Hurry up and wait, the timeless expression applies to so many situations, but it always applies to batteries! Have you ever rushed to the airport, checked in, anxiously worked through security, and then ran to your gate just to find out the flight is delayed? Now you sit around and wait for your moment to board. When they do finally announce boarding Group 5 you walk up to the counter, boarding pass in hand, and beep beep, you are on your way! Most batteries live their entire life in this perpetual purgatory just waiting for their moment to board. Usually they are in a flashlight, remote control, or garage door opener, but batteries also backup critical communications systems, emergency lighting, and other critical appliances we need when the grid goes out. Too often though many of these batteries don’t work when they are needed the most! Sometimes these batteries fail, and there are many reasons why. Most importantly, there are a few things we can do to prevent these failures and make sure the lights stay on!

Batteries are used to provide backup power for mission critical communication systems, utility control systems and industrial plant emergency control systems. These systems traditionally have used some form of lead acid batteries for backup. Lead acid batteries come in a few distinct types but the two most common are flooded (FLA) and valve regulated (VRLA). Both of these types operate by combining lead plates with sulfuric acid and distilled water. During the charge process the positive plates change to lead peroxide, and the negative plates turn to pure lead. During the discharge process the acidity of the electrolyte solution increases and the specific gravity of the solution decreases. Both of the lead plates now begin to change to lead sulfate. This continues until there is no potential difference between the plates, though this should be avoided as we will discuss later.

Figure 1: Valve Regulated Lead Acid Battery

Both of these types of lead acid batteries require maintenance and monitoring to ensure optimal performance and prevent damage. All batteries are considered wear items and wear out as they are used. Just like brake pads on a car, batteries can wear out faster or slower depending on a number of use factors. When a lead acid battery is used, the battery will form hydrogen and oxygen gasses. In a flooded battery these gases are allowed to vent to the atmosphere and must be exhausted from the container to prevent creating an explosive atmosphere. Usually, battery rooms or cabinets have special ventilation systems to exhaust this explosive gas mixture. This gas generation also uses some of the water (H2O) in the electrolyte mix and therefore the operator must periodically check the water level in each battery. If the water level ever goes below the top of the lead plates the lead can become damaged and you will lose performance. Flooded lead acid batteries are also subject to sulfation precipitation. This is when lead sulfate forms on the plates of the batteries, and then falls off into the electrolyte solution. This sulfate over time can build up at the bottom of the flooded cell “jar” (a term used to describe the usually plastic container the plates and electrolyte are in. At one point in time these were glass “jars,” and the term has stuck.). This build-up can decrease performance of the cell and eventually cause shorts and cell failures. In summary, the main failure modes of flooded lead acid batteries low water, sulfate build up, and over sulfation are all preventable with constant monitoring and maintenance.

Valve regulated lead acid (VRLA) batteries were developed in the later half of the 1900’s to alleviate some of these maintenance and performance issues. VRLA’s use the same lead plate and sulfuric acid combination but introduce two key mechanical features. The lead plates are separated by an absorbent fiberglass mat that is saturated with electrolyte, and the entire “jar” is sealed from the atmosphere to contain the hydrogen and oxygen gas generation. This allows for pressure to build up inside of the battery and the hydrogen and oxygen to recombine into water (H2O). In terms of maintenance this eliminates the need to monitor water levels and replace used up water. This does pose a potential explosion risk due to pressure, which is why a pressure relief valve is included to release excessive pressure. VRLA’s are touted as virtually maintenance free since they don’t lose water. They do require some maintenance though. It is important to verify the connections are torqued properly as they are directly next to a seal that can leak acid if over torqued or fatigued. Also, if the battery is left at a high state of charge for a long time or exposed to equalization charges the electrolyte solution can be turned into gas, build pressure, and activate the pressure relief valve. This can vent too much gas, and then the electrolyte in the battery will no longer be sufficient to carry charge, and the battery will fail to operate. Since you cannot add more distilled water to a VRLA this type of failure is quite common and is a main contributor to power failure events.

Both of these lead acid battery types suffer from well understood failure mechanisms. The primary lead acid battery failure mechanisms are internal corrosion, sulfation, and loss of electrolyte as described above. These failures are caused by lack of maintenance, deep frequent discharges, and prolonged elevated temperature exposure.

Figure 2: Failed Lead Acid Batteries at Ajo Historical Museum, Creative Commons License, Brewbooks

If a lead acid battery is over charged, positive plate grid corrosion and gases are generated, causing pressure and venting, reducing electrolyte levels and grid damage. If a lead acid battery is under charged it is prone to sulfation, blocking the ability to discharge. There is an exceedingly small voltage window (2.25-2.27V) to maintain a charge voltage, and this must be adjusted for temperature. Battery experts call this “dancing on the head of a pin.” It is quite common to overcharge or under charge lead acid batteries, and then when you need them to discharge, they will not operate due to sulfation, grid damage or loss of electrolyte.

