December 20, 2009      

Once cell phones became small enough to easily fit into a pocket, the market for them exploded. One key enabler was battery technology, which will also play a critical role in the growth of the mobile robotics market. The demand robotic systems place on batteries is far greater than that of cell phones, which require only that batteries be small and remain charged for long periods of time.

Compared with cell phones and other consumer electronic devices, robots and robotic technology are more complex, performing many energy-intensive tasks. Battery and fuel cell companies have struggled to deliver energy storage solutions that meet the power, capacity, and weight requirements for large classes of mobile robotics systems. The result: Power storage technology remains a major gating factor to the expansion of most segments of the mobile robotics market.

Not every robotic system presents the same challenges to battery development. For some market segments, battery technology will be less of a limiting factor than for others. Applications are the real drivers of power requirements.

Regardless of their individual power requirements, all segments of the mobile robotics market will benefit from recent political developments in the United States and other industrialized countries that resulted in a rapid increase in funding for battery and fuel cell research and development, along with financial support for expanding manufacturing capacity.

The consequence of this influx in funding will be new technology, as well as increased availability of power solutions formerly limited by high cost to the military and other price-insensitive markets, thereby overcoming the limitations of current power storage technology and its stranglehold on the commercial mobile robotics industry.

Power to the robots

At a minimum, mobile robots need power for locomotion. In most cases, that power source must be incorporated into the robot itself. The power source can be a battery or a fuel cell. The former can be electrically recharged after discharge, while the latter is refueled periodically. Either can produce the electrical energy necessary to drive the robot’s various on-board systems. Another source of power for mobile robots is photovoltaic cells, or solar cells, which convert solar radiation into electricity that is stored in batteries (or less typically in capacitors).

Before discussing the political, commercial, and technical trends driving the power storage industry, particularly as it applies to enabling new or enhanced applications for commercial mobile robots, it is necessary to discuss the relevant technologies and some of the key technical characteristics robot developers consider.

Battery and fuel cell technology

To understand power storage technology, one must understand elemental chemistry, specifically the simple chemical reactions that yield free electrons using two electrodes (a cathode and an anode) separated by a medium called the electrolyte. This is the case for batteries that can be recharged after use.

Fuel cells also have two electrodes separated by an electrolyte and use chemical reactions to generate the free electrons. In this case, however, fuel, usually hydrogen, has to flow into the cell to provide the energy source. Because they use fuel, fuel cells do not have to be recharged. For commercial mobile robot applications, developers have a number of choices.


There are many variations on a theme, but the primary battery technologies for commercial-class mobile robots are:

  • Lead-Acid. This mature technology, which is readily available and relatively low cost, provides 30-50 watt hours/kilogram (Wh/Kg). Lead-acid batteries boast a large charge time but are heavy and not eco-friendly. Unlike the “dry cell” discussed below, lead-acid batteries contain corrosive, caustic liquids that can leak out if the battery is punctured or flipped. Sealed lead acid (SLA) batteries overcome this problem by permanently confining their acidic contents in a rugged plastic case. SLA batteries are common to military robots and other types of larger mobile services robots such as lawn mowers and search-and-rescue robots. Like traditional lead-acid batteries, SLAs provide high current but are heavy.
  • Nickel-Cadmium. Nickel-cadmium (NiCad) batteries are also a mature technology that is easily obtainable, but at a moderate to high cost. While not eco-friendly, NiCad batteries supply a 45-80 Wh/Kg energy density, excellent peak current, long cycle life, and a rapid charge time. One issue with NiCad batteries is the gradual loss of capacity over the life of the battery due to memory effects.
  • Nickel-Metal Hydride. Nickel-metal hydride (NiMH) batteries are based on mature technology and are available at a moderate to high cost. The technology provides an energy density of 120 Wh/Kg, low to moderate cycle life, modest peak current, and a middling charge time. NiMH batteries generate more heat than other battery types, and must be fully discharged before recharging.
  • Lithium-Ion. Lithium-ion (Li-Ion) batteries, a relatively new technology, deliver approximately 110-175 Wh/Kg energy density. While the batteries are readily available and charge in a moderate amount of time, they are high priced and have safety issues. In the future, higher energy density will be possible with lithium-iron phosphate (LiFePO4) batteries and nanophosphate batteries.

