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In commercial vehicles, the need for extra power is small, but military vehicles are different as they provide a platform for different mission applications. Combat vehicles are often loaded with energy-hungry electronics.

A vehicle automotive powertrain is designed with adequate power to deliver the vehicle’s needs. While vehicles built for the 2020s have increased power to drive the computers and electronic systems on board, and also to provide spare electrical power for other electronics, there is a limit to that power draw. Military vehicles are often loaded with energy-hungry mission electronics, such as navigation, communications and survivability systems.

Some equipment may use a special generator to provide electricity, but some systems, such as Threat Detection, Active Protection Systems and data communications are operating continuously, draining the vehicle’s energy reserve. In order to meet this demand, vehicle designers integrate more powerful alternators, auxiliary power units, energy generation systems and high capacity batteries to increase the energy available on board.

Vehicles typically carry two to eight batteries, which can cost US$4,000 each when life-cycle costs are accounted for. As a result of a lack of confidence in power availability,
crews continuously idle their engines to ensure battery banks are charged, thus requiring frequent refuelling of their combat vehicles.

Vehicle Modifications and Improvements

While adding more power is a trivial solution, a comprehensive power management approach is likely to deliver the optimal result. As each combat platform uses different methods of electrical power generation, distribution, conversion, and storage, BAE Systems, for example, approaches a vehicle energy management design with the specific power system architecture of the platform.

Traditional vehicle electrical systems are designed for peak events, but power management systems allow power to flow differently during acceleration, road march, attack/defend mode, and braking. When the vehicle is stationary, power can be provided from an energy storage system to operate in pure electric ‘stealth mode’, also known as ‘silent watch’, with the engine off to reduce the vehicle’s noise and heat signature.

For example, mechanical accessory packages are implemented differently on each platform. Traditional mechanical auxiliary subsystems, whose unco-ordinated and continuous operations are inherently inefficient, generate waste heat that must be exhausted through already burdened cooling systems. Electrifying and managing power-hungry mechanical accessories, such as pumps and cooling fans, can contribute to heat reduction and fuel saving. Addressing the system, and finding the optimal level of integration, ensures that energy loss and inefficiency are minimised.

Li-ion batteries provide higher capacity but require special containers to protect the vehicle and crew from fire hazard in case of battery failure or combat damage.(Photo: Brentronics)

On-Board Power Generation

Leonardo DRS has designed a specialised power system known as TITAN On-board Vehicle Power (OBVP). As the permanent magnet generating system is integrated within the transmission, OBVP turns the power train into an efficient electricity generator delivering more energy than a standard vehicle alternator without significant load increase. OBVP is designed to support the increased demand for electricity in command vehicles and command posts, missile launchers and high-energy weapon carriers.

Optimal Battery Charging

Discharging a lead-acid battery to less than half of its capacity is not recommended because it might cause internal damage to the battery. Ideally, a 100Ah battery should not be discharged below 50Ah. Therefore, measuring the battery State of Charge (SoC) and State of Health (SoH) are key indicators for effective power management on board. SoC is often thought of as the equivalent of a fuel gauge for a battery bank. In this analogy, SoH would be the size of the fuel tank. Merlin Power Systems, a company offering battery monitoring systems for combat vehicles, defines 50Ah remaining as 0% SoC; thus, automatically topping up the battery when needed.

Energy Storage Upgrades

Revision’s SILENT WATCH Battery Pack (SWBP) is a modular lithiumion power platform composed of up to 10 independent 28V, 160Ah modules (SWatPacks), plus a power manager (SMS). This device was designed as a power enhancement for the STRYKER 8×8 armoured vehicle. (Photo: Revision)

The majority of combat vehicles in the US Army and NATO use the 6T battery format, the most common of which are lead-acid batteries, such as EnerSys’ 6T Hawker ARMASAFE Plus battery. At a unit weight of 40 kg, the battery’s capacity is 120Ah and it is designed for a life of 120 deep recharging cycles. Multiple batteries are installed in a vehicle to achieve the voltage and energy capacity required for the specific vehicle design.
In recent years, the introduction of Lithium-based batteries has provided higher capacities and power management capabilities, enabling users to better support combat missions. Li-ion batteries suitable for powering combat vehicles were introduced in 2015. The new batteries contain hundreds of standard rechargeable lithium cells interconnected in multiple groups to deliver the required voltage and current. Designed to maintain a safe operating environment through physical isolation and using battery management systems to monitor and regulate electrical voltage, currents and temperature on charging and discharge, batteries are designed to prevent user abuse as well as meet harsh operating conditions.

As the power delivered by such batteries quickly degrades in cold temperatures, built-in heaters are used to maintain an optimal operating environment under extreme temperatures. Unlike lead-acid batteries, smart battery features enable communication with the end user, which provides information about the battery SoH, SoC, and other functions.

Batteries of this type integrate into modern vehicles communicating with vehicle systems over the CAN BUS, relaying vital information, including SoC, SoH, cell voltages, temperatures and battery diagnostics. Since some Li-ion chemistries are more flammable than others, users and manufacturers opt to use 6T packs utilising Lithium Iron Phosphate (LFP) that are non-flammable. Li-ion batteries provide higher capacity but require special containers to protect the vehicle and crew from fire hazard in case of a battery failure or combat damage.

