Contemporary Operating Environment
Power Management and Silent Watch – Challenges and Opportunities for the Digitised ForceBy Lachlan Window August 28, 2019
The Royal Australian Artillery has been at the forefront of the transition to a digitised force and is today capable of a fully digital fires chain from sensor to shooter. Adoption of digitisation has improved speed of processing, enhanced situational awareness and enabled application of precision fires through a protected and resilient network. However, this transition has not come without a cost. The deployed gunline of 2019 is a far cry from the silent threat posed by an analogue L119 or M198 gunline at the turn of the century. Instead, today’s gun position is too often a high signature presence thanks to the near constant drone of diesel generators, the hum of vehicle main engines and the snorts and hisses of automatic tyre inflation systems. A gunline unable to maintain silent watch for a useful duration becomes vulnerable to location and interdiction.
What has brought about this shift? The proliferation of digital systems including radios, computers and positioning systems has placed increasing demands on electrical power supply. Current and future technologies such as electro-optics, remote weapon stations and active protection systems promise yet greater demands. Against these developing requirements the ability of our principal equipment to store electrical power has not been similarly enhanced – the end result are systems which require frequent charging using in-service high signature systems such as diesel generators or vehicle main engines.
The purpose of this paper is to examine the power requirements of a deployed gunline and consider potential improvements to silent watch duration available in the short, medium and long term. Although predicated on the tactical employment of an M777 battery, the considerations and conclusions are likely to be relevant to other deployed ground elements who rely on platform based power storage systems.
Current power requirements and systems of the deployed gunline
The 53 Battery gunline consists of four M777A2 lightweight howizters, four protected HX77 trucks with gun pack, four Protected Mobility Vehicle – Medium Troop variants (PMV-T), two PMV Command Post (PMV-CP) and one G-Wagon ambulance. The gunline is supported by logistics elements including quartermaster and ammunition lift. However, these are seldom static at the gunline. The most power intensive platforms are the PMV-CP which contain the radios and battle management systems.
Each of the radio systems are capable of transmitting simultaneously. However, most transmit only intermittently. While differing employment profiles and transmit power settings will yield differing average power consumptions, the PMV specification states average power consumption of 20.45W for a HF system and 55W for a VHF system. The UHF data system does not specify an average power consumption but given its transmit power, and the burst nature of the mesh network, a power consumption not less than that allowed for the VHF sets (55W) is reasonable. The battle management and fires terminals are effectively relatively low power laptops and civilian models typically consume between 20W and 30W depending upon workload: for planning purposes a figure of 25W will be used. The following table shows the planning power consumption of the complete communications set for the PMV-CP:
Testing by Land Systems Division in conjunction with Headquarters Forces Command in 2017 found a typical resting power draw of 25amp-hr/hr (a 600W momentary load) and as such the planning power consumption figures should be regarded as conservative. The report did however note power leakage issues likely due to wear and tear on the tested vehicles.
Other minor components of the PMV-CP consume electrical power including the intercom, lighting, battery chargers and any ancillaries connected to the 12V power sockets. However, these should be considered an optional load in comparison to the primary equipment.
Continuous power to operate these systems is provided by the integral batteries with charging provided by either the PMV engine or an external generator. The battery capacity consists of four flooded cell lead acid batteries which can be electrically isolated to two radio batteries and two crank batteries. The crank batteries can be used to supplement the radio batteries if longer duration silent watch is required, although this is usually not desirable as it may leave the vehicle unable to self-start. Commercial off the shelf (COTS) deep cycle batteries are used with a rated capacity of 125Amp-hr (20 hour rate) at 12V. Each weighs 34kg and has dimensions 345mm x 175mm x 260mm. As the radio batteries are employed in series the total capacity is still 125Amp-hr at 24V. On paper this capacity seems adequate, however application leads to a significantly reduced capacity in practice.
Lead acid battery voltage decreases as energy is withdrawn and allowing the voltage to fall below 10.5V causes permanent damage to the battery. For this reason operators are taught not to allow the series voltage of the system to fall below 21V before commencing a recharge cycle. Power draw in excess of the planned 20 hour cycle (6.25Amp-hr/hr) leads to reduced capacity due to Peukert’s Law. The specification sheet for the Century branded battery in use shows that with a discharge time of 5 hours the actual capacity is reduced to 101Amp-hr. Lead acid batteries are also vulnerable to damage due to vibration and capacity reduces with repetition of the charge / discharge cycle with most batteries of this type having a 500 to 1000 cycle life. A typical day on a static position in the field requires 8 full cycles and as such the capacity of the batteries is rapidly diminished. These factors combined mean that the experience of operating a PMV-CP yields time to minimum voltage (cut-off voltage) of between 1.5 and 2 hours.
