For many years lead acid and nickel based batteries dominated the industry. Nickel batteries (NiCd, NiMH) are being phased out due to a combination of cost and environmental factors. Lead acid has been around for over 100 years and will be a market force for the foreseeable future due to its low cost and established manufacturing base. Lithium-ion is a well-established technology for portable electronics and it is increasing its role in larger scale applications; it is emerging as a contender in certain stationary applications where volume, weight, temperature sensitivity or low maintenance is more important than initial cost. The following chart illustrates how lead acid and lithium-ion fit into the rechargeable battery world. Today, lithium-ion is the fastest growing and most promising battery chemistry.
Pioneer work with the lithium battery began in 1912 under G.N. Lewis but it was not until the early 1970s when the first non-rechargeable lithium batteries became commercially available. lithium is the lightest of all metals, has the greatest electrochemical potential and provides the largest energy density for weight.
Attempts to develop rechargeable lithium batteries failed due to safety problems. Because of the inherent instability of lithium metal, especially during charging, research shifted to a non-metallic lithium battery using lithium ions. Although slightly lower in energy density than lithium metal, lithium-ion is safe, provided certain precautions are met when charging and discharging. In 1991, the Sony Corporation commercialized the first lithium-ion battery. Other manufacturers followed suit.
The Li-Ion battery employs a Lithium metal oxide cathode and a carbon anode with an organic electrolyte. The high single cell voltage not only results in high performance, but also allows the use of fewer cells, compared to other battery systems.
In lithium based batteries, the anode is made of carbon, while the cathode is a lithiated metal oxide (LiCoO2, LiMO2, etc.). The electrolyte is made up of lithium salts (such as LiPF6) dissolved in organic carbonates. When the battery is being charged, the Lithium atoms in the cathode become ions and migrate through the electrolyte toward the carbon anode where they combine with external electrons and are deposited between carbon layers as lithium atoms. This process is reversed during discharge. Because lithium reacts to water, non-aqueous solutions are used.
The energy density of lithium-ion is typically twice that of the standard nickel-cadmium. There is potential for higher energy densities. The load characteristics are reasonably good and behave similarly to nickel-cadmium in terms of discharge. The high cell voltage of 3.6 volts allows battery pack designs with only one cell while a nickel-based pack would require three 1.2-volt cells connected in series.
Lithium-ion is a low maintenance battery, an advantage that most other chemistries cannot claim. There is no memory and no scheduled cycling is required to prolong the battery's life. In addition, the self-discharge is less than half compared to nickel-cadmium, also lithium-ion cells cause little harm when disposed.
Despite its overall advantages, lithium-ion requires a protection circuit to maintain safe operation. Built into each pack, the protection circuit limits the peak voltage of each cell during charge and prevents the cell voltage from dropping too low on discharge. In addition, the cell temperature is monitored to prevent temperature extremes. The maximum charge and discharge current on most packs are is limited to between 1C and 2C.
Some capacity deterioration is noticeable after one year, whether the battery is in use or not. At the same time, lithium-ion packs are known to have served for five years in some applications.
Storage in a cool place slows the aging process of lithium-ion (and other chemistries). Manufacturers recommend storage temperatures of 15°C . In addition, the battery should be partially charged during storage. The manufacturer recommends a 40% charge.
The most economical lithium-ion battery in terms of cost-to-energy ratio is the cylindrical 18650 (size is 18mm x 65.2mm). This cell is used for mobile computing and other applications that do not demand ultra-thin geometry. If a slim pack is required, the prismatic lithium-ion cell is the best choice. These cells come at a higher cost in terms of stored energy.
- High energy density - potential for yet higher capacities.
- Does not need prolonged priming when new. One regular charge is all that's needed.
- Relatively low self-discharge - self-discharge is less than half that of nickel-based batteries.
- Low Maintenance - no periodic discharge is needed; there is no memory.
- Specialty cells can provide very high current to applications such as power tools.
- Requires protection circuit to maintain voltage and current within safe limits.
- Subject to aging, even if not in use - storage in a cool place at 40% charge reduces the aging effect.
- Transportation restrictions - shipment of larger quantities may be subject to regulatory control. This restriction does not apply to personal carry-on batteries.
CEGASA SPECIFIC CHEMISTRY
There are many Li-ion chemistries each one with different characteristics. CEGASA use depending on the specific application a high energy chemistry (NMC) or a high number of cycles (LFP).