Fuel Cells

A fuel cell derives the energy it delivers from whatever “fuel” is provided, which, as previously noted, is typically hydrogen. The electrical output is generated at a constant level until the fuel runs out. There are many types of fuel cells, characterized by the type of electrolyte they use, and all are relatively new choices for robot developers. Products for the robotics market will come primarily from small start-ups heavily dependent on military contracts and venture capital. Three types of fuel cells are:

  • Polymer Electrolyte Membrane. With polymer electrolyte membrane (PEM) fuel cells, hydrogen protons are transferred through the electrolyte membrane. Most PEM fuel cells use hydrogen, although methanol and other biofuels are possible if they are reformulated to remove residual sulfur atoms. The PEM electrolyte membrane most commonly used is DuPont Nafion.
  • Direct Methanol. Direct methanol fuel cells (DMFCs), as their name implies, process methanol directly. This simplifies their design but results in a less efficient conversion. DMFCs, however, convert their fuel more cleanly than PEMs and are less expensive.
  • Solid Oxide Electrolyte. With solid oxide electrolyte (SO) fuel cells, oxygen is removed from air at the cathode and is transmitted through a solid electrolyte, usually a filter made of ceramic material, to the anode. The fuel is then oxidized at the anode, forming water and carbon dioxide. Hence, any hydrocarbon gas such as propane or butane may be used for fuel. Fuel cells tend to generate a lot of heat and operate at temperatures of 650 to 1,000°C. This imposes a warm-up period (anywhere from 20 minutes to two hours) before full power can flow, which can be shortened in a hybrid solution that incorporates a battery.

Any application of a fuel cell in a mobile robot, regardless of the fuel used, would typically require a hybrid power solution that incorporates a battery or an ultra-capacitor to accommodate peak currents. Just as with motor vehicles, hybrid power solutions are likely to be one of the important future technology directions for mobile robots.

Advantages of fuel cells

The two main advantages of fuel cells are their quick refueling compared with the long recharge times of battery-only solutions, and they operate two or three times longer than conventional batteries. Just how much longer depends on the size of the fuel supply. Energy densities of 300 to 700 Wh/Kg average for 12 hours have been demonstrated in military applications, but with larger fuel tanks that number could eventually be three or four times greater depending on the fuel used.

Most research and development efforts for fuel cell design are in the area of automotive power, which requires several kilowatts of power, or for applications that demand less than 20 watts. There is much less development (and fewer sales) for fuel cell technology suitable for typical mobile robotics applications. The exception is military robotics.

Because weights and runtime trade-offs are more critical than cost for military robotics applications, producers of fuel cell technology have focused on this lucrative market. Also, in the United States, much fuel cell technology research is funded through the Department of Defense. Commercial versions are a lower priority for product development.

When fuel cell manufacturers do come to market with products targeted to the commercial space, they will likely seek vehicle use, such as warehouse forklifts, as their first market. As a result, most robot developers are taking a wait-and-see approach and relying on batteries for the foreseeable future. An increase in funding for fuel cell R&D efforts, which is occurring at this time (see “Expanded R&D and Manufacturing Initiatives” below), should speed the development of fuel cell solutions suitable for mobile robotics systems.

Applications and impact

Robot developers typically look at five selection criteria when choosing battery type: 1) high capacity (energy density), 2) longer life cycle, 3) physical dimensions (weight and volume), 4) cost, and 5) peak current. For most mobile robots, weight and volume requirements can be satisfied by low-cost lead-acid batteries, which, although heavy, provide a stable center of gravity when placed in the base of the robot.

