A confluence of events is setting the stage for what may well be dramatic change in a key component of uninterruptible power supply (UPS) technology that really hasn’t changed much for 40 years or more.
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I’m referring to the lead acid battery, the energy storage technology used in UPS systems, which remains much the same now as it did decades ago. But as the industry develops new types of batteries for devices ranging from smart phones to electric automobiles, we can clearly see the day when UPSs take advantage of these developments.
It’s coming at a good time, because customers are facing some difficult challenges with respect to UPSs, whether they’re for data centers, critical buildings, industrial processes or critical infrastructure. These challenges are driving the need for specific UPS requirements, including:
I believe lithium ion (Li-ion) batteries on Wikipedia hold great promise to address all of these challenges and requirements. In this post I’ll explain the four main reasons why.
First, Li-ion batteries provide multiple times the energy and power density as compared to valve-regulated lead-acid batteries (VRLA), which are the most common type currently used in UPS systems. As a result, UPSs built with Li-ion batteries take up only about one-third the space or less of a VRLA-based solution that delivers the same power.
That smaller footprint translates to reduced cooling requirements as well as about a two-thirds reduction in weight, at least. That means customers have more flexibility in terms of where they install the systems and can often avoid costly building modifications.
Li-ion batteries can also withstand a wider temperature range than VRLA batteries. The rule of thumb is that VRLA battery life is reduced by half for every 10°C (18°F) increase above 25°C (77° F) ambient temperature. Li-ion batteries when use at low discharge rate are far less sensitive to temperature fluctuations and can accept spikes in temperature with almost no effect on battery life. For very short run time and high discharge rates like in UPS application the Li-ion are still sensitive to temperature but to a lesser extent.
A third benefit is that Li-ion batteries always come with sophisticated battery monitoring systems (BMS) that provide a clear picture of battery runtime and health. It’s essentially the same technology that enables you to easily see how much battery life is left in your smart .
In contrast, VRLA batteries rely on chemistry that makes it hard to accurately predict when they’re going to fail. Think about your car battery: it may crank perfectly fine one day but the next it’s a little chilly and the battery fails, without warning. That won’t happen with Li-ion batteries.
Which leads to the final benefit of Li-ion batteries for UPSs: increased life expectancy. In theory, VRLA batteries used in UPS systems have a life expectancy of 10 years. But due to the constraints around being able to determine their actual health and life expectancy, in practice most customers replace them after 5 or 6 years.
In contrast, Li-ion batteries of the sort best suited for UPSs are expected to last for more than 10 years, reducing the burden and cost of battery replacements, as well as the risks of down time or load interruption during maintenance.
Of course no new technology comes without certain implementation challenges and Li-ion batteries are no different. First is the need to find the type of Li-ion battery that’s best suited for UPS applications. UPS requirements are quite different from those for, say, an electric car battery. Car batteries are designed to store lots of energy so the car can travel as many miles as possible before recharging. With UPS batteries, the concern is not length of run time so much as the need to deliver a lot of power quickly for a short period of time, usually just a few minutes until the backup generators kick in.
For a UPS we’re also not really interested in a battery that can cycle on and off thousands of times, because a UPS kicks in only occasionally. Rather, we need it to be highly reliable and safe, with a long life expectancy.
Secondly, we need a battery that can deliver a lower total cost of ownership (TCO) as compared to VRLA batteries. Li-ion batteries are already competitive on that front. They may cost more up front, but will last about twice as long as VRLA batteries. Li-ion batteries also have a far smaller footprint, which drives down both space and cooling requirements – delivering further cost savings.
I expect the TCO story to get even better in coming months and years, since Li-ion technology is still quite new with respect to UPSs. Prices should fall at a much faster rate than that for the mature VLRA technology.
I’d love to hear your thoughts on Li-ion technology and whether you think it could help address your own UPS challenges and requirements. Please let me know what you think using the comments below.
Charging and discharging batteries is a chemical reaction, but Li-ion is claimed to be the exception. Battery scientists talk about energies flowing in and out of the battery as part of ion movement between anode and cathode. This claim carries merits but if the scientists were totally right, then the battery would live forever. They blame capacity fade on ions getting trapped, but as with all battery systems, internal corrosion and other degenerative effects also known as parasitic reactions on the electrolyte and electrodes still play a role. (See BU-808b: What causes Li-ion to die?)
