FAQ: Using lithium-ion batteries in UPS for data centers

Lithium-ion batteries have a number of advantages over traditional valve-controlled lead-acid batteries (VRLA), which are widely used in UPS today. Much longer life, smaller size and weight, faster recharge times, and lower prices have made lithium-ion batteries an attractive energy storage technology. This article provides brief answers to the most common questions about lithium-ion batteries and their use by UPS and is intended to help the user decide which battery technology is best suited for use in backup power supplies.

What is a lithium-ion battery and how does it differ from a lead-acid battery?

Simply put, a battery is an electrochemical device that stores energy and releases it as electricity. Batteries are usually connected in series, parallel, or in combination to achieve any voltage and current. This simple description applies to both lead-acid and lithium-ion batteries. Each battery contains a cathode (positive electrode) and an anode (negative electrode), which are located in the electrolyte. The electrolyte acts as a catalyst for an electrochemical reaction that leads to charging and discharging as ions flow from one electrode to the other. It also prevents the reverse flow of electrons accumulated at the anode to the cathode inside the battery itself when there is no load. The chemical reaction results in a charge potential difference (i.e., voltage) between the cathode and the anode when electrons accumulate on the anode. When a load is connected by a wire to the battery terminals, a current is induced that discharges the battery as a stream of electrons (i.e. current) from the anode (negative terminal) to the load, and then to the cathode (positive terminal). The chemical composition of the battery changes as this ion flow occurs until more electrons are applied to the anode, which leads to battery discharge. The battery can be recharged using an external power source to reverse the flow of electrons through the electrolyte from the cathode to the anode.

Property Lithium-Ion batteries Lead-acid (VRLA)
Energy density, W/Kg 70 - 60 15 - 50
life Cycle 10 - 15 years 4 - 6 years
Number of charge/discharge cycles >1000 200 - 400
Recharge time 1 hour 6 - 12 hours

Table 1. Typical performance ranges for the type of battery used in the UPS today.

So, the main difference between a lithium-ion battery and a lead-acid battery is the chemical composition of the materials used in the electrodes and electrolyte. Most modern lithium-ion batteries use metal oxide for the cathode and carbon-based material for the anode. The electrolyte solution is a lithium salt dissolved in an organic solvent. A lead-acid battery, on the other hand, uses lead dioxide for the cathode, lead anode, and a form of sulfuric acid as the electrolyte. This chemistry largely determines the battery’s operational capabilities. Table 1 highlights some of the high-level performance differences between the two types of batteries used in UPS applications. However, it is important to understand that there can be large differences in performance between one battery and another of the same type due to differences in chemistry (for example, in the composition of the electrolyte and electrodes) and the general type and quality of materials used, as well as the design of the cell. This makes it difficult to generalize or generalize the characteristics of lithium-ion batteries unless their specific application and design is intended.

How do the costs of lithium-ion battery systems compare with the costs of lead-acid battery systems when using a UPS?

In General, we can say that the installed cost of a lithium-ion system is about 1.2-2 times more than that of a lead VRLA at the same power level and operating time. The high purchase price was the main reason why the use of lithium-ion batteries in UPS was so limited. However, it is worth noting that just a few years ago, this difference in cost was 10 times. with this reduction in cost, combined with high operational advantages, lithium-ion systems are increasingly becoming a viable energy storage option for an increasing number of people. Growing demand, future technological improvements, and additional improvements in production efficiency are likely to lead to further cost savings. Lead-acid batteries, on the other hand, represent an older, more Mature technology with less chance of significant evolution, which would significantly change the cost per kWh much further down.

On the operating cost side (OPEX), the lithium-ion system has a clear advantage. This is mainly due to the fact that the life span is approximately twice (or more) that of VRLA systems. Over a 15-year period, lead-based batteries will probably need to be replaced 2-3 times, while lithium-ion batteries may not need to be replaced at all (or perhaps just once), which will significantly save money and reduce maintenance. The service life of a lithium-ion battery is often calculated for operating conditions at a higher temperature (40°C/104°F) than lead-acid conditions (20-25°C/68-77°F).