Charging these batteries is achieved either by running the PMV engine or attaching an external generator; neither option is desirable due to the noise produced. The PMV engine provides the simplest option as it is contained within the vehicle and uses the vehicle’s primary fuel tank. However, the engine is significantly more powerful than required leading to inefficient use of fuel and production of noise. An operating PMV also cycles the automatic tyre inflation system which leads to characteristic snorts and hisses as air is pressurised and released. Maintenance liability of the PMV must also be considered when running the engine for extended periods.
A more efficient option is the use of an external generator with the current in-service solution the 1.3kW Generator Set, Diesel Engine. This system consists of an alternator direct coupled to a single cylinder 4-stroke air-cooled diesel engine capable of producing either 14V or 28V direct current. While the system’s fuel consumption is efficient compared to the PMV engine, it has a high signature when operating. Noise is the most significant issue as the system is rated at 79dB at 7m, however its thermal signature is also high as the system runs hot due to being air cooled. Digging-in the system does help to reduce noise but care must to be taken to ensure the pit is sufficiently large to enable airflow for cooling and exhaust and this diminishes the effectiveness of noise abatement. The generator set also imposes a technical compliance burden due to its short servicing interval of only 100 hours. The generator set appears to have no smart charging capabilities and this causes over-charging which further reduces battery lifetime and capacity.
On the gunline it is not only the PMV employing a noisy generator as the M777A2 also requires electrical power to operate at full capacity. The M777A2 is a fully digitised platform supported by on-board electronic systems including fixation and orientation, a VHF radio with 50W amplifier, mission computer and various displays. Battery charge and discharge is managed through an integrated Power Conditioning and Control Module (PCCM). This module regulates power supplied either externally or internally from the twin 12V absorbed glass matt lead acid batteries, manages charging cycles and reports battery state to the detachment commander’s display. Absorbed glass matt batteries are maintenance free and have a longer charge / discharge life cycle than flooded cells however are more costly. In the case of the M777A2, the orientation of the batteries in their cradle (lying on their side) prevents the use of flooded batteries. Each battery has a 55amp-hr rating at 20 hours and a 50amp-hr rating at 5 hours. As the batteries are arranged in series the total capacity at 24V is still 55amp-hr (at twenty hours). One battery weighs 20kg and has dimensions 255mm x 175mm x 201mm.
Work done by Warrant Officer Class Two Troy Charters last year in conjunction with Capability and Sustainment Group tracked battery power over time and found that the complete M777A2 at rest consumed 5amp-hr/hr (120W). Tracking of terminal voltage over time showed a time to cut-off voltage (10.5V per battery) of just over 6 hours. The PCCM also appears to report battery percentage remaining relative to cut-off voltage. This causes detachment commanders to apply charge earlier than technically required due to an understandbly reasonable fear that a low percentage is leading to battery damage. Whilst the capacity of the battery pack verse the required power draw would seem to imply a silent watch capacity of 6 hours the PCCM charge rate, even when connected to regulated mains power, did not exceed 10amp-hr/hr. This leads to the conclusion that, for a period of arbitrary length, one third of the operating cycle must be spent on charge. In effect maintaining charge on the gun requires at least 8 hours of charge time per operating day, a conclusion supported by analysis of the usage data of the 1.3kW generator sets allocated to each gun detachment.
In practice, and given the above considerations and power requirements, a deployed gunline using the in-service equipment is running six 1.3kW generator sets for at least eight hours each per day. This creates a significant noise signature and imposes a high maintenance burden. A commander looking to maintain a gunline at silent watch can achieve about 2 hours of silent time at best. The number of hours in the day for which some noise is being made can be reduced by implementing a program of controlled start-ups and shutdowns. However, this is constrained by the most power hungry platforms resulting in other systems running more often and for longer than they require – essentially trading off signature management against maintenance liability.