The Li ion charger is a voltage-limiting device that has similarities to the lead acid system. The differences with Li-ion lie in a higher voltage per cell, tighter voltage tolerances 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 gain extra capacity with pulses and other gimmicks does not exist. Li-ion is a “clean” system and only takes what it can absorb.
Li-ion with the traditional cathode materials of cobalt, nickel, manganese and aluminum typically charge to 4.20V/cell. The tolerance is +/–50mV/cell. Some nickel-based varieties charge to 4.10V/cell; high capacity Li-ion may go to 4.30V/cell and higher. Boosting the voltage increases capacity, but going beyond specification stresses the battery and compromises safety. Protection circuits built into the pack do not allow exceeding the set voltage.
Figure 1 shows the voltage and current signature as lithium-ion passes through the stages for constant current and topping charge. Full charge is reached when the current decreases to between 3 and 5 percent of the Ah rating.
Li-ion is fully charged when the current drops to a set level. In lieu of trickle charge, some chargers apply a topping charge when the voltage drops.
The advised charge rate of an Energy Cell is between 0.5C and 1C; the complete charge time is about 2–3 hours. Manufacturers of these cells recommend charging at 0.8C or less to prolong battery life; however, most Power Cells can take a higher charge C-rate with little stress. Charge efficiency is about 99 percent and the cell remains cool during charge.
Some Li-ion packs may experience a temperature rise of about 5ºC (9ºF) when reaching full charge. This could be due to the protection circuit and/or elevated internal resistance. Discontinue using the battery or charger if the temperature rises more than 10ºC (18ºF) under moderate charging speeds.
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Full charge occurs when the battery reaches the voltage threshold and the current drops to 3 percent of the rated current. A battery is also considered fully charged if the current levels off and cannot go down further. Elevated self-discharge might be the cause of this condition.
Increasing the charge current does not hasten the full-charge state by much. Although the battery reaches the voltage peak quicker, the saturation charge will take longer accordingly. With higher current, Stage 1 is shorter but the saturation during Stage 2 will take longer. A high current charge will, however, quickly fill the battery to about 70 percent.
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 a high voltage stresses the battery. Choosing a lower voltage threshold or eliminating the saturation charge altogether, prolongs battery life but this reduces the runtime. Chargers for consumer products go for maximum capacity and cannot be adjusted; extended service life is perceived less important.
Some lower-cost consumer chargers may use the simplified “charge-and-run” method that charges a lithium-ion battery in one hour or less without going to the Stage 2 saturation charge. “Ready” appears when the battery reaches the voltage threshold at Stage 1. State-of-charge (SoC) at this point is about 85 percent, a level that may be sufficient for many users.
Certain industrial chargers set the charge voltage threshold lower on purpose to prolong battery life. Table 2 illustrates the estimated capacities when charged to different voltage thresholds with and without saturation charge. (See also BU-808: How to Prolong Lithium-based Batteries)
Adding full saturation at the set voltage boosts the capacity by about 10 percent but adds stress due to high voltage.
When the battery is first put on charge, the voltage shoots up quickly. This behavior can be compared to lifting a weight with a rubber band, causing a lag. The capacity will eventually catch up when the battery is almost fully charged (Figure 3). This charge characteristic is typical of all batteries. The higher the charge current is, the larger the rubber-band effect will be. Cold temperatures or charging a cell with high internal resistance amplifies the effect.
The capacity trails the charge voltage like lifting a heavy weight with a rubber band.
Estimating SoC by reading the voltage of a charging battery is impractical; measuring the open circuit voltage (OCV) after the battery has rested for a few hours is a better indicator. As with all batteries, temperature affects the OCV, so does the active material of Li-ion. SoC of smartphones, laptops and other devices is estimated by coulomb counting. (See BU-903: How to Measure State-of-charge)
Li-ion cannot absorb overcharge. When fully charged, the charge current must be cut off. A continuous trickle charge would cause plating of metallic lithium and compromise safety. To minimize stress, keep the lithium-ion battery at the peak cut-off as short as possible.
Once the charge is terminated, the battery voltage begins to drop. This eases the voltage stress. Over time, the open circuit voltage will settle to between 3.70V and 3.90V/cell. Note that a Li-ion battery that has received a fully saturated charge will keep the voltage elevated for a longer than one that has not received a saturation charge.
