© Toshiba International Corporation The Comparison of Valve Regulated Lead Acid and Lithium Titanate Batteries for UPS Applications Ajinkya Savant, Product Manager, Toshiba International Corporation Published 10th January, 2019 © Toshiba International Corporation Revision: 0 Toshiba Abstract: Uninterruptible Power Supplies specificity, this paper shall focus on VRLA and (UPS) have evolved over the years to become LTO offering for a short duration (less than 15 increasingly more efficient in terms of their minutes) UPS application and compare data energy usage and footprint, but batteries, from an unspecified VRLA manufacturer and essential components of this back-up power Toshiba’s SCiB LTO battery. All data, figures, system, have not been updated with the latest and numbers cited in this paper have been technology for several decades. This paper measured empirically. aims to highlight the differences in performance of valve-regulated lead acid Background: To be able to accurately (VRLA) and lithium titanate (LTO) batteries compare these two different chemistries, it is with respect to their discharging rate, cycle necessary to understand how they are called and shelf life, safety, and specific energy in an into service, how they are manufactured, how UPS application with the goal of demystifying the requirements on them are applied, and the battery selection process between these why they have come to be as they are. Since two options so that customers can make VRLA batteries predate lithium batteries by informed choices. more than a century, most of the requirements in the industry have been set up according to Introduction: Lead acid batteries have the specifications and short-comings specific dominated the UPS application landscape for to the VRLA battery. The background of the several decades and are the archaic default for LTO battery shall be considered first, followed most applications. However, given the by VRLA’s background and finally a quick advancements in lithium-ion battery overview of the UPS market as it stands today. development, specifically with LTO cells, it is worth considering what these cells could offer There have been several LTO manufacturers to the end users and how they could who have tried to innovate and establish revolutionize the market. For the purposes of themselves in the market. Most manufacturers 1 © Toshiba International Corporation have faced difficulties in developing LTO cells have forgotten the reason behind why 10 due to the hefty capital investment, the high minutes is considered the standard back-up degree of technical know-how, and the requirement or why it is prudent to have an meticulous monitoring and control of the n+1 redundancy built into the design. The delicate manufacturing process. LTO archaic 10 minute back-up time requirement manufacturing is especially tricky when trying was derived as a rule of thumb by those who to manufacture in a pouch cell format given understood VRLA degradation, in that 8 the outgassing issues arising out of stringent minutes beginning of life capacity would purity requirements of the anode material and translate to 4 minutes at the end of life for the the electrolyte. Hence, few manufacturers battery. This degradation in capacity coupled remain in this niche and as a result this with the Coup de Fouet effect, renders VRLA chemistry sees limited exposure to UPS batteries unable to provide the required kW applications. Yet, LTO chemistry has proven needed to support the load if the initial battery itself to be a leader in fast charge and runtime was below 10 minutes. As a refresher, discharge applications such as start-stop in Coup de Fouet is the observable drop in passenger vehicles and frequency regulation battery voltage when the batteries are called for utilities. Extrapolating off of this into service. This phenomenon ranges from a performance, LTO seems to be the ideal few milliseconds to a few seconds depending lithium-ion candidate for short duration on the battery chemistry, the state of health, applications. the state of charge and the sizing of the load in comparison to the battery. Typically, as the Since their inception in 1859 at the hands of battery ages, this effect becomes more French physicist Gaston Planté, lead acid prominent and severe. In terms of UPS batteries have found their way into myriad applications, if the generators are unable to applications and have changed form several come online in 2 minutes, then it is doubtful times since. Today, they can be widely broken that they would come online at all in time to down into two categories: VRLA and flooded avoid dropping the load. Hence, the 10 minute or sealed lead acid (SLA) batteries. A relatively back-up time VRLA requirement shields an recent development has been the absorbent actual 2 minute back-up time requirement glass mat (AGM) lead acid batteries and