The mobile robot equivalent of the “pocket-size mobile phone” is the ability to pass easily through standard doorways and hallways. That ability is available with existing batteries, so it would seem that improvements in battery technology focused strictly on size reduction are not going to spur the growth of the mobile robotics market in a major way.

However, lighter batteries would mean that battery-powered motors would require less effort, and hence less power, to drive the lightened mobile platforms. Longer operating times, coupled with shorter recharging times, cannot hurt.

Reduced battery weight would have a major impact on a specific subset of mobile robots, namely portable mobile robots. Battery weight is a critical factor for portable robots that are used at multiple sites and must be transported, perhaps in a backpack. Today’s battery technology for these applications places clear limits on portability and the number of hours of operation between recharges.

Since improvements in portable power solutions will provide smaller, lighter, higher energy-density solutions, Robotics Business Review believes it is in this submarket in which advances in battery technology will have the greatest impact on market growth.

Applications for portable mobile robots are easy to imagine. The commercial equivalent of the Roomba vacuum cleaner is one, if the owner is an office cleaning service with multiple clients and must move the cleaning robot from site to site.

Other examples of portable mobile robots include search-and-rescue robots deployed by fire departments at disaster scenes, surveillance robots installed by temporary contract services (for example, events), robots used by caterers, mobile robots used by police to defuse bombs, mobile kiosks used temporarily at events to dispense information and guide attendees to booths, and scout robots performing any number of search-and-assist tasks for police entering a potentially hostile environment.

The point is that the robot must be mobile, easily portable, perform for a practical number of hours, and have a reasonable total cost of ownership. This is not the case today, but newer Li-Ion batteries or emerging fuel cell products will gradually improve the situation.

Market dynamics

Today, most commercial-class mobile robots run on lead-acid batteries. NiMH runs a distant second and NiCad and Li-Ion account for the tiny remainder. Robotics Business Review expects this to change modestly over the next five years, as Li-Ion gradually takes market share from NiMH, and as Li-Ion variants, which deliver higher power densities but cost more, capture much of the growth associated with new mobile robot applications.

Given that fuel cell usage in military applications is still in the demonstration phase, widespread use of fuel cells as an alternative to batteries for mobile robots is still a few years off.

Worldwide, R&D money is intensely targeting fuel cell and battery technical nuances (nanowires that could improve the cost/benefits of Li-Ion batteries, for example) to provide low-weight, low-cost power solutions that operate for a long period of time. Technical breakthroughs will occur faster with a significant increase in funding for R&D and commercialization efforts. This is exactly what is occurring now.

Expanded R&D and manufacturing initiatives

Governmental funding for battery and fuel cell R&D across most industrialized countries has historically been strong. This is particularly true in the security/defense and energy arenas. Venture capital and other private investment sources have also supported battery and fuel cell R&D and commercialization efforts.

Although the recent worldwide economic downturn has negatively affected battery and fuel cell production and sales in the short term, it has-through the U.S. government’s American Recovery and Reinvestment Act (ARRA), for example-greatly accelerated R&D efforts and the long-term commercialization prospects of the technologies.

In February 2009, the ARRA was signed into law, providing massive investment in power technology, including batteries and fuel cells. Under ARRA’s Electric Drive Vehicle Battery and Component Manufacturing Initiative (DE-FOA-0000026), for example, 48 new battery and electric-drive projects would receive $2.4 billion in funding, $1.5 billion of which would be grants to U.S.-based manufacturers to produce battery and fuel cell technology and to expand battery recycling capacity.

These grants are already being awarded. Polypore International Inc., a supplier of high-performance microporous membranes, for example, announced that its Celgard LLC business unit has been selected for a grant award of $49 million to expand Li-Ion battery production under the program. Similarly, BASF’s BASF Catalysts LLC division will receive $24.6 million to build cathode material production facilities for Li-Ion batteries.

In addition to increasing manufacturing capacity, ARRA also provides huge amounts for basic, university-level battery and fuel cell research and development.