NMC chemistry provides the greatest balance of power, energy, safety, and overall performance compared to other lithium-ion technologies, including Lithium Manganese Spinel.
NMC provides significantly greater specific energy compared to lithium iron phosphate technology. while lithium iron phosphate technology provides more safety and cycle than NMC technology.
CEGASA use two cell types:
- The cylindrical 18650 & 26650 with metallic case.
- The prismatic lithium-ion cell with plastic case ( pouch type).
CHARGING LITHIUM-ION BATTERY
The Li‑ion charger is a voltage-limiting device that is similar to the lead acid system. The difference lies in a higher voltage per cell, tighter voltage tolerance and the absence of trickle or float charge at full charge.
While lead acid offers some flexibility in terms of voltage cut‑off, manufacturers of Li‑ion cells are very strict on the correct setting because Li-ion cannot accept overcharge. The so-called miracle charger that promises to prolong battery life and methods that pump extra capacity into the cell do not exist here. Li-ion is a “clean” system and only takes what it can absorb. Anything extra causes stress.
Most cells charge to 4.15 V/cell with a tolerance of +/–50mV/cell. Higher voltages could increase the capacity, but the resulting cell oxidation would reduce service life. More important is the safety concern if charging beyond 4.20V/cell.
Li-ion is fully charged when the current drops to a predetermined level or levels out at the end of Stage 2. In lieu of trickle charge, some chargers apply a topping charge when the voltage drops to 4.05V/cell (Stage 4).
Li-ion does not need to be fully charged, as is the case with lead acid, nor is it desirable to do so. In fact, it is better not to fully charge, because high voltages stresses the battery. Choosing a lower voltage threshold, or eliminating the saturation charge altogether, prolongs battery life but this reduces the runtime. Since the consumer market promotes maximum runtime, these chargers go for maximum capacity rather than extended service life.
Relying on the closed circuit voltage (CCV) to read the available capacity during charge is impractical. The open circuit voltage (OCV) can, however, be used to predict state-of-charge after the battery has rested for a few hours. The rest period calms the agitated battery to regain equilibrium. Similar to all batteries, temperature affects the OCV. Read "How to Measure State-of-Charge".
Li-ion cannot absorb overcharge, and when fully charged the charge current must be cut off. A continuous trickle charge would cause plating of metallic lithium, and this could compromise safety. To minimize stress, keep the lithium-ion battery at the 4.20V/cell peak voltage as short a time as possible.
Battery professionals agree that charging lithium-ion batteries is simpler and more straight forward than nickel-based systems. Besides meeting the voltage tolerances, the charge circuits are relatively simple. Limiting voltage and observing low current in triggering “ready” is easier than analyzing complex signatures that may change with age. Charge currents with Li-ion are less critical and can vary widely. Any charge will do, including energy from a renewable resource such as a solar panel or wind turbine. Charge absorption is very high and with a low and intermittent charge, charging simply takes a little longer without negatively affecting the battery.
Lead acid batteries have many disadvantages; it is heavy and is less durable than nickel- and lithium-based systems when deep-cycled. A full discharge causes strain and each discharge/charge cycle permanently robs the battery of a small amount of capacity. This loss is small while the battery is in good operating condition, but the fading increases once the performance drops to half the nominal capacity. This wear-down characteristic applies to all batteries in various degrees.
Depending on the depth of discharge, lead acid for deep-cycle applications provides 200 to 300 discharge/charge cycles. The primary reasons for its relatively short cycle life are grid corrosion on the positive electrode, depletion of the active material and expansion of the positive plates. These changes are most prevalent at elevated operating temperatures and high-current discharges.
Charging a lead acid battery is simple but the correct voltage limits must be observed, and here there are compromises. Choosing allow voltage limit shelters the battery but this produces poor performance and causes a buildup of sulfation on the negative plate. A high voltage limit improves performance but form grid corrosion on the positive plate. While sulfation can be reversed if serviced in time, corrosion is permanent.
Lead acid does not lend itself to fast charging and with most types, a full charge takes 14 to16 hours. The battery must always be stored at full state-of-charge. Low charge causes sulfation, a condition that robs the battery of performance. Adding carbon on the negative electrode reduces this problem but this lowers the specific energy.