While the advantages of digital systems, and the need to maintain critical components of the fires chain, require the maintenance of power at all times, the capacity of the platforms to store and generate power are insufficient to the needs of tactical employment. This paper is certainly not the first to consider this problem; units have purchased silent petrol generators and 24V rectifiers, Smart Soldier covered improvised extended lead acid battery packs for the PMV, and Capability Acquisition and Sustainment Group (CASG) trialled integrated silenced 5kW diesel powered auxiliary power units in the PMV-CP replacing the winch and main bin with good results. Defence Scinence and Technology (DST) has also looked at this problem including a paper of January 2017, “Review of Battery Technologies for Military Land Vehicles” and the September 2014 paper, “Power Options for the M777A2 Howitzer Digital Fire Control System.” Yet the problem persisits. The next section looks at new opportunities to improve storage and generation of electrical power.
Short term opportunities
In order to be implemented in the next twelve months, improvements to gunline silent watch time must be achievable either through changes to procedure or procurement of COTS components with, at most, minor engineering modifications. Short term opportunities considered below include improved training, improved maintenance, drop-in replacement batteries and improved power distribution. The previously mentioned trials of quiet petrol fuelled generators produced good results. However, the diversification of fuel types (particularly with petrol being a more volatile fuel) and lack of commercially available 24V petrol generator systems make this option difficult to support at the whole of army level.
Soldiers operating PMV and other electrical systems are generally taught not to allow battery voltage to fall below 10.5V per battery. Unfortunately, few understand the reason for this limitation. Similarly there is little to no understanding of the underpinning battery chemistry or the factors that lead to reduced capacity over time. Maintenance of electrolyte levels in flooded lead acid batteries is an uncommon skill set generally reserved for Transport Corps soldiers and so becomes infrequently performed. Improvements in training which explain the operating principle and failure modes of these type of batteries, as well as how to check, refill and balance electrolyte, would enable operators to limit capacity reduction over time.
Maintenance - Smart Charger
The periodic nature of field deployments and annual programs means that batteries, particularly flooded lead acid batteries, are without maintenance charging for long periods. Flooded lead acid batteries have a high self-discharge rate which can lead to sulfation and loss of capacity / life cycle if left idle. Capacity and battery life would be enhanced through purchase and distribution of smart chargers on a one-per-PMV basis. These would be connected to the PMV in the hanger whenever the PMV was not operating for a multi-day period. Modern smart charger systems not only charge and maintain voltage (float) in the batteries, but perform an electrolyte mixing equalisation phase that can remediate light sulfation. Although 24V systems are less common than 12V systems, a good quality charger can still be purchased for less than $600. Purchasing and employing smart chargers would assist in maintaining capacity over time and extend battery life.
Whilst lead acid batteries are a mature technology and relatively inexpensive, there are other battery chemistries which offer better performance in terms of capacity, cycle life and charge/discharge speed. DST’s 2014 report recommended consideration of replacement of the M777A2 sealed lead acid battery with a hybrid lead acid / ultracapacitor battery, an Australian invention marketed under the name “Ultrabattery”. These batteries provide two to three times the life cycle of the lead acid battery (offsetting their higher cost) with dramatically improved charge/discharge speed and a claimed higher capacity for size and weight. A comparatively sized Ultrabattery for the M777 claims to extend silent watch time by up to 70% whilst offering more rapid recharging. However, a later review concluded that Ultrabatteries were unlikely to significantly improve silent watch performance. Noting the relative sophistication of the M777A2 PCCM a different battery technology may require recalibration or replacement of the PCCM which may rule out this approach in the short term. Australia is not alone in considering new battery chemistries for the M777A2 and the United States enhanced power pack program implements a new single 28V lithium iron phosphate battery of 80amp-hr capacity which replaces both existing lead acid batteries. While this would provide at least a 25% improvement in silent watch, the key will be the revised or reprogrammed PCCM which enables charging at the higher rate the lithium iron phosphate battery affords. The enhanced power pack modification should be tested and approved as soon as possible for Australian systems.
DST’s 2017 review of battery technology for vehicles concluded that further investigation of drop-in replacements utilising lithium iron phosphate or lithium titanate batteries was warranted. These battery chemistries were seen as more desirable than the otherwise mature lithium ion battery type due to improve safety considerations. One advantage of lithium iron phosphate and lithium titanate batteries for application to PMV silent watch is their comparable voltage with standard lead acid cells. This leads to the opportunity to purchase and install COTS batteries of this type without modifying surrounding electrical systems. COTS replacements of the PMV radio batteries with lithium iron phosphate batteries of the same weight and rough dimensions would at least double the amp-hr capacity of the pack. For instance a currently commercially available 12V lithium iron phosphate battery of 37.5kg with dimensions 520mm x 267mm x 228mm has a rated capacity of 300amp-hr (compared with the 125 amp-hr radio pack currently fitted to the PMV). Given the ability to discharge this type of battery further than lead acid without degrading capacity the actual improvement would be to more than double the silent watch period of the vehicle.
