How to Choose Batteries

• Opinions and Recommendations
• Battery Basics
• Single-use Batteries
• Rechargeable Batteries
• Rechargeable Batteries: Q&A
• Battery Capacity Ratings
• Battery Tips
• Solar Chargers
• Batteries for Wrist-top Altimeters, Fitness Watches
• Recycling/Disposal
• Quick-reference Guides
  • Rechargeable Batteries Quick-reference Guide (PDF Download)
  • Single Use Batteries Quick-reference Guide (PDF Download)

Batteries tag along on all my outdoor excursions—in my headlamp on every hike; in my bike light on every ride; in my Steri-PEN (a water purifier that disinfects water with ultraviolet light) on every backpacking trip. If I add a GPS receiver, a two-way radio or a cell phone, all of them need batteries. I carry backup batteries, too.

Ideally, a good battery will offer a balance of the following factors:

• Long duration
• High performance
• Fair cost
• Environmental friendliness

While not perfect on all these fronts, batteries are inching toward that elusive ideal.

We begin with Opinions and Recommendations, a summary of my research. The detailed rationale that supports this thinking comes next, starting with Battery Basics. Want the short version? Many key details are summed up in 2 handy Quick-reference Guides.

• Rechargeable Batteries Quick-reference Guide (PDF Download)
• Single Use Batteries Quick-reference Guide (PDF Download)

Opinions and Recommendations

Rechargeable batteries are the preferred battery type: Within this group, Nickel Metal Hydride (NiMH) batteries are the current standard-setter. They offer an excellent long-term value for powering GPS receivers, headlamps, flashlights and bike lights. They're also a good choice for small household items used frequently or continuously (toys, for instance) and "high-drain" devices such as digital cameras.

The upside of NiMH batteries:

• They typically can be recharged and reused 150 to 500+ times. One set could do the work of hundreds and hundreds of single-use ("disposable") batteries. Nearly 3 billion single-use batteries, mainly alkaline batteries, are sold in the U.S. each year. The majority wind up in landfills. Ouch.
• They outperform single-use batteries in "high-drain" devices such as digital cameras and GPS receivers. Early in their life cycle, NiMH batteries discharge energy more steadily (and thus longer) than single-use batteries. In a camera (which involves repeated power bursts), alkaline batteries start strong but fade quickly. In GPS units, alkalines generally perform well.

Downside:

• They require fairly regular maintenance. NiMH batteries lose power when sitting idle, 1% or more per day. To keep them operating near their peak, standard NiMH batteries need to be recharged (and preferably used) every 1 to 2 months.
• They grow less predictable as they age. Late in their life cycle, NiMH batteries hold charges for shorter periods. Tip: If you own many NiMH batteries, avoid mixing older and newer cells—keep them paired or grouped. Tape ID tags on them.

Precharged NiMH batteries are the best rechargeables now available: Precharged NiMH batteries are also called "hybrid," "ready-to-use" or "low self-discharge" batteries.

Upside:

• Ready for action. They can go straight from the package into a device. No initial charging needed.
• Lower "self-discharge" rate than standard NiMH batteries. All batteries experience "self-discharge"—a loss of stored power when a battery is not in use. Standard NiMH batteries may lose up to 40% of their stored energy in a month and be fully empty in as little as 2 months. Precharged NiMHs, which employ a type of modified ion, minimize that loss, surrendering only 10% to 25% of their stored power over 6 months. They can serve as backup batteries for trips lasting a couple of weeks or a few months.

Downside:

• Some maintenance is still required. If left idle, recharging is suggested every 6 to 9 months.
• Slightly less energy capacity than standard NiMH batteries. During periods of continuous or intense activity in a short time frame (shooting photos at a wedding, for instance), standard NiHM batteries will probably outlast precharged NiMHs due to their modest advantage in energy capacity.

