Energy Supply

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Real energy
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Canned food?
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Introduction

Robots move. In particular, they move in non-constant ways. From basic physics, changing the movement of a massive body requires the expenditure of energy. Robots also compute. Physics dictates that this too requires some energy, though the connection is not so obvious, and the theoretical lower bound is small compared to existing devices.

So robots need energy.

These days, most computation is done electronically. Electrical power is also used, almost exclusively, for controlling sensors and actuators, even when non-electrical energy provides the main motive force. So robots generally need electrical energy.

Energy for movement can be obtained from electricity, or from other sources such as gasoline engines or compressed gasses. For small, indoor robots, of the sort most likely to be created by the bginning constructor, electric actuators are most common. Flying machines used to require combustion-powered motors, but new battery technology has made autonomous electric flyers feasible, at least for short-duration (10-20 minute) missions. Extended or long-distance flight still needs combustion engines, but who knows? That cheap, lightweight fuel cell may be just around the corner.

Dinnertime

As a philosopher I once heard about put it, "the ultimate question of life, the universe, and everything is, 'what am I going to eat today ?' ". For the robot constructor, this translates to "where will my robot get its energy?"

A robot can, of course, be tethered to a power source such as an electrical cable or a pneumatic line, in which case energy supply issues can be decoupled from the robot design. However, power tethers severely limit the range of operation, and the need to avoid twisting and tangling of the tether creates additional problems.

A good method for wireless transmission of power would be fantastic, but current technology in this regard is primitive, especially compared to wireless transmission of information, which is highly developed. Wireless power transmission modes that have been explored for use in robots include light (sun, lasers) + solar cells, microwave beams, wind (windmills, sailboats), and inductive coupling (sometimes with resonance). All have significant limitations.

In practice, for real autonomy, an onboard source of energy is needed. There may be a need to periodically "refuel", but between fillups, such a robot has much more freedom than it would if tethered.

For the amateur constructor, on-board energy often comes down to one option: batteries. So most of the material here is about just that.

Battery Basics

Batteries provide instant, on-demand access to electrical energy, long storage times (days to years), reasonable power density, and silent, vibration-free operation. They have no moving parts, no emissions, no hot components, and produce little waste heat. They are maintenance-free, and mechanically simple, robust and reliable. They can be made in virtually any size. No other onboard source of electrical power has this combination of characteristics.

Batteries produce electric power through complex electrochemical reactions. Luckily, the chemical details are rarely relevant to the robot constructor (except maybe in situations where battery contents, which can contain toxic and hazardous substances such as lead, cadmium, and lithium metal, might be released to the environment).

The basic unit of a battery is the electrochemical cell. For a given battery technology, each cell generates a characteristic voltage (currently limited to the 3.7 volts of lithium cells). A battery as purchased may consist of a single cell, or of multiple cells connected in series (and sometimes parallel) to obtain higher voltages (and currents). In fact, the term "battery" refers to the multiple-cell nature of many devices, in analogy to an artillery battery.

A very wide range of batteries is commercially available. They vary in size, shape, electrical characteristics, chemical technology, rechargeability, and may other respects. The robot constructor needs to select appropriate batteries for each of the various capabilities they power. If this is not carefully done, the robot will either simply not work, or it will work badly (which might include burning up or exploding). The following sections try to elucidate some of the more important considerations.

Battery Ratings

Batteries as units are rated by their voltage, their storage capacity, their ability to deliver current, and their self-discharge rate. Chemical technologies are rated rated by specific energy.

Voltage

Battery voltage is specified (naturally enough) in volts. A properly selected battery will maintain close to its initial (rated) voltage during use, declining only slowly, until nearly exhausted, at which point the voltage drops rapidly. The correct voltage should ALWAYS be used for a device. Too low, and the device will not operate. Too high and it may burn out or worse (explode or start a fire). Some devices have internal regulators and will accept a range of voltages. If so, it will be prominantly noted.

Capacity

Battery capacity is usually specified in milliamp-hours (mAh), or sometimes amp-hours for larger devices. This rating is a rough measure of how long a battery can supply a specified current before it is exhausted (i.e. the voltage supplied to a device drops below a specified level). For example, a 1000 mAh rating would imply that a battery can supply 100 mA for 10 hours, or 1000 mA for 1 hour. For purposes of comparison, a standard 1.5V AAA alkaline cell can deliver about 1000 mAh, and a D cell 10,000 mAh. This rating is not really a constant. The actual time-integrated current a battery can supply depends on the discharge rate, "rest" periods, and temperature, among other factors.