Following DST’s review, Land Systems Division, in conjunction with Headquarters Forces Command, tested replacement of the four PMV lead acid batteries with four lithium ion replacements. The report noted that the higher charging speed of the replacement batteries necessitated replacement of the PMV alternator with a higher capacity unit. Seizing the opportunity to fast charge lithium based batteries contributes to reducing the engine or generator run time and increased silent watch periods over the duty cycle. Total estimated per vehicle cost to replace the lead acid batteries with lithium ion systems in 2017 was $25,000; it should be noted that the cost price for Lithium Iron Phosphate batteries has since substantially decreased and continues to fall.
Simple Power Distribution
The HX77 gun tractor is fitted with gun pack (primarily concerning the connection hardware for M777A2 power and data) as well as an additional power unit which is hydraulically driven by the main engine. In addition to the gun power output connector, the chassis mounts two 24V 60A sockets (each capable of providing 1.4kW). The HX77 at idle is significantly quieter than the PMV although still suffers from the inherent inefficiency of employing a 10L 324kW diesel engine to maintain a 120W load. The vehicle has a large visual signature and when employed tactically is usually parked remotely to the gunline to enable concealment. The relatively quiet operation of the HX77 and run time insensitivity of its maintenance schedule make it an otherwise desirable charging source except for its large visual signature. In order to enable the HX77 to be better concealed away from the gunline, longer (more than 100m) power cables could be fabricated or purchased. These cables would be simple power cables and so should not be expensive or delicate.
Medium term opportunities
To be suitable for implementation in the next three years, opportunities to improve gunline silent watch time need to employ existing mature technologies and not require substantial modification of existing platforms. Medium term opportunities considered below include custom battery packs, flow batteries, replacement diesel generator systems, networked power distribution and solar power supply.
Rechargeable battery packs can be manufactured in almost any dimension with the required voltage regulation and charging systems integrated in to the pack. Large battery applications, such as electric vehicles, employ numerous smaller cells. A common cell type weighs 48g and has dimensions 18.5mm diameter x 65.3mm length for a capacity of 3.25amp-hr at 3.6V. Seven of these cells in series would provide a 3.25amp-hr 25V battery weighing 0.34kg. A custom battery pack of the same weight as two PMV flooded lead acid batteries could therefore provide 650amp-hr less some loss for the associated housing, voltage regulation and charging systems. This represents a more than fivefold increase in silent watch time without increase to vehicle weight. Such a pack would occupy more volume than the current batteries but the ability to custom assemble the pack means that it could be placed elsewhere on the vehicle. Options for placement might include replacing the void of the internal water storage tank or a conformal pack externally mounted such as a bulge on the rear armour plate of the vehicle.
With regard to the PMV water tank, the availability of a protected void in the vehicle affords the opportunity to consider implementation of a flow battery system. Flow batteries are a special case of fuel cell – effectively a rechargeable fuel cell. A current commercially available zinc bromine flow battery holds 10kWh of power using a 100L electrolyte tank and runs at 48V with up to a 3kW normal continuous output. This type of battery can be fully discharged without reduction in life and can be stored in any charge state indefinitely without damage. A unique capability of flow batteries is that energy storage capacity can be increased simply by increasing the volume of electrolyte – if the PMV water tank space was to be used a capacity of well over 100L is available. Even at 10kWh the battery would provide the PMV-CP with a silent watch capacity of more than 24 hours.
Advanced Power Distribution
The uptake of quiet petrol generators by some units as a power source enabling vehicle level silent watch reflects the lack of small quiet diesel generators commercially - small and light diesel engines are invariably noisy. Larger generators can afford the weight and complexity of sound suppressing systems, however these are substantially more powerful than required by an individual vehicle. Still, such a system remains attractive for its quiet operation, potential fuel efficiency and maintenance gains over running vehicle main engines. A 5kW system would be sufficient for the whole of the gunline and could be connected to each vehicle via a power distribution network. Operating at 24V such a network would not require specialist electrician installation and should be engineered to be as plug and play as possible. Creating and connecting such a network also creates the opportunity to integrate two wire communications in the cabling reducing the position’s electromagnetic signature. Such a system would only be suitable when multiple platforms are deployed close together (battery tight deployments). Notably, this is the dominant deployment mode during training and the more likely deployment mode for prolonged static occupation on operations.