When lithium-ion batteries must be left in the charger for operational readiness, some chargers apply a brief topping charge to compensate for the small self-discharge the battery and its protective circuit consume. The charger may kick in when the open circuit voltage drops to 4.05V/cell and turn off again at 4.20V/cell. Chargers made for operational readiness, or standby mode, often let the battery voltage drop to 4.00V/cell and recharge to only 4.05V/cell instead of the full 4.20V/cell. This reduces voltage-related stress and prolongs battery life.
Some portable devices sit in a charge cradle in the ON position. The current drawn through the device is called the parasitic load and can distort the charge cycle. Battery manufacturers advise against parasitic loads while charging because they induce mini-cycles. This cannot always be avoided and a laptop connected to the AC main is such a case. The battery might be charged to 4.20V/cell and then discharged by the device. The stress level on the battery is high because the cycles occur at the high-voltage threshold, often also at elevated temperature.
A portable device should be turned off during charge. This allows the battery to reach the set voltage threshold and current saturation point unhindered. A parasitic load confuses the charger by depressing the battery voltage and preventing the current in the saturation stage to drop low enough by drawing a leakage current. A battery may be fully charged, but the prevailing conditions will prompt a continued charge, causing stress.
While the traditional lithium-ion has a nominal cell voltage of 3.60V, Li-phosphate (LiFePO) makes an exception with a nominal cell voltage of 3.20V and charging to 3.65V. Relatively new is the Li-titanate (LTO) with a nominal cell voltage of 2.40V and charging to 2.85V. (See BU-205: Types of Lithium-ion)
Chargers for these non cobalt-blended Li-ions are not compatible with regular 3.60-volt Li-ion. Provision must be made to identify the systems and provide the correct voltage charging. A 3.60-volt lithium battery in a charger designed for Li-phosphate would not receive sufficient charge; a Li-phosphate in a regular charger would cause overcharge.
Lithium-ion operates safely within the designated operating voltages; however, the battery becomes unstable if inadvertently charged to a higher than specified voltage. Prolonged charging above 4.30V on a Li-ion designed for 4.20V/cell will plate metallic lithium on the anode. The cathode material becomes an oxidizing agent, loses stability and produces carbon dioxide (CO2). The cell pressure rises and if the charge is allowed to continue, the current interrupt device (CID) responsible for cell safety disconnects at 1,000–1,380kPa (145–200psi). Should the pressure rise further, the safety membrane on some Li-ion bursts open at about 3,450kPa (500psi) and the cell might eventually vent with flame. (See BU-304b: Making Lithium-ion Safe)
Venting with flame is connected with elevated temperature. A fully charged battery has a lower thermal runaway temperature and will vent sooner than one that is partially charged. All lithium-based batteries are safer at a lower charge, and this is why authorities will mandate air shipment of Li-ion at 30 percent state-of-charge rather than at full charge. (See BU-704a: Shipping Lithium-based Batteries by Air)
The threshold for Li-cobalt at full charge is 130–150ºC (266–302ºF); nickel-manganese-cobalt (NMC) is 170–180ºC (338–356ºF) and Li-manganese is about 250ºC (482ºF). Li-phosphate enjoys similar and better temperature stabilities than manganese. (See also BU-304a: Safety Concerns with Li-ion and BU-304b: Making Lithium-ion Safe)
Lithium-ion is not the only battery that poses a safety hazard if overcharged. Lead- and nickel-based batteries are also known to melt down and cause fire if improperly handled. Properly designed charging equipment is paramount for all battery systems and temperature sensing is a reliable watchman.
Charging lithium-ion batteries is simpler than nickel-based systems. The charge circuit is straight forward; voltage and current limitations are easier to accommodate than analyzing complex voltage signatures, which change as the battery ages. The charge process can be intermittent, and Li-ion does not need saturation as is the case with lead acid. This offers a major advantage for renewable energy storage such as a solar panel and wind turbine, which cannot always fully charge the battery. The absence of trickle charge further simplifies the charger. Equalizing charger, as is required with lead acid, is not necessary with Li-ion.
Consumer and most industrial Li-ion chargers charge the battery fully. They do not offer adjustable end-of-charge voltages that would prolong the service life of Li-ion by lowering the end charge voltage and accepting a shorter runtime. Device manufacturers fear that such an option would complicate the charger. Exceptions are electric vehicles and satellites that avoid full charge to achieve long service life.
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