Current Delivery

Deliverable current is specified in Amperes (or milliamps). Ratings may refer to either sustainable current (the maximum current that can be safely delivered for an extended period of time), or to burst capacity, (the maximum current that can be delivered for a short period of time, typically a few seconds). These values can differ by an order of magnitude or more. The robot constructor will generally be most interested in sustainable current. Burst capacity is important for applications like starting a gasoline engine where units such as "cold cranking amps" (CCA) are used. A 1.5V AAA alkaline cell can deliver around 100 mA continuously, a D cell, about 500 mA.

For rechargable batteries, current delivery ability is sometimes specified as a C-value. C1 means delivery of a battery's rated capacity in 1 hour. C10 means 10 times that rate, or delivery of the capacity in 1/10 of an hour. C values for rechargables range from C1 to C50. The concept can be applied to single-use batteries as well, but for alkaline cells, and some other technologies, the capacity at slow drain rates can be several times the capacity at high drains, so the number is poorly defined.

A common mistake among beginning constructors is to select a battery for running a motor only on the basis of the voltage required by that motor; for example using a standard 9-volt "transistor" battery for wheel motors rated 6-12 volts. The problem is that the wheel motors may well draw over an Ampere of current when driving the vehicle. A 9-volt battery is intended to sustain a few tens of milliamps at most, and burst capacity is limited to a few hundred (and only when the battery is fresh). It just can't supply enough power to drive the motors. A good rule of thumb is that your batteries should weigh significantly more than your motors.

Self Discharge

All batteries eventually self discharge, that is, they lose their charge due to various internal side reactions and slow diffusion effects. The rate may vary from several percent per month to a fraction of a percent per year. A discharged battery not only will not provide electric power as expected, but may be a chemical corrosion or even a fire hazard. Most modern batteries can be left at least a year if fully charged initially, and some designs are expected to last 20 or 30 years (though it is unclear if this has been completely tested empirically, since most designs have not been around that long).

Energy Density

The energy density or specific energy rating for a battery technology describes the (maximum) electrical energy production for the chemical reaction and packaging employed. It is commonly specified in Watt-hours per kilogram (Wh/kg) or kilo-joules per kilogram (kj/kg). 1 Watt-hour is 3.6 kilojoules. The theoretical energy density for common batteries ranges approximately from 40 to 300 Wh/kg (150 to 1100 kj/kg). This compares to an energy density for fuels like gasoline of approximately 11,000 Wh/kg (40,000 Mj/kg). (So you can see why fuels are sometimes used despite their messiness)

Single-Use and Rechargeable Batteries

Batteries are classified as single-use (primary, disposable, replaceable), or rechargeable (secondary).

Single-use smorgasbord

Single-Use Batteries

Single-use or primary batteries are what you usually buy at the store for your flashlight. When they run out, they must be discarded and replaced with new ones. This is an expense, but if batteries last for weeks to months of normal use, or if a device is only used occasionally, they may be a better choice than rechargeables. Common examples include alkaline batteries in AAA-D formats, and "button" or "watch" cells. Replaceable batteries will usually hold energy for a period of years, compared to days to months for rechargables. They may also have better energy density than comparable rechargeables. They are typically more expensive to use on an on-going basis than rechargeables, and some expended batteries are classified as toxic waste.

Practical robots rarely use single-use batteries as a primary energy source, but may use them to maintain state, or for certain startup operations.

No attempt should ever be made to recharge a single-use battery. The chemistry in the cell just does not work in reverse. At best, the unit will get hot and leak nasty chemicals. At worst it may explode and burn. Another caution: old and new batteries should never be used together in a device that needs multiple cells. The older ones run out first, and are driven into "reverse polarity" by the newer ones, which again, can result in chemical leaks, overheating, and explosion or fire.

Replaceable "Alkaline" batteries are familiar consumer commodities in 1.5 V AAA, AA, C, and D cylinders, and 9V "boxes". Newer lithium cells are available in 1.5V - 3.6V AAA and AA, and 9V box formats. Compared to alkaline cells, Lithium batteries have better high-drain performance, better cold-weather performance, and potentially longer shelf life (12 vs. 6 years). They are also several times the price.

