All About Helicopter Batteries, Present and Future
By Tracy Martin
Rotary-wing aircraft and automobiles do not have all that much in common, except for the fact that they both use an on-board battery. The job a battery performs in a car or truck is simple: starting the engine and providing power to accessories, such as the lights or power windows, when the engine is not running. By contrast, batteries used in helicopters are used to start engines or auxiliary power units (APUs), provide emergency backup power for essential avionics equipment and lighting systems, ensure uninterrupted power for navigation units and fly-by-wire computers, and provide ground power capability for maintenance and preflight checks.
Not all helicopters use batteries in the same way. For example, the U.S. Navy’s Sikorsky MH-60R Seahawk has two GE T700-GE-401 turbine engines that are too large to start with any battery. To start its main engines, the Seahawk uses an APU, which is started using power from an onboard battery or a plug-in from a ground-based electrical source. The APU produces air pressure that is used to start one of the main engines, which, in turn, produces bleed air to start the second main engine.
In another example, the U.S. Navy’s single-engine Bell TH-57 Sea Ranger is powered by an Allison 250-C20BJ turbofan engine. Because the engine is relativity small, the onboard battery is able to supply enough electrical energy to spin the rotor fast enough to start the engine.
How a Battery Works
It is important to understand that a battery does not actually store electricity. Rather, it only stores chemical energy that is converted to produce electricity. To explain this process, let’s start by taking a look inside aircraft batteries that use lead plates and sulfuric acid. Batteries that use this technology are known as “flooded” and “maintenance-free” batteries. We also will discuss lithium-ion batteries, which use a different chemical process.
The battery case is divided in sections called cells, with a 24-volt battery having twelve cells that produce approximately 2.2 volts each (depending on the battery type) for a total of just over 26 volts. The cells consist of lead plates, half of which have a positive charge, while the other half have a negative charge. Within each cell, the plates are stacked alternately: negative, positive, negative. Insulators or separators (usually made from fiberglass or treated paper) are placed between the plates to prevent contact.
The alternating plates in each cell are connected at the top into two groups, one positive and one negative. Each cell’s groups of plates are then connected in series (positive to negative) to those in the next cell. The “active material” (lead peroxide on the positive plate and metallic sponge lead on the negative plate) in these positively and negatively charged groups of plates produce electricity when immersed in an electrolytic solution of sulfuric acid and water, often called battery acid. The potential to produce electrical energy is directly related to the amount of active material and total plate surface area — the larger the plate surface, the higher the battery’s capacity to produce electrical current.
An aircraft battery’s capacity is rated in ampere hours (Ah) or the flow of electrical energy. One ampere hour is defined as a current flow of 1 ampere for a period of 1 hour. The capacity of an aircraft battery is usually based on a 1-hour discharge rate. So a 17 Ah battery will supply a current of approximately 17 amperes for a period of 1 hour. A 34 Ah battery will deliver twice that amount of current over the same period of time.
Charging and Discharging
Batteries are constantly charging or discharging. During either process, ions (both positively or negatively charged) are transferred between the positive and negative groups of cell plates. The insulators or separators between the plates are permeable and non-conductive and thus allow this transfer of ions.
During charging and discharging, the ratio of acid to water changes. This ratio is expressed as “specific gravity” or SG. The SG for pure water is 1.000, and sulfuric acid has an SG of 1.835. Combined, their SG ranges around 1.275 to 1.300.
As the battery discharges, and ions move from the positive plates to the negative plates, there is less sulfuric acid and more water, the specific gravity of the electrolyte solution is lowered as well. The process is reversed when the battery is charged. The SG becomes higher as the ratio of acid to water changes back to mostly acid.
When a battery discharges, and the SG ratio reflects more water and less acid, a chemical byproduct called lead sulfate is produced and starts coating the cell plates, reducing the surface area over which the chemical reactions occur that produce electrical current. This is the reason that an engine’s starter motor cannot be cranked indefinitely and other electrical loads cannot be left on for long periods of time without the battery going dead. Recharging the battery reverses this process. However, if the battery becomes too discharged, the lead sulfate deposits cannot be removed, no matter how much the battery is charged, resulting in total failure of the battery.