Single-use batteries make excellent backup batteries: I always carry some as spares, though precharged NiMHs will also do. They store well for years, are always ready for action and offer high energy capacity. Pricey lithium batteries are superb performers (especially in cold weather), but check your gizmo's instructions first before using them. Lithium batteries (at 3 volts per cell) can overpower some devices (headlamps in particular) and fry their circuitry. Alkaline batteries, meanwhile, are tried-and-true workhorses suitable for any device. In a GPS unit, for instance, they typically deliver about 2 days' worth of continual service. Their chief downsides: 1) rapid depletion when used in a digital camera and 2) their unending cycle of use-discard-replace.

No ideal battery exists: As the charging cycles add up, rechargeable batteries hold charges for progressively shorter stretches. No rechargeable battery lasts forever, though they can usually be counted on for years. Single-use batteries, meanwhile, are predictable and convenient, but over the long long-term are more expensive due to the endless need to replace them. Plus, each year billions wind up in landfills because many people aren't aware they can be recycled or don't make the effort to do so.

Battery performance is not predictable: Many factors—the type of device being powered; the frequency or strength of a power drain; the temperature; battery age—make it difficult to forecast how consistently a battery will perform from one application to another or from one device to another. Please be aware that the guidelines offered in this article are all presented with an implied fudge factor. The old consumer bromide rings true when it comes to batteries: Your results may vary.

Batteries are evolving: Just like the microelectronic devices they power, today's mass-selling batteries will become tomorrow's dinosaurs. On the horizon: fuel cell batteries, thin-film polymers and batteries modified by nanotechnology.

Solar chargers are worth considering: For extended stays in remote corners of the planet, these ever-improving energy collectors can supply a moderate amount of power to your devices each day.

Now for a little Battery 101:

Battery Basics

Batteries are portable storehouses of energy. When activated, they furnish a flow (or discharge) of electric current. They date back to 1800 when Italian physicist Alessandro Volta invented the "voltaic pile"—a stack of blotting papers saturated by a briny stew of silver and zinc. In 1896 the National Carbon Company (which became Eveready, then Energizer) is credited for introducing the first battery marketed for consumer use: the 1.5-volt Columbia dry cell. In those days it measured 6 inches long.

Modern microelectronic devices such as headlamps and GPS receivers are powered by dry cell batteries such as the cylindrical AA batteries we all know. They are available in single-use or rechargeable models. In technical battery jargon, single-use batteries are known as "primary" batteries. Rechargeables are "secondary" models.

A dry cell battery has positive and negative terminals. Internal layers (electrodes) include a cathode (which transports a positive charge) and an anode (to carry a negative charge). They are divided by some type of barrier called a "separator." (Technical footnote: With rechargeable batteries, the cathode and anode carry reverse charges during recharging.)

In single-use alkaline batteries, the cathode is usually powdered manganese dioxide (sometimes mixed with graphite). The anode is zinc. Nickel, along with various alloys, is the dominant component of most cylindrical rechargeable batteries.

All batteries contain some type of electrolyte—a substance that conducts electricity (a flow of electrons) between a battery's terminals. Motor vehicles use large "wet cell" batteries where the electrolytes are liquid. In dry cell batteries, the electrolyte is more of an aqueous paste or gel.

When a battery is activated, the electrolyte, cathode and anode interact and a chemical reaction (basically oxidation) occurs. Ions (positively charged) and electrons (negatively charged) flow through the electrolyte, exit via the negative terminal and enable a device such as a headlamp (the "load") to function.

Over time, a battery's internal chemicals begin to degrade and interaction diminishes. Eventually they can no longer retain a charge. In this depleted state, the battery is "dead."

The mix of chemicals in dry cells aims to provide some combination of the four holy grails of the elusive "ideal" battery—long life, high performance, reasonable cost and low environmental impact. A frustrating fact of battery life is that advancements in battery technology have not come close to keeping pace with the rapid rate of progress (and increasing power demands) in microelectronics.