Replaceable "button" or "watch" batteries are available in a huge variety of (small) sizes, so many that it often seems that no two devices use the same battery. They are intended for long term, very-low-current (microamps) applications, sometimes with brief (a few seconds) currents of a few milliamps. They utilize a variety of chemical technologies, including alkaline, lithium, silver oxide, and zinc-air.

NiCad and lithium-polymer battery chargers



Automotive lead-acid battery charger

Rechargeable Batteries

Rechargable batteries can have their energy replenished when exhausted, generally hundreds of times. This is done by essentially pushing electricty through them in reverse using a device known as a charger. Rechargeables are usually the choice in any application that runs down batteries on a daily or weekly basis, as they are much more economical in this situation. Practical, battery-powered robots almost always use rechargeable batteries for primary energy.

The charging process typically takes several hours, which can be an inconvenience. Chargers also need an external power supply, are often bulky with awkward attached cords, and may be expensive. Rechargeable batteries can be damaged by incorrect recharging procedure, and may even explode and/or catch fire. For this reason, it is essential to always use a charger designed for the exact battery type and size you are using. All rechargeables eventually wear out and fail to hold adequate charge, but usually they are good for a couple years of regular use if treated properly.

Common rechargeable technologies include lead-acid, nickle-metal-hydride (NiMH), and lithium-ion/lithium-polymer. Most large batteries are rechargeables, as it would make little sense to discard multiple kilograms of relatively expensive materials on a regular basis. Conversely, button-scale rechargeables are uncommon, though they do exist in applications such as solar-powered watches.

Easily available lead-acid batteries range in size from car batteries weighing 50 pounds or more down to D-cell format. NiMH and lithium-ion batteries range from portable tool power packs and laptop batteries weighing a few pounds, down to AAA format. Lithium polymer (LiPo) battery packs are available down to a few tenths of an ounce (5g). Very large (e.g. multi-kilogram) lithium batteries are a serious hazard if they catch fire, and are not readily available to consumers.

Battery Technologies

Very briefly, a battery consists of one or more "electrochemical cells" that produce the electical power. Every cell has an anode, the "-" terminal of the battery, and a cathode, the "+" terminal. In use, the cathode(+) delivers positive electric current, (which, under our somewhat confusing electrical conventions, means it sucks up electrons) and the anode(-) receives it (which means it produces electrons).

The terminology is a bit confusing, since we are used to thinking of the negative terminal of a device like a diode as the cathode. The explanation is that the cathode is defined as the terminal through which electrons flow into a device (regardless of polarity). To confuse the issue even more, during charging, the current flow is reversed, so the + terminal is the anode and the - terminal the cathode :( .

Internally, the anode (-) is a material with loosely bound electrons, i.e. a reducing agent in the oxidation-reduction sense, often a metal. The cathode (+) is an oxidizing agent with a strong chemical affinity for electrons, frequently a metal in a high oxidation state. If mixed together, the two materials could react with each other, but are kept physically separate, so they cannot. If put in electrical contact through a conductor, the cathode will grab a few electrons from the anode, until unbalanced charge keeps it from taking any more.

Ordinarily, everything would stop here. However, in a battery, the cathode and anode are also in contact through an "electrolyte bridge" which contains ions in solution. These ions, being in intimate contact with the electrode materials, can react. One ionic reaction removes those "extra" electrons from the cathode(+), allowing it to pull more from the anode through the conductor. A symmetric reaction dumps electrons on the anode(-), replenishing those stolen by the cathode. Thus electrons move through the conductor from anode(-) to the cathode(+), creating electric current. Movement of ions in solution creates a balancing counter-current inside the battery.

The above explanation is a bit backwards from what is sometimes encountered in chemistry books, where the emphasis is on the reactions of the electrolyte with the electrodes. However, energetically, the entire process is driven by the difference in electronegativity (or electropositivity) between the anode and cathode materials. The electrolyte, which is usually fairly neutral chemically, can be viewed as just the messenger. So it makes sense to first look at the process from this perspective.