This can lead to additional savings in operating costs by reducing cooling energy. And since lithium-ion battery systems are 50-80% smaller, the available space could be used more productively. The financial value of this can further increase the total cost of ownership of lithium-ion systems. In General, if we consider a 10-year cycle, UPS systems based on lithium-ion batteries have a TCO of 10-40% less than systems with lead batteries.

How safe are lithium-ion batteries?

All types of batteries, by definition, store chemical energy, so each battery, if handled incorrectly (for example, thrown into a fire) or overcharged, can potentially present a hazard, releasing hazardous materials or igniting a fire. Lithium-ion batteries are considered more unstable due to reports of fires and because of their much higher specific energy combined with greater sensitivity to overcharging. An improperly used lithium-ion battery is easier to achieve a “thermal separation” state because it has a lower cell resistance and a higher energy storage capacity than a lead-acid battery. However, significant progress has been made over the years, making them safer and much more comparable in terms of safety to other widely used battery types. Chemical changes and improvements in cell packaging have made them more stable. Production processes have matured, and the materials used have become more durable. Battery management schemes that prevent overheating or overcharging are well tested and proven to be effective in the field.

The widespread use of lithium batteries in the production of millions of portable electronic devices, smartphones and electric vehicles is the best evidence of their high level of safety. Because lithium-ion battery systems are much more sensitive to how they charge and discharge, they all include a battery management system, or BMS, consisting of microprocessors, sensors, switches, and related circuitry. It constantly monitors the battery temperature, charge level, and charge rate at the cellular level to protect against short circuits and overcharging. The system is also a tool to protect cells from damage by preventing too low voltage when discharging. The BMS provides the UPS and the user with accurate information about the battery status, health, and available runtime. Although BMS makes lithium - ion battery systems much safer, they themselves have a high cost. Lithium-ion batteries do not contain mercury, lead, cadmium, or any other material considered dangerous. Of course, this doesn’t mean that recycling or not recycling doesn’t have an environmental impact, but the US government considers them safe enough to be disposed of in landfills.

What is the difference between the types of lithium-ion batteries?

All lithium-ion batteries today use a non-metallic solution containing lithium-ions as the electrolyte. This layer is an electron conductor in which current flows between two electrodes, the cathode (+) and the anode (-). the Cathode is a metal oxide, and the anode is made of a porous carbon material. By changing the type of materials used, you can change the characteristics of the batteries themselves.

Some batteries, for example, are designed in such a way as to maximize the energy capacity and provide a long operating time, measured in hours. They are often called “energy cells”.

On the other hand, “power elements” are configured to provide a very high power density (i.e. specific power), but with a lower energy capacity, when all of its power can be applied to the load in a short period of time. Since UPS systems are usually configured to consume their batteries for a short period of time (minutes), power cells are used here. Thus, batteries are designed for their intended use. And new chemical technologies are being developed and tested to further push the boundaries of modern lithium-ion battery technology. A common way to differentiate types of lithium-ion batteries is based on their main active chemical material, which gives the battery its unique, inherent properties compared to other lithium-ion batteries. According to Battery University3, six of the most common types are:

  • Lithium-cobalt (LiCoO2)
  • Lithium-manganese (LiMn2O4 or “LMO”)
  • Lithium-Nickel-manganese-cobalt-oxide (LiNiMnCoO2 or “NMC”)
  • Lithium-iron-phosphate (LiFePO4)
  • Lithium-Nickel-cobalt-aluminum-oxide (LiNiCoAIO2)
  • Lithium-titanate (Li4Ti5O12)

Table 2 shows how different types of chemistry differ from each other in terms of their key attributes

Сhemical composition Using for the battery high capacity Using for the battery high current Safety Efficiency Life cycle Expensiveness
LiCoO2 (LCO) High Average Average High Medium Medium
LiMn2O4 (LMO) High High High Average Average Average
LiNiMnCoO2 (NMC) Highest High High High Long Average
LiFePO4 (LFP) Average High High High Very long Average
LiNiCoAIO2 (NCA) Highest High Average High Long High
Li4Ti5O12 (LTO) Medium High Highest Top Very long Top

There are other aspects besides chemistry that distinguish one battery from another. Individual battery cells can be Packed differently, and this has an impact on battery performance. There are prismatic bags and cans, as well as cylindrical cans. The shape of the elements, as well as the type and quality of materials used in their construction, depends on the weight of the battery, energy density, ability to conduct heat, durability (safety) and price.