For example, Saft’s XCELION 6T leverages the company’s Super-Phosphate technology that supports the recharging cycle, longer life, and higher energy density over lead-acid batteries. The Saft XCELION 6T is a 28V, 60Ah Li-ion battery system that is designed within the dimensions of a traditional lead-acid battery, enabling easy installation into a vehicle. Addressing the US Army requirement for vehicle batteries to meet MIL-PRF-32565, Saft redesigned its XCELION 6T introducing the 6T Type 1-A variant of the MIL-PRF-32565 Rev B standard, withstanding extreme cold temperature performance requirements without the need for pre-heating. The battery provides slightly lower energy capacity – 58 Ah with a nominal voltage of 26.4 volts. Designed to support a cold start without pre-heating, XCELION 6T Type 1-A delivers 1,100 amps at a temperature of -18 C° for 30 seconds, and 400 amps at -40 C° for 30 seconds.

Saft received several contracts to supply its XCELION 6T batteries to foreign customers, including Thales Australia, for use in the Australian Hawkei protected tactical vehicle. The Switzerland-based General Dynamics Land Systems Europe company has also placed a multi-million-dollar contract for batteries to equip 300 PIRANHA V armoured personnel carriers.

A major producer of Li-ion batteries, Bren-Tronics has embarked on a US$6M project establishing a high-volume automated manufacturing facility for military 24V 6T Li-ion batteries for the US Army. The line is expected to produce up to 2,000 6T batteries per month. The company offers two ‘Brenergy’ brand BT-70939 models of Li-ion vehicle batteries formed in the 6T standard, one optimised for high-power (AP-APH) delivering 103 Ah, and the other for high-energy (CU/CUH) delivering 126 Ah.

The 6T battery from Denchi Power weighs 25 kg and delivers 85Ah (2.142 kWh) with 900 A cold cranking Amps. The battery is designed to support 3,500 deep charging cycles at charge rates of up to 270 A. As the battery management system limits battery to 45 A, Denchi also offers higher charging rates at ‘bypass mode’, enabling the battery to charge directly for fast charging at rates up to 270 A. This mode enables the battery to absorb energy during turret braking, sinking energy pulses up to 500 A for five seconds.

In Search of New Cell Chemistries

Leonardo DRS has designed a specialised power system known as TITAN On-board Vehicle Power (OBVP), which turns the power train into an efficient electricity generator. (Graphic: via author)

The Li-ion 6T packs hundreds of rechargeable Li-ion cells to deliver the energy it is designed to provide. The type of cell chemistry determines the energy density – some provide higher energy level, others are less vulnerable to damage and fire. Obviously, increasing the energy density and preserving safety would be highly beneficial for the military user. A new cell developed by the US Army Research Laboratory (ARL) implements aqueous electrolyte could provide reliable high energy source with robust damage tolerance. The new cell employs several technologies developed by the team, including the intrinsically safe “water-in-salt electrolytes” (WiSE) and the technique to stabilise graphite anodes in WiSE, and a novel cathode chemistry that further extends available energy for aqueous batteries to a previously unachievable level. The energy output delivered by the experimental water-based battery is comparable to conventional Li-ion batteries based on flammable organic liquids other than water, but is much safer. The new system is able to hold 240 milliamps per gramme for an hour of operation, delivering about 25% extra the energy density.

A different approach to vehicle power storage is reflected in Revision’s SILENT WATCH Battery Pack (SWBP), a modular lithium-ion power platform composed of up to ten independent 28V, 160Ah modules (SWatPacks), as well as a power manager (SMS). Although this device was first designed as a power enhancement for the STRYKER 8×8 armoured vehicle, according to the company, it can be customised to fit different enclosures and attachment methods. The system was mounted externally in order to avoid potential hazards or flammability issues with Li-ion cells.

An Epsilor ELI-52526 fire-protected battery pack (Photo: Epsilor)

Individual SWatPacks are made of seven high-end lithium polymer cells rather than hundreds of smaller cells, typical of most current systems. Fewer connection points mean less potential for failure, thus greater reliability and lower lifetime maintenance costs. Each SWatPack can last up to 6,500 cycles (approximately ten years), and multiple redundant safety measures have been incorporated. SWBP systems and individual SWatPack cells are available now for global forces. Just like the XCELION 6T Type-1A, SWBP is designed to be MIL-PRF-32565 compliant.

Another new concept for vehicle battery is the dual voltage battery developed by Spear Power Systems LLC addressing a US Army requirement. The new battery that Spear is developing will support multiple voltages, avoiding the need for multiple fielded batteries to address multiple voltage requirements. The 24/48 Volt battery conforms to 6T standard offering a drop-in replacement for 12 Volt batteries. Configured in packs of multiple 48 V batteries, the new system will provide a ‘building block’ for higher voltage systems up to at least 300V.

While the unit cost of Li-ion batteries is higher in comparison to lead-acid batteries, a comparison of life cycle costs shows that the cost of ownership over the battery life cycle of Li-ion 6T batteries is slightly lower than similar lead-acid batteries. This LCC is expected to decrease even further, as Li-ion cell prices drop in tandem with wide penetration of electric cars. With current battery cost reduced to around US$100/kWh, it is expected that in the long-term, military vehicle batteries will further reduce in price, and reduce logistical burdens to stow, transport and distribute replacement batteries. According to estimates by Saft, this translates to over US$200M of savings in the total cost of ownership for a fleet of 20,000 vehicles over a 20-year life span.

Tamir Eshel is a security and defence commentator based in Israel.