The tricky part to all this is finding combinations of anode, cathode, and electrolyte so that all the reactions are energetically favored to move in the right direction. Also, a host of other properties are needed for a practical battery (high capacity, chemical stability, high current potential, low self-discharge via secondary paths, mechanical robustness, no sloshing liquids...) Finding all these together is tough. There is a reason there are only a few commercially sucessful battery technologies.

Wikipedia on batteries

Single-Use Technologies

Despite a very large number of electrochemical reactions that can produce electrical power, and a substantial number that have been used in practical batteries, only a few single-use technologies are widely marketed to retail consumers. These are "zinc-carbon" or "heavy-duty", "alkaline", and "lithium". Carbon-zinc and alkaline actually use the same anode and cathode materials, and there are two separate mass-market lithium technologies. A few other technologies (silver oxide, zinc-air) have niche markets in button cells for watches, hearing aides, and other miniaturized self-contained devices.

Wikipedia on single-use batteries

Zinc-Carbon Batteries

Cell voltage: 1.5V
Energy density: 35 Wh/kg (125 kJ/kg)
Self-discharge: 4%/year

Zinc-Carbon batteries were the first dry cell to be commercially produced and marketed. Invented in 1886, they provided the technological basis for the Ever Ready Battery Company, founded about 1900. The term "dry cell" means no sloshing liquids. Instead, the electrolyte is contained in a paste or gel. This meant that, for the first time, batteries could be truly portable, and useful for a device such as a flashlight.

Zinc-carbon batteries have an anode(-) consisting of a can of zinc metal, and a cathode(+) consisting of a paste of manganese dioxide (MnO2) mixed with carbon to make it conducting. The electrolyte is ammonium chloride in the original version, zinc chloride in the modern "heavy-duty" version, in the form of an aqueous paste that impregnates a paper separator. Contact with the cathode paste is made via an embedded carbon rod (which is the origin of "carbon" in the name). The anode can itself provides the negative contact. Initial voltage of a cell is about 1.5 volts. This decreases as the cell is used up, apparently due to the accumulation of reaction products in the pastes employed.

"Heavy-duty" zinc carbon batteries are widely available in 1.5 V AAA, AA, C, and D cells, and in 9V rectangular cans. Large 6V rectangular versions are also available, but are used mostly by traditionalist campers for old-fashioned "giant" flashlight lanterns. From a practical standpoint, such lanterns have been entirely superceded by modern LED technology, which needs far less current, coupled with modern battery technologies that can deliver higher currents in a small package.

Zinc-carbon batteries have a number of limitations compared to more modern alternatives. First, the current they can supply over an extended period (more than a few minutes) is limited, a few tenths of an Ampere for a D-cell. Comparable lithium cells can deliver well over 10x the current. Second, their capacity is less than 1/2 that of a comparable alkaline cell, and more like 1/4 in applications with high current drains. Third, the open-circuit voltage decreases almost linearly as the battery is used up, compared to more constant voltages provided by alkaline, and especially lithium cells. Fourth, their shelf-life is suspect. Originally, the zinc cans could corrode through in a year or less, especially if the batteries were near exhaustion. Modern versions are better, but not nearly as good as alkaline or lithium cells.

So why are these batteries still marketed? One reason. They are cheap to make. If you want to include batteries with a toy or a throw-away novelty gizmo you are selling, you can save a lot of money by using plain carbon-zinc. For your own devices, ProfRC recommends that you never buy "heavy-duty" batteries, which name aside, are actually the lightest duty batteries available.

Wikipedia on zinc-carbon batteries

Alkaline disposable batteries

Alkaline batteries

Cell voltage: 1.5V
Energy density: 85-190 Wh/kg (310-680 kJ/kg)
Self-discharge: 2%/year

Alkaline bateries are the modern replacement for the old zinc-carbon technology. They were developed in the 1950s and 60s by (you guessed it) the Ever Ready Battery Company (by then part of Union Carbide) among others, and brought to market in the late 60s.

Like zinc-carbon, alkaline batteries use zinc as the anode(-) material, and manganese dioxide (MnO2) as the cathode(+). However, rather than being a can of sheet material, the zinc is finely divided and suspended in am electrolyte gel. This makes it more accessible, allowing higher currents and use of more zinc mass. The electrolyte is potassium hydroxide (KOH) rather than ammonium or zinc chloride, and it is not consumed overall during battery operation. In aqueous gel and paste, KOH is a good conductor, so the MnO2 cathode(-) needs less carbon in the mix than a zinc-carbon cell. This allows more MnO2 to be packed into a given casing, providing more battery capacity.