Until the perfect battery arrives, here's a look at the portable power options now available to you:

Single-use Batteries

• Single Use Batteries Quick-reference Guide (PDF Download)

Alkaline

The most commonly used battery of all contains an alkaline electrolyte, usually potassium hydroxide.
Nominal voltage (the force that "pushes" electrons through a battery): 1.5 (though it gradually declines to less than 1 volt as battery discharges its energy).
Estimated shelf life (at 68°F/20°C): 5-7 years.
Best use: "Low-drain" devices such as LED headlamps, LED flashlights, toys, remote control devices, clocks and radios, even moderate-drain items such as lights using incandescent bulbs. Can be used in high-drain devices (digital cameras, for instance), though life expectancy will be sharply reduced. Why? Even though alkalines have high initial energy capacity, high-drain devices exert such a substantial draw that energy swiftly gets slurped out. As mentioned earlier, in a GPS receiver they typically deliver 2 days of continual use, or many days of periodic use.

PROS:

• Moderately priced.
• Widely available.
• Estimated to provide about 300% more energy capacity than zinc chloride (so-called "heavy duty") or antiquated zinc carbon ("general purpose") batteries.

CONS:

• Perpetual cycle of use-disposal-replacement. Most wind up in landfills.

Note: Some rechargeable alkaline batteries exist, but they typically can accept only a few charges. They are widely viewed as a failed technology.

Lithium

Lithium, an exceptionally light metal, gives lithium batteries the highest energy density of any battery cell. Thus they can store more energy than alkaline batteries or any single-use battery of a comparable size. USE WITH CAUTION: Their higher voltage capacity makes lithium batteries too powerful for some devices and may damage circuitry. Read manufacturer instructions for battery recommendations for individual products.
Nominal voltage: 3 (though it gradually declines as battery discharges).
Estimated shelf life (68°F/20°C): 10-15 years.
Best use: "High-drain" devices (digital cameras) and most (but not all) lower-drain microelectronic devices. They are too powerful, for example, for some headlamps.

PROS:

• Longest life (by far) in single-use category; in a digital camera, lithium batteries hypothetically may produce 100-200+ flash photos; alkaline batteries, 20-40+.
• Superior functionality in cold weather (particularly subzero temperatures) and hot conditions.
• Estimated to provide about 300% more energy capacity than zinc chloride (so-called "heavy duty") or antiquated zinc carbon ("general purpose") batteries.
• Longer shelf life.
• Lowest weight.

CONS:

• More expensive.
• WARNING: Higher voltage may damage some devices. Read manufacturer instructions that accompany each device to determine if they can handle lithium batteries.

Note: What is the difference between lithium and lithium-ion batteries? Lithium batteries cannot be recharged. Lithium-ion batteries can.

Heavy Duty or General Purpose

These are the ancestors of alkaline batteries. The electrolytes used are zinc chloride (heavy duty) or zinc carbon (general purpose).
Nominal voltage: 1.5 (gradually declines to less than 1 volt as battery discharges its energy).
Best use: Clocks or similar low-drain, low-use devices.
Estimated shelf life (68°F/20°C): 3-5 years.

PROS:

• Cheap.

CONS:

• Shortest life of any battery.
• Not recommended for extended use in outdoor electronic devices.
• Not stocked at REI.

Rechargeable Batteries

• Rechargeable Batteries Quick-reference Guide (PDF Download)

Nickel-Metal Hydride (NiMH)

As the name suggests, a NiMH battery consists of:

• nickel (typically nickel hydroxide; used for the cathode/positive electrode)
• an alloy (a mixture of metals or metal mixed with other elements; used for the anode/negative electrode)
• potassium hydroxide (an alkaline) as an electrolyte.