If an additional generator is not considered suitable, then employing the HX77 as the primary power source for the gunline may still be viable. With a total power output of more than 2.8kW one HX77 is sufficient for most gunline power demands and runs a relatively silent engine. Ideally linked to a power distribution network such as considered above the HX77 employed as the primary power source could be periodically rotated to spread the engine run time across the fleet.
Solar power provides a power source which requires no consumable fuels and is completely silent. However, it is limited to functioning during daylight hours and, even with the latest non-specular coatings, creates a significant visual signature particularly against airborne threats. Nonetheless, the effectively unlimited power available and silent operation may make it attractive on long deployments, particularly when the air threat is low. On average, a 1m square solar panel produces about 1kW during good conditions so approximately 5 such panels would suffice for gunline requirements during daylight. However, due to the limited window to produce power actual requirements would be higher: a solar system would need to generate not just enough power to run the gunline but also enough power to charge batteries for the night hours and these batteries would need to be upgraded to the point that they can support a silent watch of the full period of darkness. While such a large system would be cumbersome, the availability of HX77 and flat racks to the gunline mean that system size is not necessarily a problem if the advantage gained can justify the use of cargo space. Such a system might require substantial setup and tear down time so might only be useful when prolonged periods of static occupation are anticipated. Nonetheless, under such prolonged deployment a solar generator has the potential to reduce the logistics burden of the deployed element.
Long term opportunities
Long term opportunities to improve gunline silent watch rely on technologies which are not yet fully mature or which would require substantial modification of even complete replacement of fleet and platform. Options considered include advances in alternative power generation technology and the hybridisation or full battery electrification of vehicle drive trains.
Fuel cell technologies offer power generation that is effectively silent with the energy density and refuelling advantages of traditional generators. Whilst hydrogen fuel cells (polymer electrolyte membrane type) are a relatively mature technology already employed in commercially available electrically driven vehicles, hydrogen is a relatively difficult fuel to handle and store and is highly volatile making it less suitable for military employment. Fuel cells capable of employing denser fuels such as natural gas exist (molten carbonate and solid oxide types) however operate at temperatures over 650 Celsius making them high signature and unsuited to vehicle employment. The best military option in the future is likely Direct Methanol Fuel Cell (DMFC) technology. Methanol has the advantage of being a relatively cheap liquid fuel but current generation DMFC are expensive and require high purity methanol. DMFCs are able to operate at room temperature although are more efficient at around 90 Celsius. Advances in DMFC technology are focussed on reducing the amount of precious metal required in the catalyst (to reduce the cost of the system) and increasing impurity tolerance. If this technology reaches the point where it is affordable and can accept military grade fuels then it is likely to be a highly suitable replacement for high signature diesel generators.
Drive Train Electrification
Allied military vehicle acquisition and development programs are currently pursuing hybridisation and full battery electrification of planned future fleets. In the United States this effort is led by the Combat Capabilities Development Command Ground Vehicle Systems Center (CCDCGVSC). In addition to improvements in efficiency (and hence endurance), the CCDCGVSC notes the desirability of incorporating large battery packs to enable protracted silent watch and silent attack. Works undertaken by the Japanese Ministry of Defence by their Ground Systems Research Center created a hybrid drive M113 which displayed performance characteristics as good or better than a traditional diesel drive only version and employed a 32.6kWh lithium ion battery pack. The use of an M113 is significant in that it represents both an in-service vehicle with the Australian Defence Force and a vehicle of comparable weight (at 13 metric ton) to a PMV. A comparable hybrid drive system then would afford several days of silent watch capability whether employed in a CP or connected to an M777A2.
An operationally deployed gunline seeks to minimise its chances of detection until it can be employed at a decisive moment in the battle. The current short duration of silent watch achievable with in-service power packs and generator technology undermines this objective. Immediate action to improve battery maintenance, replace low energy density batteries with more modern chemistries and improve power distribution is recommended. As the demands on vehicle power increase the Army should purchase and integrate high capacity custom energy storage solutions, particularly on the PMV, as well as replacing the 1.3kw generator set with a quieter and more powerful system linked to a distribution network. In the longer term the Australian Defence Force should closely follow developments in fuel cell technology as a generator replacement option and consider hybridisation in future fleet acquisitions and life extension programs.