Wikipedia on alkaline batteries

Lithium disposable batteries

Lithium Single-Use

Li-FeS2
Cell voltage: 1.5V
Energy density: 300 Wh/kg (1080 kJ/kg)
Self-discharge: 0.6%/year

Li-MnO2
Cell voltage: 3.0V
Energy density: 280 Wh/kg (1000 kJ/kg)
Self-discharge: 5-10%/year

Lithium single-use batteries (aka just "lithium" batteries as distinguished from "lithium-ion" rechargeables) were developed in the 1970s and introduced to the consumer market in the early 1990s. They offer higher energy density (2x), much higher currents (10x), more stable voltage, and better cold-weather performance than comparable alkaline cells. Some varieties have significantly longer shelf lives. They are also typically 5-10x as expensive.

Lithium batteries have an anode(-) of lithium metal, and a cathode of one of several different materials depending on the battery, including iron disulfide (FeS2) and manganese dioxide (MnO2). The electrolyte consists of lithium salts (e.g. perchlorate, hexaflourophosphate) dissolved in polar, aprotonic, organic solvents (e.g. propylene carbonate, dimethoxyethane). At the anode(-) lithium gives up an electron and goes into solution as Li+. At the cathode, Li+ from solution grabs an electron and reduces a component of the cathode, for example Mn(IV)O2 to LiMn(III)O2.

There are over a dozen cathode materials employed in lithium batteries, most for specialized applications. The only ones widely marketed to consumers are lithium-iron disufide (1.5V, Li/Fe, Li-FeS2), and lithium-manganese dioxide (3.0V, Li/Mn, Li-MnO2). Both have energy densities approaching 300Wh/kg, which is double that of alkaline batteries.

Li-FeS2 batteries have a cell voltage of 1.5V, and are sold in AAA, AA, and 9V formats for replacing standard alkaline cells. These batteries have a very low self-discharge rate, and a predicted shelf-life of 15-20 years. Li-MnO2 batteries have 3.0 volt cells, and are generally used in flashlights or other devices designed to accept them. They have higher self-discharge rates, especially at elevated temperatures, and should not be kept for more than a few years.

Lithium is a very reactive metal, and can catch fire if exposed to air or moisture. The electrolyte solvents are flammable. So it is not a good idea to cut or open a lithium battery. Some jurisdictions have recently imposed limits on their sale, supposedly because illicit manufacturers of methamphetamine extract the lithium for use as a reducing agent in their syntheses.

Because of the hazardous components, large-size lithium batteries (bigger than AA format) are not easily available to retail consumers. You sometimes see "C" format lithium batteries in stores, but (as of 2016) these are AA size cells in an expanded package, not actually higher-capacity batteries.

Robots rarely use disposable batteries as a primary power source, as the ongoing expense is high. More often they are used as backup power to maintain clocks or other state while primary power is switched out, or the robot is powered down. For the robot constructor, the price of lithium might be justified if a lightweight, high-current source that is reliable over long off-times is needed (e.g. emergency generator or aircraft engine startup, or emergency lighting).

Wikipedia on lithium batteries

Rechargeable Technologies

Rechargeable battery technologies are much harder to come by than disposable ones. The big hurdle is reconsituting the reactants into usable physical configuration after they have consumed - a process that tends to destroy the electrode shape along with its material. Only a handful have ever made it into widespread commercial use, and most of these are available to the consumer market. The principle current (2016) technologies are lead-acid, nickel-metal hydride (NiMH), and lithium-ion. At one point nickel-cadmium (NiCad) rechargables were common, but they have been largely superseded by higher-energy (and less toxic) NiMH batteries. On the other hand, lead-acid batteries are still widely used, despite the high toxicity of lead.

Wikipedia on rechargeable batteries

Lead-acid automotive batteries



Compact lead-acid battery

Lead-Acid

Cell voltage: 2.1V
Energy density: 110-140 kJ/kg = 30-40 Wh/kg
Self-discharge: 3%-20%/month
Durability: 500-800 cycles

Lead-acid batteries were the first, and for a long time, the only commercial rechargeable technology. They were invented in 1859, commercialized in 1881, and have been continually refined ever since. They are still used today (2016) for automotive starters, backup electrical systems, and other applications. They are inexpensive, reliable, and capable of delivering very high currents. On the down side, they have relatively low energy density, cannot be charged rapidly (restoring a depleted battery typically takes several hours), and present a toxic waste disposal problem.