NiMH batteries have supplanted nickel cadmium (NiCd) batteries as the preferred cylindrical rechargeable battery. They offer higher energy capacity (up to 50% more) than NiCd batteries and avoid the high toxicity of cadmium.
Volts: 1.2 (steady voltage is generally sustained throughout a cycle, dropping to 1.1 before a charging cycle is complete).
Estimated number of recharging cycles: 150 to 500, maybe more, perhaps fewer.
Self-discharge rate: Loses 1% (or more) of stored power per day, roughly 40% per month.
Maintenance: Recharge whenever energy capacity drops 30% to 50% below its peak capacity. If left unused, it should be recharged every 1 to 2 months. Performs best over the long haul if used frequently. After many months or years of disuse, NiMHs may require a "conditioning" cycles (an option found on "smart" chargers) to revive its usefulness.
Storage: Store fully charged at 60°F/15.5°C.
Best use: High-drain devices (digital cameras, flash units) or devices that experience prolonged or continuous use (GPS receivers). Not recommended for items that are rarely used or infrequently inspected, such as smoke detectors or a flashlight in an emergency kit.

PROS:

• Delivers energy capacity at a more constant rate (technically, a flatter discharge rate) than single-use batteries—for example, the light from a headlamp using alkaline batteries starts brightly and progressively grows dimmer. With NiMHs, the light level remains stable due to the steady voltage delivered by rechargeable batteries.
• Delivers substantially more current (electron flow) than an alkaline battery, boosting its performance when servicing high-drain devices.
• Energy capacity is up to 50% higher than NiCd batteries.
• No measurable "memory effect" (explained later) that NiCd batteries exhibit.
• Performs reasonably well in colder weather.
• Better long-term value than single-use batteries.
• Recyclable.

CONS:

• Fairly fast rate of "self-discharge" (loss of power when not in use)—idle NiMH batteries may lose between 1% and 5% of their stored power in a day, between 30% and 40% in a month (and potentially more in warm conditions).
• Should not be stored in very warm areas (affects longevity).
• Moderately expensive initially.
• Must be charged before first use.
• Accommodates fewer recharging cycles than NiCd batteries.
• Should be charged every 1 to 2 months.
• Energy capacity declines by 10% to 15% after 100-plus recharges.
• Performance may diminish if dropped or handled roughly.

Notes: Best when charged in a "smart charger" designed for NiMH batteries. Some specialized chargers can restore NiMH batteries in 15 minutes. One innovative NiMH battery can be recharged by plugging it into a computer's USB slot.

Precharged NiMH

Also called "hybrid," "ready-to-use" or "low self-discharge" batteries, this NiMH battery comes precharged in its package so it's ready for action. It offers a very low self-discharge rate (power loss when not in use), which makes it best-of-breed in the rechargeable category for cylindrical batteries (AA, AAA, C and D cells).
Volts: 1.2 (steady voltage is generally sustained throughout a cycle).
Estimated number of recharging cycles: 150 to 500, maybe more.
Self-discharge rate: Much better than standard NiMH batteries, roughly 10% to 20% over 6 months.
Maintenance: If left unused, should be recharged every 6 to 9 months. Benefits by being used frequently.
Storage: Store fully charged at 60°F/15.5°C.
Best use: High-drain devices (digital cameras, flash units) or moderate-drain devices that experience prolonged or continuous use (GPS receivers). Its lower self-discharge rate also makes it suitable for smoke detectors, though the batteries much be recharged every 6 months to optimize power.

PROS: Same as standard NiMH, plus:

• Can go straight from the package and into your device.
• Much lower self-discharge rate than standard NiMH batteries (making this design an excellent choice for headlamps or any device that may be actively used for a week, then left untouched for months).

CONS:

• Moderately expensive initially.
• Accommodates fewer recharging cycles than NiCd batteries.
• Energy capacity declines by 10% to 15% after a few hundred recharges.