Lead acid has a moderate life span and is not subject to memory as nickel-based systems are. Charge retention is best among rechargeable batteries. While NiCd loses approximately 40 percent of its stored energy in three months, lead acid self-discharges the same amount in one year. Lead acid work well at cold temperatures and is superior to lithium-ion when operating in subzero conditions.
The specific energy of Li‑ion is twice that of NiCd, and the high nominal cell voltage of 3.60V as compared to 1.20V for nickel systems contributes to this gain. Improvements in the active materials of the electrode have the potential of further increases in energy density. The load characteristics are good, and the flat discharge curve offers effective utilization of the stored energy in a desirable voltage spectrum of 3.70 to 2.80V/cell. Nickel-based batteries also have a flat discharge curve that ranges from 1.25 to 1.0V/cell.
COMPARING LITHIUM-ION TO LEAD ACID BATTERIES
The following tables provide a brief comparison of lead acid to lithium-ion (NCM) on a pack level. It should be noted that both chemistries have a wide range of parameter values, so this table is only a simplified representation of a very complex comparison.
An interesting point in this table is that the different chemistries have different typical state of charge windows. The implication of this is that a lead acid system must have a larger nameplate energy capacity than the lithium-ion system to have the same amount of available energy.
Given the significant differences in technical and economic characteristics of the battery types, it stands to reason that the “best” solution for which battery type to use is application specific. Following is a more in-depth look at some of the topics addressed.
Cycle Life Comparison
Lithium-ion has significantly higher cycle life than lead acid in deep discharge applications. The disparity is further increased as ambient temperatures increase.
When determining what capacity of battery to use for a system, a critical consideration for lead acid is how long the system will take to discharge. The shorter the discharge period, the less capacity is available from the lead acid battery.
Cold Weather Performance
Both lead acid and lithium-ion lose capacity in cold weather environments, but lithium-ion loses significantly less capacity as the temperature drops into the -20°C range.
Lead acid batteries compare poorly to lithium-ion with regards to environmental friendliness. Lead acid batteries require many times more raw material than lithium-ion to achieve the same energy storage, making a much larger impact on the environment during the mining process. The lead processing industry is also very energy intensive, leading to large amounts of pollution. Although lead is highly hazardous to human health, the manufacturing methods and battery packaging make the human risk negligible. On the plus side, over 97% of lead acid batteries in the United States are recycled, which makes a huge impact on the environmental equation.
Lithium is not without its own environmental problems. The major components of a lithium-ion cell require the mining of lithium carbonate, copper, aluminum, and iron ore. Lithium mining specifically is resource intensive, but lithium is only a minor portion of the battery cell by mass, so the aluminum and copper environmental impacts are much more significant. The lithium-ion recycling industry is only in its infancy right now, but the cell materials have shown high ability for recovery and recyclability, so it is expected that lithium-ion recycling rates will rival lead acid.
Lead acid and lithium-ion cells are both capable of going into “thermal runaway” in which the cell rapidly heats and can emit electrolyte, flames, and dangerous fumes. The likelihood and consequences of an event are higher for lithium-ion as it has a higher amount of energy in a smaller volume. Multiple cell and pack safety precautions shown in Figure 18 Lithium-ion safety mechanisms are taken to prevent trigger events, such as short circuits and overheating, but incidents still occur.
RESTRICTIONS ON SHIPMENT OF LITHIUM-ION BATTERIES
- Anyone shipping lithium-ion batteries in bulk is responsible to meet transportation regulations. This applies to domestic and international shipments by land, sea and air.
- Lithium-ion cells whose equivalent lithium content exceeds 1.5 grams or 8 grams per battery pack must be shipped as "Class 9 miscellaneous hazardous material." Cell capacity and the number of cells in a pack determine the lithium content.
- Exception is given to packs that contain less than 8 grams of lithium content. If, however, a shipment contains more than 24 lithium cells or 12 lithium-ion battery packs, special markings and shipping documents will be required. Each package must be marked that it contains lithium batteries.
- All lithium-ion batteries must be tested in accordance with specifications detailed in UN 3090 regardless of lithium content (UN manual of Tests and Criteria, Part III, subsection 38.3). This precaution safeguards against the shipment of flawed batteries.
- Cells & batteries must be separated to prevent short-circuiting and packaged in strong boxes.