Modern lead-acid batteries are mostly "sealed cell", meaning that, unlike old-style automobile batteries, they do not need to have their electrolyte level monitored and periodically replenished. Some have gel electrolytes and can be operated in any orientation. They are available in sizes from "D" cell format up to automotive batteries weighing 50 pounds or more. Automotive batteries are often designed for occasional delivery of high current pulses for starting, and will degrade quickly if they are repeatedly called upon to deliver close to their capacity. Batteries intended for extended power delivery between charges are referred to as "deep cycle" batteries and are more expensive. A battery intended as the primary energy source for a robot should be of this variety.

Lead-acid batteries have an anode(-) of metallic lead and a cathode(+) of lead dioxide (PbO2), generally mixed with carbon and coated onto a lead-alloy grid to provide conductivity. This is an unusual example of battery chemistry where putting anode and cathode active materials can be put in direct contact without reacting. The electrolyte consists of dilute sulfuric acid (H2SO4) in water. During power generation, both electrodes react with the electrolyte to produce lead sulfate. The lead Pb(0) at the anode(-) is oxidized to Pb(II)SO4, and the Pb(IV)O2 at the cathode is reduced, forming the same product.

The overall forward electrochemistry can be written as follows.
Anode(-): Pb + H2SO4- --> PbSO4 + 2e- + 2H+.
Cathode(+): PbO2 + H2SO4- + 2H+ + 2e- --> PbSO4 + 2H2O
The actual reactions are a bit more involved, since dilute sulfuric acid consists mainly of HSO4- and H3O+ in solution.

Lead-acid batteries fail mainly due to "sulfation", which is the formation of crystalline PbSO4 that cannot be forced back to Pb and PbO4, and detaches from the electrodes in an inert mass that ties up the active materials. The problem is exacerbated if batteries are allowed to remain in a discharged state for extended periods of time. Attempting to charge batteries too fast or at too high a voltage (cells above 2.4V) can electrolyze the water in the electrolyte to hydrogen and oxygen gas, which can explode violently, blowing the battery apart. Chargers thus need to control voltage carefully.

In robotics, lead-acid batteries are usually used as primary energy sources for larger wheeled vehicles. They are too heavy for flight applications, and do not come in small enough form factors for mini or micro robots. The hours-long recharging time can be an issue. If a robot is employed in anything like continuous service, extra batteries recharged offline are a necessity.

Wikipedia on lead-acid batteries

Nickel-Cadmium (NiCad)

Cell voltage: 1.2
Energy density: 40-60 Wh/kg (140-230 kJ/kg)
Self-discharge: 10%/month
Durability: up to 2000 cycles

Nickel-Cadmium batteries were demonstrated in wet-cell form in 1899. Dry-cell forms were commercialized in 1946. The batteries were widely used as a rechargable power source for portable power tools in the 1960s through the 1980s, seeing continual improvement in the technology. They lost market in the 1990s to nickel-metal hydride technology, which had better energy density and less toxic components.

NiCad batteries have a metallic cadmium anode(-), a nickel(III) oxide/hydroxide (NiO(OH)) paste cathode(+), and employ a basic electrolyte, usually potassium hydroxide (KOH) in solution. The overall forward reactions are:
Anode(-): Cd(0) + 2OH- --> Cd(II)(OH)2 + 2e-
Cathode(+): 2Ni(III)O(OH) + 2H2O + 2e- --> 2Ni(II)(OH)2 + 2OH-
Unlike lead-acid batteries, the electrolyte is not consumed overall.

NiCads can deliver very high currents, up to 15C continously, and are very robust to abusive treatment, both electrically (high-current shorts) and physically (knocks, thermal stress). The voltage is very stable right up until depletion, and the number of recharge cycles is as high or higher than any other commercially available battery. They are still used sometimes where this combination of characteristics is at a premium. Because they contain significant amounts of cadmium, used NiCads constitute toxic waste in most countries and locales. This contributed to the rapid decline in their use when NiMH batteries came on the scene.