these that VRLA cannot meet. On the other hand, can fall into either of the above mentioned LTO chemistry need not be as grossly categories depending on their construction. oversized and does not suffer from Coup de AGM batteries typically involve less Fouet to the same extent. However, customers maintenance and are more expensive. continue to conflate this issue with runtimes However, lead acid batteries are reaching and to worry about outage scenarios where a saturation in terms of potential advancements longer back-up runtime would be beneficial. and improvements. There are established Such fears are unfounded as the 2003 manufacturers who have been in place for publication from Berkeley National decades and advancements are limited to Laboratories shows that over 67% of total cost increasing surface area of lead plates or the impact by power interruptions is caused by absorbency of glass mats. momentary interruptions, those lasting less than 5 minutes, which are more frequent than In fact, lead acid batteries have been the sustained interruptions, those longer than 5 immutable standard for so long that people 2 © Toshiba International Corporation minutes. In fact, when looking at total compared to lithium based chemistries), incidences of power quality and downtime, the making it essential to oversize the batteries to data is skewed more in favor of short outages, meet the requirements of the application. As a power sags or fluctuations from the utility side note, memory effect is the physical effect lasting shorter than 10 seconds; 96% of all whereby batteries are able to “remember” incidences affecting commercial and suppressed capacities owing to ambient industrial applications are short outages. conditions and then fail to recover capacity Hence, battery back-up should ideally provide when those conditions return to normal. only a few seconds worth of power; enough to However, lead acid batteries are inexpensive provide the generator and back-up generator and hence find use in applications where other two chances to come online. This adds up to be chemistries may not be financially feasible. between 45 seconds and 90 seconds, depending upon the availability of a As seen from the above exposition, the two redundant generator, and can be easily chemistries in consideration are vastly provided by a two minute battery. Thus, an different and as a result have differing ideal setup with an UPS and generator only properties. We will look at a few important needs the batteries to provide 2 minutes of properties and see how this has an impact on back-up runtime, thereby minimizing the the application in the next section. battery requirement and the footprint utilized. Chemistry Comparison: Lithium-ion Similarly, strings of lead acid batteries batteries are more energy dense, lighter, and continue to fail when called into service, discharge faster than VRLA, and LTO is no especially as they age and the load is higher different. However, lithium chemistries differ than normal, so an n+1 redundancy is, in fact, in how safe they are, how long they can last, a necessity. Lithium batteries do not suffer and the temperature ranges in which they can from the same issue and can be called into operate. LTO is leagues ahead of its lithium-ion service in a matter of 10 milliseconds, namely peers in these arenas and is more power dense in grid connected ancillary services and compared to other lithium chemistries, but auxiliary power applications. VRLA cells suffer that is a discussion for a different paper. Please from several short-comings such as Coup de see the following chart for an overview of the Fouet effect, memory effect, and an properties and costs of VRLA and LTO accelerated degradation of capacity (as batteries: Chemistry VRLA LTO Cost Comparison X 2.5X Cell Voltage 2.4V 2.3V Specific Energy 40Wh/kg 90Wh/kg Charge rate 0.5C 7C* Discharge Rate 1C 7C* Cycle Life 200 15000 Operating Temp. 10° to 30°C -30° to 55 °C Table 1: Chemical Properties of LTO and VRLA cells 3 © Toshiba International Corporation As seen on the previous page, VRLA costs less savings in terms of floor loading and the ability compared to LTO. To be precise, LTO is 2.5 to have battery rooms vertically distributed in times the cost of VRLA. There may be some urban landscapes. Another notable difference variance in price which arises out of different is the C-rate between the two; we find VRLA at configurations, run-times, and heat or below 1C while we find LTO at 7C. C-rate, is dissipation requirements; however, that is the the inverse proportion of nominal battery only area where VRLA fares better than LTO. capacity and the time taken to charge (or LTO energy density is 90Wh/kg compared to discharge) that capacity. Hence, we can see VRLA’s 40Wh/kg, which means that LTO is that LTO is capable of discharging the battery more than twice as energy dense as VRLA.