Type of lithium-ion batteries: a) - cylindrical, I) - prismatic, C) - bag

But even two batteries of the same type with the same type of chemistry can have different characteristics. UPS manufacturers must select the cell design and material quality level appropriate for the intended application.

Can I use Li-ion batteries with an existing UPS?

Only if the UPS manufacturer says that the specific lithium-ion battery in question is compatible. As explained above, lithium batteries come in a wide variety of types, sizes, voltage ranges, form factors, and connector types. These differences should be taken into account when implementing the UPS inverter, charger, mechanical device, and embedded software. However, assuming that the battery system voltage is within the capabilities of the UPS, it is possible that the existing UPS could be made compatible by updating the UPS firmware to ensure that charging procedures are correctly implemented, run-time calculations are correct, and charge status is accurately reported. Always contact the manufacturer to determine which batteries are safe and compatible.

How do lithium-ion batteries differ in size and weight from lead-acid batteries?

In General, a Li-ion battery system for UPS will take up 50-80% less floor space and weigh 60-80% less than a similar lead-acid system. This significant savings are due to the very high specific energy (energy density) characteristic of lithium-ion batteries. The specific power of lithium batteries available today ranges from 70 WH/kg to 260 WH/kg, with most in the 120-200 range. For comparison, a typical lead-acid battery is in the range of 30-50 WH/kg.

What is the service life of lithium-ion batteries?

The cycle life is the number of times the battery can be fully discharged and charged over a certain temperature range before it needs to be replaced. As soon as the battery capacity reaches 60-80% when fully charged, this battery becomes unusable and must be replaced. For a traditional sealed lead-acid battery, the service life is between 200 and 400 cycles. A typical lithium-ion battery is already designed for more than 1000 cycles. This number depends on several factors, including the specific chemistry used in the design. Some lithium-ion batteries today can do more than 5,000 cycles.

What type of service is required?

Lithium-ion batteries have very low maintenance requirements. There is no liquid level to measure and maintain (as for valve-controlled lead-acid batteries). There is no battery “memory " to worry about, nor the need to cycle the batteries periodically to calibrate their running time. And their long service life-10 years or more-reduces the need to replace them during the entire life of the UPS. By the time the batteries need replacing, it will probably be time to replace the UPS as well. The included battery management system (BMS) automatically collects and reports all the necessary data to accurately understand the state and condition of the battery system, further reducing maintenance time.

Do I need to force-cool the lithium-ion batteries used in the UPS?

Both lead-acid and lithium-ion batteries will suffer from high temperatures. However, in General, the life of lithium-ion batteries is less affected by higher temperatures than lead-acid batteries. Many Li-ion batteries used in UPS are designed for higher average temperatures (for example, 40°C/104°F). For both types of batteries, the capacity (ampere-hours or “Ah”) actually increases with increasing ambient temperature, but higher temperatures may limit the available battery life even within the acceptable operating range specified by the manufacturer. This happens rarely and depends on several factors, including the discharge rate, the heat sink capability of the specific battery design in question, and, of course, the temperature. In particular, at higher discharge rates, the battery may reach its thermal limit and shut down before its available capacity is actually used.

Most UPSs with lithium-ion batteries do not require additional cooling to maintain the specified durability, but keeping the battery at a temperature (for example, 25°C) can help achieve the reliability that the manufacturer has set.

How should lithium-ion batteries be stored?

storing them in a cool place slows down the aging process of lithium-ion (and other chemical) compounds. Manufacturers recommend a storage temperature of 15°C. In addition, the battery must be partially charged before being stored. Many manufacturers recommend a 20% -40% charge level.

Is it possible to Hot Swap them ?

No, lithium-ion batteries by their nature are not amenable to hot swap. This is due to their high sensitivity to excessive and insufficient charge (low impedance).

Are lithium-ion batteries environmentally friendly?