Besides sulfation concerns, other detrimental chemical reactions take place inside the battery while it is in a discharged condition. The corrosive effect caused by acid on the lead plates and connections within the battery is increased due to the reduced specific gravity of the electrolyte. This corrosion can result in a gradual reduction in battery performance; corroded connectors may lead to reduced discharge current that is enough to power low-drain accessories, such as lights and instruments, but insufficient for engine starting. If the corrosion is bad enough, it also can cause the inter-cell connectors and connecting welds to fail. This creates an open circuit within the battery, resulting in sudden battery failure.
Another condition that frequently occurs in a discharged battery is freezing of the electrolyte solution. In a deeply discharged battery, the specific gravity is lowered, resulting in a higher percentage of water than sulfuric acid. During this condition, the battery can freeze at temperatures of 32 degrees Fahrenheit (F) and below, while the electrolyte solution in a fully charged battery will not freeze until temperatures drop to –75 degrees F.
“Wet-cell” or “flooded” batteries can be identified by the cell fill caps located on the top of the battery case. These types of batteries used to be common in automobiles. Gas stations even had battery fillers, containing tap water, located at the pumps, which customers could use to top off the batteries when electrolyte levels became low.
When this type of battery is discharged or charged, it outgases a highly flammable mixture of hydrogen and oxygen. Therefore care was required when jump-starting a car, as a spark from the jumper cables could cause an explosion.
When a battery is charged, and especially when excess voltage is applied, some of the water in the electrolyte is converted to gaseous hydrogen and oxygen, which exits through the vents in the battery fill caps. Out-gassing causes some of the electrolyte solution to be lost to the atmosphere, thus the normal requirement to replenish the water in the cells. (Only distilled water should be added to aircraft flooded batteries.) Aircraft battery caps have additional internal stoppers to help prevent excessive loss of electrolyte solution when flying at extreme attitudes.
Individual battery cells also may feature a pressure relief valve (PRV) that is designed to open when the internal pressure of a cell is approximately 1.5 pounds per square inch (psi) above the external pressure. The PRV prevents excessive pressure buildup when the battery is being charged, and it automatically reseals once the pressure is released.
A bulge in the battery case may be present when the internal pressure increases slightly but is not enough to open the PRV. Alternatively, if the PRV opens at altitude, and the battery is then returned to the ground, the external pressure can be greater than the internal pressure, resulting in a concave battery case. Both of these conditions are normal and do not affect the battery’s operation. Not all aircraft batteries use vented cell caps, and those that do not are often called “maintenance-free” batteries.
To reduce aircraft battery maintenance, and to eliminate the needs for external battery vents, absorbed glass mat (AGM) sealed battery technology was developed around 1980. Introduced in 1985 for military aircraft, where power, weight, safety, and reliability were paramount considerations, the AGM design was an improvement over wet-cell battery designs. AGM batteries are also known as Valve Regulated Sealed Lead-Acid Battery or VRSLAB.
Within an AGM battery, both positive and negative plates are sandwiched between layers of mats constructed from glass micro fibers of varying length and diameters. Unlike a flooded battery, the electrolyte is not suspended in a liquid form; instead, it is absorbed and held in place by the capillary action between the fluid and the AGM fibers. The plate separators used in an AGM battery are only saturated around 90 percent with electrolyte, providing space to allow oxygen to travel from the positive to the negative plates. When the oxygen reaches the negative plates, it reacts with lead to form lead oxide and water.
This reaction at the negative plate suppresses the generation of hydrogen gas and eliminates virtually all of the out-gassing from the battery. At the same time, the lead/acid cells are sealed with a pressure relief valve that regulates the internal cell pressure and prevents gases from escaping, and the design recombines the water inside the cells. The result is a battery that never needs to have water added, and thus is maintenance-free.
First developed in the 1970s, lithium-ion batteries (often referred to as LIB) are common in consumer electronics. LIB cells use positive and negative electrodes that transfer ions between them during discharge and charging modes. The chemistry features a cathode material made of lithium iron phosphate, which is more chemically stable than the oxide-based cathodes used in lead-acid batteries. Lithium-ion batteries do not use an electrolyte solution (battery acid) and have no lead plates, so they are about one quarter of the weight of a lead-acid battery of the same capacity, making them an attractive choice for aircraft applications.
Because safety is paramount, these batteries use a microprocessor-based Battery Monitoring System (BMS) that tracks individual cell activity and protects the battery from abnormal conditions, such as excessive electrical current during charging or discharging, over/under voltage, and extreme operating temperatures. The BMS on some batteries controls battery heating for operation in cold environments, or the system may provide cooling if battery’s internal temperature becomes too high. In addition, the system balances the battery’s cells to maximize usable capacity and monitors overall battery health. It also can interface with both digital and analog output controls for chargers and other external devices.
