can also lead to a thermal run-a-way event with NMC batteries in which the temperature of a battery can no longer be controlled and rises to the point that equipment is damaged, or a fire is started.

The design of lithium-ion batteries and the presence of a BMS systems means that the batteries are designed to function at a set nominal voltage and are not intended to be wired in series like is done with lead acid batteries. They are sold in nominal voltages that can be wired in parallel to achieve the desired Ah or kWh capacity. One of the advantages of the BMS systems that accompany each battery is that proper charging is ensured for all batteries connected in parallel meaning that there is typically no limited to the number of batteries that can be put in parallel like exists with lead acid batteries (it is best to limit the number of lead acid batteries in parallel to no more than three strings to ensure proper charging).

Lithium ion battery types

Diagrams comparing different lithium-ion battery types. The outermost ring is the highest rating or performance. Innermost ring is the lowest rating or perfomance.
(1) Energy density (2) Power density (3) Safety (4) Toxicity (5) Cycle life (6) Cost

Lithium iron phosphate (LFP) and Lithium nickel manganese cobalt (NMC) batteries each have distinct advantages that lend themselves to different applications.

LFP

LFP batteries have emerged as the preferred technology for stand-alone systems as they offer several advantages when compared to NMC batteries: a longer cycle life, a lower price point per kWh, a less toxic chemistry, and a lower probability of thermal runaway events. The characteristics of LFP batteries enable manufacturers to build smaller batteries that can function as drop-in replacements for lead acid batteries in many different systems.

NMC

The main advantage of NMC batteries is that they have a higher energy density (can store more energy for the same volume and weight) and can function with higher charge/discharge currents. NMC batteries are typically found in larger integrated systems like those built by LG Chem and Tesla.

Compatibility with other equipment

It is important to make sure that any equipment used with lithium-ion batteries is compatible with them. Many have different parameters and voltages than lead acid batteries that will require additional programming. These parameters vary significantly - consult the manual or manufacturer of any battery being considered.

Battery storage

Lithium-ion batteries often come with enclosures or racks provided by the manufacturer. The manual and manufacturer should be consulted to determine the appropriate means of storage. They should be stored in an enclosure or room that prevents unauthorized access via a lock. If being installed in a harsh environment, it is ideal if they are kept in a climate-controlled environment. They cannot be charged below 0°C.

Safety

Lithium-ion batteries are always sealed and do not, except during an a thermal runaway event or an accident, present a risk for exposure to hazardous chemicals. In the case of electrical hazards, they still carry risks of arc flashes and a potential for shocks with higher voltage systems, but they are also typically safer than lead acid batteries when it comes to electrical hazards as they often have additional safety measures provided by the battery management system, overcurrent protection devices, or an integrated disconnect. These measures work to ensure that the battery does not provide current unless properly connected and activated by the installer.

Recyclability

Recycling lithium-ion batteries can be somewhat of a challenge when compared to lead acid batteries which have a global recycling network. This is partly because many of materials that they contain are not as valuable as lead, but also because the recycling processes are more complicated which has limited the number of facilities globally that are capable of handling them. The appropriate way to recycle a battery depends upon the manufacturer and its chemistry - they manufacturer will be the best resource for determining how to dispose of batteries. Many LFP batteries contain no hazardous chemicals, which is a distinct advantage over lead acid batteries and NMC batteries.

Comparison with lead acid batteries

Lithium-ion batteries have both advantages and disadvantages with compared with lead acid batteries.

Advantages:

• Higher energy density. They can store significantly more energy in the same space/weight.
• Longer cycle life. They can provide 2-3 times as many cycles as lead acid batteries.
• Can accept a high charge current until nearly completely full - no absorption phase is required. This allows allows more rapid and efficient charging during the limited daylight hours when the PV source is operating or when running a generator.
• Higher round trip efficiency. 95-98% as compared to 80-90% efficiency for lead acid batteries.
• Higher depth of discharge. 80-100% with each cycle is possible as compared to recommended maximum of 50% with lead acid batteries.
• Voltage is not very responsive to temperature, which means that lithium ion batteries perform better in warm temperatures. Temperature compensated charging is not required.

Disadvantages:

• Higher upfront investment. Although the cost per cycle of some lithium-ion energy storage systems is comparable to that of lead acid batteries.
• Very sensitive to over-discharging and over-charging. The input and output current is limited and if exceeded can irreparably damage the batteries.
• Require a battery management system.
• Risk of thermal runaway events.
• Far more sensitive to low temperatures. Cannot be charged below 0°C.
• Limited availability.
• Limited compatible equipment availability.

Notes/references

Hydrowires - Energy Storage Technology and Cost Characterization Report
Chemical and Engineering News - It’s time to get serious about recycling lithium-ion batteries
Isidor Buchman - Batteries in a Portable World
Thomas Reddy - Linden's Handbook of Batteries, 4th Edition