Nickel Cadmium (NiCd)

This is the ancestor of NiMH batteries. Contains cadmium, a highly toxic component.
Volts: 1.2 (steady voltage is generally sustained throughout a cycle).
Best use: Power tools; two-way radios; high-temperature situations.
Estimated number of recharging cycles: Up to 1,500.
Self-discharge rate: Loses 1% of stored power (or more per day), roughly 40% per month.
Maintenance: If left unused, should be fully discharged and recharged every 2 to 3 months. Performs best over the long haul if used frequently.
Storage: Store fully discharged at 60°F/15.5°C. Recharge before next use.

PROS:

• Less expensive than NiMH batteries.
• Accepts more charging cycles.
• Very rugged.
• Moderate self-discharge rate (approximately 20% in a month).

CONS:

• Contains a highly toxic component (cadmium).
• Less energy capacity than NiMH batteries (typically 50% less).
• Often vulnerable to "memory effect," which causes a shortened run time (a flaw that sometimes can be corrected by 2, 3 or more repeated cycles of full discharging-full recharging).
• Not available at REI.

Lithium-ion

They're not yet widely available in the cylindrical shape of AA, AAA, C or D batteries, but battery insiders say they are on the horizon. Lithium-ion batteries today are more commonly found in the form of a slab, block or battery-pack. They are used extensively in mobile phones, single-reflex digital cameras, computers, camcorders and other consumer electronics.
Volts: 3.6 (with some variations).
Estimated number of recharging cycles: 500 to 1,000+.
Self-discharge rate: Very low, but age is the enemy of Li-ion batteries. Even if unused, the simple passage of time robs them of some energy capacity. The quantity of the loss varies according to the size and configuration of the battery.
Maintenance: Should recharged frequently (even after shallow discharges of 10% and 20%). It is considered impossible to overcharge most Li-ion batteries, since they are designed to stop accepting energy when fully recharged. Recharge whenever it reaches 50% of its capacity. No damage will be done to the battery, however, if it is fully discharged.
Storage: Store at roughly 60°F/15.5°C, either fully charged or at 50% of capacity (opinions vary on this topic).
Best use: Notebook computers, camcorders (some camcorders use NiMH batteries), mobile phones, single-lens reflex cameras, some bike lights.

PROS:

• Offers the lowest self-discharge rate (less than 10% in a month) of any rechargeable battery.

CONS:

• More expensive.
• Even if left unused, is negatively impacted by age.

Alkaline

Rechargeable alkaline batteries exist, but as their limitations became known, interest in them quickly flamed out. They can accommodate only a small number of recharging cycles (from 10 to perhaps a few dozen) before their usefulness is exhausted. They are also relatively expensive. REI regards them as a failed design and does not stock them.

N Cells

Created for use in extra-small devices. Available in both single-use and rechargeable models. Volts: 1.2-1.5. PROS: Small and light. CONS: Limited energy capacity, requiring more frequent replacement.

Rechargeable Batteries: Q&A

Q: How often should batteries be recharged? What is the best way to store them?

A: Here's some pure-gold advice: Before using either a rechargeable battery or a charger, read and follow the manufacturer instructions. Still have questions? Contact the manufacturer.

Beyond that recommendation, here are the prevailing points of view (though opinions vary) on strategies for recharging and storing mainstream rechargeable batteries:

NiMH batteries (including precharged/hybrid varieties):

• Can be recharged at any time, no matter what level of energy capacity they retain.
• Periodically, it's OK to fully discharge and recharge batteries. This is known as "conditioning." See the next question for details.
• To begin a prolonged period of storage, all NiMH batteries first should be fully recharged.
• If left unused for long stretches, recharge accordingly: Standard NiMH batteries: every 1 to 2 months Precharged NiMHs batteries: every 6 to 9 months

NiCd batteries:

• Best maintained when completely drained before recharging is attempted.
• It is possible you to reduce the performance capabilities of NiCd batteries if they are recharged before being fully exhausted.
• Doing so could result in the dreaded "memory effect"—a loss of energy capacity after a NiCd receives a charge in a partially depleted state. A NiCd battery tends to "remember" to only store the amount of energy it delivered during its most recent discharge.
• NiMH batteries demonstrate no measurable evidence of a memory effect.
• For prolonged periods of storage, NiCd batteries should first be fully drained.