The amateur robot constructor will probably not have reason to employ NiCads, since both NiMH and lithium-ion rechargeables have better energy density, less toxic components, and are widely available.

Wikipedia on NiCad batteries

Nickle-metal hydride batteries

Nickel-Metal Hydride (NiMH)

Cell voltage: 1.2V
Energy density: 100Wh/kg (360 kJ/kg)
Self discharge: 15-70%/month (1-3%/month special)
Durability: 500-2000 cycles

Nickel-metal hydride (NiMH) battery technology was first demonstrated in 1967. Commercial batteries appeared in 1990, quickly displacing NiCads as the standard power source for portable power tools and other consumer rechargeable applications.

NiMH batteries have an anode(-) composed of a metal hydride. Chemically, the anode material is hydrogen, and the metal hydride serves as a compact means of storing it. The metal in modern cells consists of complex intermetallic compounds containing rare-earth elements (e.g. lanthanum or Misch metal) combined with transition elements (e.g. nickel, cobalt, manganese) and/or aluminum. The cathode(+) is a nickel(III) oxide/hydroxide (NiO(OH)) paste, the same anode material used in NiCads. The electrolyte is usually potassium hydroxide (KOH) in aqueous solution.

The overall forward electrochemistry is:
Anode(-): MH + OH- --> M + H2O + e-
Cathode(+): NiO(OH) + H2O + e- --> Ni(OH)2 + OH-
As with NiCads, the electrolyte is not consumed overall.

NiMH batteries have good energy density, can deliver reasonably high currents (5C continuous), and they are cheaper and less prone to catastrophic explosion and fire than lithium-ion batteries. On the down side, they have a high self-discharge rate, especially at elevated temperatures (5-20% on day 1, 1-4%/day thereafter), which means they cannot relied on to retain power for more than a week or so. Special low self-discharge versions are available for additional cost.

NiMH power is a good choice for small to medium-size robots where weight and raw energy density are not a crucial issue. They are less expensive, less tricky to charge, less prone to fire, and more robust overall than Lithium-ion batteries. The latter are a real advantage only in electric aircraft, high-performance racing vehicles, or other applications where minimizing weight is a primary design consideration.

Wikipedia on NiMH batteries

Lithium-polymer (LiPo) batteries



LiPo on the grill



Be careful out there...

Lithium-Ion

Cell voltage: 3.2-3.9V
Energy density: 360-950 kJ/kg = 100-265 Wh/kg
Self-discharge 8%/month
Durability: 400-1200 cycles, 500 typical

Lithium-ion battery technology was developed in the 1980s, and the first commercial cells marketed in 1991. Since lithium is a very light and highly electropositive metal, batteries employing it have the potential for a very high energy density. However, the same reactivity (lithium will react with water) makes battery development difficult. To date (2016), efforts to develop a commercial rechargable battery based on metallic lithium have not been successful. However, a variant known as lithium-ion technology has seen commercial success.

Rather than metallic lithium at the anode(-), lithium-ion batteries use graphite that has been "intercalated" with lithium, which means that lithium atoms squeeze into lattice spaces in the graphite. With the release of an electron to the graphite, the lithium atoms can enter the electrolyte as lithium ions (Li+).

The cathode(+) is made of special lithium-trapping crystalline materials generally involving a metal in a high oxidation state (e.g. cobalt oxide, iron phosphate, manganese oxide). These also intercalate lithium, and in doing so, the highly-oxidized metal effectively oxidizes the lithium, which makes the process very energetically favorable. By grabbing an electron from the cathode, lithium ions (Li+) in the electrolyte can enter the crystal matrix. The cathode reaction is very similar to what happens in disposable lithium cells. The difficult bit of technology is making cathode material in a form that can incorporate the lithium atoms without expanding and flaking off, so that the process can be reversed during recharge.

The electrolytes are also special. The voltages present will electrolyze water to hydrogen and oxygen, so aqueous solutions are not usable. Instead, complex mixtures of organic carbonates (e.g. ethylene carbonate, diethyl carbonate) with dissolved lithium salts of large, fully oxidized anions (e.g. perchlorate, hexafluorophosphate) are used. Under cell voltage, the organic carbonates form a thin film on the graphite electrode, which is permeable to the small litium ions, but impermeable to the large negative ions and solvent molecules. This prevents further electrodecomposition of the electrolyte.