There are many different ways that you can consider a product more environmentally friendly or not than another. Lithium-ion batteries do not contain hazardous materials, while lead-acid batteries do (for example, lead). Both types of batteries are recyclable, but it is now much easier to dispose of lead in most regions of the world than the larger-format lithium-ion batteries used in UPS and electric vehicles. However, to get a complete picture of the environmental impact, consider the entire carbon footprint over the battery’s life cycle. Carbon use accumulates throughout the product life cycle:

  1. extraction of raw materials
  2. energy for production and transportation
  3. operating energy to keep the batteries charged and cool
  4. recyclability and impact on the land when it’s time to recycle

The analysis shows that operational losses (i.e., the energy used to keep the batteries charged) are currently the dominant driver of the carbon footprint of the UPS and its battery system over the 10-year life cycle. However, there is not much difference in operational losses between the two systems. Which one borders on the other depends on the actual use case. Lithium-ion batteries require less energy to maintain a charge than lead-acid batteries. The charge cycle of a lithium-ion battery is 90% efficient compared to 80-85% for a lead-acid battery. In addition, lead-acid batteries self-discharge at a faster rate than lithium-ion batteries. However, this increase in efficiency is offset by the need to use the Li-ion battery management system (BMS) for short-circuit and overcharge protection, which also consumes energy. Thus, the total operating losses of the two types of batteries are very similar to each other.

Given that lithium-ion batteries containing landfill-safe materials are recyclable, as well as the fact that their service life is 2-3 times longer than that of lead-acid batteries, it can be argued that lithium-ion batteries are “greener". However, please note that the percentage of lead recycling from lead-acid batteries is 99%, and more than 90% of the batteries that have expired are sent for recycling.

However, the processing of lithium-ion batteries, especially larger-format batteries (such as those used in electric vehicles and data center UPS), is not yet fully developed.

Are lithium-ion batteries suitable for recycling?

Yes, they are recyclable. And there are many processors that will take small lithium-ion batteries. However, at the time of this writing, most of the smaller-format batteries are simply assembled and then sent for shredding and incineration, where some of the materials used in production can be recovered for later use. But still, most of the material from lithium-ion batteries ends up in landfills. From a purely financial point of view, recycling lithium-ion batteries to reuse very small amounts of lithium metal and other more common but less valuable metals (aluminum, Nickel, etc.) is not worth the effort.

Research is continuing to improve the recycling economy, and governments are beginning to encourage, in particular, or explicitly require the collection and proper recycling of batteries. Recycling larger-format lithium-ion batteries (such as those used for electric vehicles and data center UPS) is more challenging. At the time of this writing, there are several companies that can recycle large lithium-ion battery systems. The market is immature. So far, very few of these batteries have run out of life.

Are there any special restrictions for transportation of Li-Ion Batteries?

Yes, the rules vary by region, but a good guide to understanding the restrictions and requirements for air transport is the international air transport Association (IATA) and its “dangerous goods regulations” (DGR)7, which describe air transport mandates by size, weight, and quantity.

Keep in mind that all types of batteries face certain requirements and limitations.

Do I need a battery management system when using Li-ion?

Yes, and it is already included in every Li-ion battery system. As already mentioned, lithium-ion batteries are very sensitive to overcharging, short circuits, and too much discharge. The control system continuously monitors each battery element and manages the charging system to ensure that these conditions do not occur or cause damage or overheating.

If you are designing an uninterruptible power supply system using lithium-ion cells without BMS, you must install such a system separately.

How does the recharge time compare to lead-acid batteries?

the charging time in the UPS is determined not so much by the battery as by the power of the rectifier, so the time to reach 80% charge state (SOC) will be more or less the same for both types of batteries. But it takes less time for a lithium-ion battery to go from 80% to 100% charge.

If we are not limited by the power of the UPS charger, of course the Li-ion battery is much better: it will reach 100% charge in 30-60 minutes, and this will be 5-10 hours less than in the case of a VRLA battery of the same capacity.

Conclusion

the technology of lithium-ion batteries has evolved so that they are now viable and safe for use in large devices such as electric vehicles and data center UPS. Their unique chemistry and cell packaging offer advantages over more traditional battery alternatives. When used with a UPS, lithium-ion batteries are smaller, lighter, charge faster, and have twice or longer calendar life than lead-acid batteries, the dominant energy storage technology used in battery backup today.

Li-ion may be a more environmentally friendly solution. And, perhaps surprisingly, given the higher cost of acquisition, has a lower total cost of ownership (TCO).