Use of Lithium-ion Batteries in Aircraft
Lithium-ion batteries generally work well in laptops, cell phones, portable power tools, and such. Though there have been notable exceptions, including the recent issues with Samsung’s Galaxy Note S7’s battery meltdown and subsequent worldwide recall. There also have been problems with larger applications, particularly in aircraft.
For instance, Boeing’s new 787 Dreamliner had issues with lithium-ion batteries catching on fire, and aircraft were grounded in January 2013 following two LIB incidents. Battery fire containment solutions and other safeguards were put into place in April of that year, and the planes were returned to service.
Another issue is that the service life of lithium-ion batteries installed in aircraft has not yet been established. Many battery manufacturers project a service life of 5 years or more, but these projections are based on laboratory data and not field experience. Lithium-ion batteries for consumer products typically last around 2 to 3 years, but aircraft applications operate in more extreme environments, so battery service life may be significantly less.
The cost for lithium-ion batteries also can be three to six times that of a lead-acid battery. Given the possibility of a shorter service life, it may be difficult to justify the price. Nevertheless, the weight savings offered by LIB designs, even with additional safeguards in place, may offset its increased cost.
The U.S. Navy is using a lithium-ion battery, supplied by Concorde Battery, in the new, heavy-lift CH-53 helicopter made by Sikorsky. The application calls for a high-discharge-rate battery designed for APU starting and emergency power.
According to John Timmons, Vice President of Engineering for Concorde Battery, “Concorde lithium-ion batteries for aircraft are based on the safest chemistry so far developed for lithium-ion technology, featuring a cathode material of lithium iron phosphate, which precludes oxygen generation. Oxygen generation is one of the main contributors to the fire hazard in lithium-ion batteries. The lithium-ion battery for the CH-53K will be part of an integrated design with the control software and electronics of the aircraft system. Redundant safety systems built into the helicopter as well as into the battery are required to control the lithium-ion battery. The protection systems and monitoring of the battery will provide safe reliable power for this next generation of CH-53 helicopters.”
Lithium-ion, Battery of the Future?
Safety, or the perception of safety, remains a major concern for aircraft designers faced with deciding whether or not to adopt LIB technology. Only through proper design, testing, and increased data based on use in the field can LIB safety issues, as well as concerns about battery life, be fully addressed.
Bell Helicopter engineers have chosen to use a 17 Ah LIB to start the engine of the 505 Jet Ranger X. The battery used weighs less than 16 pounds, which is about 45 percent lighter than lead-acid alternatives, and is said by the manufacturer to require 60 to 90 percent less maintenance. Though this rotorcraft is not currently used in military applications, it may be predictive of the use of LIB technology for defense force rotary-winged aircraft. Lighter weight, smaller footprint, and less maintenance are factors that are likely to drive the use of lithium-ion batteries in aircraft of the future.
Image #1 - U.S. Navy Aviation Machinist Mate Vasya Mstislavski conducts maintenance on a Sikorsky MH-60S Seahawk helicopter assigned to Helicopter Sea Combat Squadron in the hangar bay of the aircraft carrier USS George Washington in the South China Sea. General maintenance items on helicopters include servicing the on-board batteries. (Photo by U.S. Navy Mass Communication Specialist Seaman Liam Kennedy/Released)
Image #2 - Photo courtesy of Concorde Battery Corporation
Image #3 – Unknown
Image #4 - Photo courtesy of Concorde Battery Corporation
Image #5 - During a 500-hour phase maintenance inspection on a Boeing AH-64D Apache attack helicopter, located at Camp Taji, Iraq, Specialist David Reed, an Apache mechanic in the 1st Cavalry Division, works on the engine compartment of the helicopter. During the 500-hour maintenance, the aircraft is stripped of all major components and thoroughly inspected, including battery testing. (Photo by Sergeant Travis Zielinski, 1st Air Cavalry Brigade, 1st Cavalry Division, Division of Public Affairs)
Image #6 - Sailors assigned to the Black Knights of Helicopter Anti-Submarine Squadron HS-4 perform maintenance on three MH-60S Sea Hawk helicopters aboard the aircraft carrier USS Ronald Reagan. During routine maintenance, the helicopter’s APU starting batteries are inspected and tested. (Photo by U.S. Navy photo by Seaman Nolan Kahn/Released)