Lithium-ion batteries have become more popular in stationary applications as a backup power supply. Lithium batteries provide more energy storage in a smaller space, weigh less, last 3-5x longer, and pack a suite of advanced features. Lithium batteries are not immune to failure, and there are four main ways that lithium batteries can fail. They do have a few extra features built in that nearly eliminate the possibility of failure and make them a true contender in the stationary market.

Figure 3: Components of a lithium-ion cell

Lithium-ion batteries are comprised of three main components, the lithium-ion cell, the battery management system (BMS), and the mechanical enclosure. The lithium-ion cell is quite different than the lead acid cell in a number of ways. A lithium-ion cell is comprised of a graphite (carbon) anode, polymer separator, cathode strip, and just enough electrolyte to saturate the separator. These components are built into either a round, square or pouch style cell, and sealed airtight. Since lithium-ion cells are sealed, no gasses are generated under normal use, they can be installed in any orientation, and do not require special explosive atmosphere venting like lead acid.

Figure 4: Mechanics of degradation in a lithium-ion cell

The main four causes of LFP battery cell failures are short circuit, deep discharge, over charge, and elevated temperature. Figure 4 shows all of the mechanics of failures and degradation in lithium-ion batteries. If you protect the cell from these four forms of abuse, you will have a well performing battery. Every Lithium-ion battery has a battery management system. The primary purpose of this BMS is to prevent these four modes of failure. The BMS continually monitors the voltage, current and temperature of the battery. If any of these parameters exceed pre-programmed thresholds the BMS will disable the battery and turn off the output. Once the fault has been removed, the battery automatically turns the output back on. A well designed BMS will not only prevent damage to the battery, but they will also provide premium features such as fuel gauging, data logging, remote monitoring, and remote control. The BMS can then be hooked into a central monitoring system or site monitor to communicate all sorts of information.

Not all lithium batteries have the same features. At a minimum the battery must protect against over and under voltage, short circuit, and high temperatures. Since this basic set of protections doesn’t prevent all forms of abuse, there are best practices to use to prevent pre-mature battery failure. There are two common circumstances that can prematurely wear out lithium-ion batteries, over discharge and elevated temperatures. If a lithium-ion battery is discharged to 0% state of charge (SOC) and left for an extended period, the cells could be over discharged and damaged. Typically, this extended period is measured in weeks. To prevent this, more sophisticated BMS will keep a small amount of charge in reserve, and enable a low power mode when the battery is at 0% SOC. This extends the time between 0% SOC and damage to many weeks or months.

Elevated temperatures are the enemy to all battery chemistries. Lithium-Ion batteries are more resilient to elevated temperatures than lead acid but are still affected by prolonged exposure. Since most BMS don’t have the ability to cool the batteries, the operator is responsible to keep the batteries in a reasonable temperature environment. Lithium-Ion batteries ideal temperature range is 10 to 35 degrees Celsius, or 50 to 95 degrees F. It is best practice to keep batteries in these temperature ranges to prevent premature failures. Lithium-Ion batteries typically begin to slowly degrade in elevated temperatures and will have lower discharge capacities after exposure. Lithium-ion batteries can and will perform at temperatures up to 65 degrees C but will suffer degradation if continually stored or used at these temperatures.

Both lead acid and lithium-ion batteries work well as stationary energy storage mediums for emergency communication systems. Both chemistries are unique and can fail due to lack of maintenance or exposure to abusive conditions. Since both of these batteries live the hurry up and wait life there is ample opportunity to fail when they are needed the most. Lithium-ion batteries have the advantage of an integrated BMS that can prevent damage to the cells due to abuse, and remotely perform maintenance tasks such as voltage, impedance, and temperature readings. Lithium batteries can last 10 years, or 3,000 cycles, and provide even more reliable power when you need it most.

ABOUT THE AUTHOR

JD DiGiacomandrea

JD DiGiacomandrea is the product marketing engineer for Green Cubes Technology. As a lithium battery and energy storage industry veteran he has more than a decade of experience designing lithium batteries and systems for the military, medical, and industrial markets. DiGiacomandrea has had roles at major battery manufactures including electrical engineer, applications engineer, and field sales engineer, bridging the gap between engineering and sales. He holds a Bachelor of Science degree in electrical engineering from Clarkson University in Potsdam, NY, and a PE license from the state of New York.

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