Lithium-ion cells have potentials between 3 and 3.7 Volts, which is about double most other common cells. They can have energy densities exceeding 250 Wh/kg (900kJ/kg), which is also double the next best rechargable technology. They can be made with very high discharge rates (exceeding 20C), and can typically be recharged about 500 times. As of 2016, the energy density is continuing to improve.

All these advantages come at a cost. Lithium-ion batteries are relatively expensive; several times the cost of NiMH. More significantly, have stability issues. If the cell voltage goes too high (above 4.2V), or too low (below 2.0 V), or if they are charged or discharged too rapidly, they can be permanently damaged. They can catch fire or explode. They can do this without warning. Because of this volatility, shipping, especially by air, is restricted. Charging is tricky, and must be done with a "smart" charger especially matched to the particular battery. These chargers carefully monitor the charging voltage and current, and switch from constant current to constant voltage at a precisely defined point.

Lithium-ion batteries are avaliable in a huge variety of shapes and sizes. There are custom batteries for laptops, cell phones, and other electronic devices, and for portable power tools. The most popular standard cell is referred to as size 18650, which is a cylinder slightly langer and wider than a AA. Typical capacities range from 1500 to 3500 mAh, with max currents from 2 to 30+ Amperes. There is a tradeoff: the high amperage batteries tend to have distinctly lower capacity and vice versa. These cells are used to construct higher voltage and amperage packs that go into everything from laptops, to power tools, to electric vehicles.

The batteries most useful to the amateur robot constuctor are so-called "lithium polymer" or LiPo packs intended for Radio-controlled hobby vehicles such as cars, boats, planes, and quadcopters. LiPo batteries are Lithium-ion batteries with physically robust (polymer) electrolytye gels that permit a large variety of shapes and even flexible packs to be made. They are high-current designs intended for driving motors, and come in sizes from a few hundred to several thousand mAh. They are readily available together with matched chargers from hobby shops or over the internet (though there may be shipping issues if you want to order more than a couple at one time).

Wikipedia on lithium-ion batteries

A Battery Table

The following table, compiled from a variety of sources, lists voltage, capacity, and current delivery ability for some widely available batteries. The values should be considered approximate/typical, since battery performance depends on usage conditions and the particular battery brand and make. Capacity given is low-current limit. (Note that robot usage tends towards the high-current-continuous domain.) Values for sustainable current are highest continous draw that can achieve about 3/4 of the low-current capacity without causing the battery to overheat. Burst current is what might be drawn for a few seconds, and the values are low end. Under some conditions, the rated batteries might deliver considerably more.

Wikipedia comparison of battery types

Wikipedia list of battery sizes

Technical specifications for Energizer batteries

Other Electrical Power Sources

Fuel-powered generator systems can have higher power densities than batteries, but must be started up, are noisy, produce vibration, emit hot and potentially toxic exhaust, are mechanically complex, often orientation sensitive, and need all sorts of maintenance. They have a lower bound on size. The smallest weigh on the order of a pound, and most are MUCH heavier. Overall chemical to electric efficiency of large modern diesel generators approaches 40%. Small gas generator systems usually operate below 25% efficiency.

Capacitors can deliver very high bursts of power, but have limited storage times (seconds to days), and low to very low power densities (compared to batteries) even in the best ultra/super capacitors.

Fuel cells are neat devices. They combine combustible fuels with oxygen from the air in a process that produces electricity directly, rather than via an electro-mechanical generator. This yields energy densities typical of fuels in a device with many of the desireable properties of a battery (few/no moving parts, instant electric power, silent, vibration-free operation etc.) Fuel cells can have theoretical chemical to electrical energy efficiencies exceeding 60%, and 50% is routinely exceeded in practice.

Unfortunately, fuel cells are not a mature small-format technology (as of 2016). Currently available devices are usually large, often expensive, need fueling (usually with liquids or compressed gasses), have maintenance and durability issues, are sometimes fragile and may be orientation sensitive. Some technologies run only at high temperatures. Hopefully, sometime in the near future, you will be able to buy a pocket-size fuel cell, fill it with an airline vodka single, and run your laptop (or portable robot companion) for 24 hours, but today is not that day.

Wikipedia on Fuel Cells

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