Lithium-ion batteries: Available for outdoor microelectronic devices, but are commonly found in computers, single-reflex digital cameras, mobile phones.

• Recharge often, even if only a modest amount of energy has been drained.
• Li-ion batteries are not vulnerable to a memory effect.
• Avoid fully exhausting a Li-ion before recharging. Doing so won't ruin a Li-ion, but it is not viewed as a recommended practice.
• If a fully charged Li-ion battery is attached to device that also uses household electrical current (a notebook computer, for example), no recharging current is applied to the battery. A battery's voltage must drop below a certain level before it will accept a recharge. In general, a Li-ion battery thus cannot be overcharged even if the device is plugged in and operated for a prolonged period. (Check your device's manufacturer instructions for its battery instructions.)
• More charging cycles can be achieved if a Li-ion battery is recharged after shallow discharges (roughly 30% of capacity, which can be determined on devices that offer an energy capacity indicator or battery "fuel gauge"). If possible, avoid scheduling recharges after medium (50%) or full (90%-100%) discharges.
• For ideal long-term storage, opinions vary. All experts recommend choosing a cool location (roughly 60°F (15.5°C). Some people in the battery industry say it is acceptable to store a Li-ion battery fully charged; others advise draining it to approximately 40% to 50% of its peak capacity prior to storage. In many cases, it is hard for the average consumer to know precise capacity levels, even with devices that offer a "fuel gauge" display. The consumer should simply make his or her best estimate.

Q: Is "conditioning" needed for all rechargeable batteries?

A: Not necessarily, though opinions can vary widely on this topic when it comes to NiMH batteries. Your best bet is to consult and follow manufacturer instructions.

Conditioning is the process of "deep-cycle exercise" for rechargeable batteries, where most (but not quite all) of the capacity is drained from a battery, then it is fully recharged. Many newer chargers that offer sophisticated features include a conditioning cycle that can be accomplished with the touch of a button. Here's some basic guidance:

• Standard NiMHs: Condition batteries after 10 normal recharging cycles. Some industry insiders say to condition NiMHs every 3 months, particularly if the batteries have been left unused or if battery performance seems sluggish compared to past performance. Conditioning prevents the formation of a crystal within the cell that can reduce its life.
• Precharged NiMHs Same as standard NiMHs; every 10 cycles or every 3 months.
• NiCds: Every recharging cycle should be a conditioning event.
• Lithium-ion: Not necessary. However if you have a Li-ion battery pack with a fuel gauge or capacity indicator, fully discharging and recharging the battery occasionally is considered a good idea. This allows the fuel gauge electronics will relearn its full-capacity levels and keeps the fuel gauge as accurate as possible over the life of the battery pack. At some point in normal use, many people commonly drain their Li-ion batteries to a near-nothing level, so the next charge serves as a "conditioning" cycle for the fuel gauge.

Tip: Conditioning is beneficial if a battery is frequently recharged after it gets regular use but only for brief intervals. Example: a security guard using short flashlight bursts during a nightly walk.

Q: How many charges can a rechargeable battery accept?

A: Based on customer feedback and manufacturer claims, my best guess for NiMH batteries is between 150 and 500. Maybe more, perhaps up to 1,000, according to some manufacturer claims. But maybe just a little over 100 if the batteries are poorly maintained. After 500 charges, even the most optimistic estimate puts a NiMH at 80% of its original energy capacity. If used constantly, a rechargeable battery will not last forever. A battery will lose some of its capacity with every charge. Various additional factors will influence its longevity:

• the severity of its power drain (the irregularity of a strobe-flashing camera vs. the steady, gentle pull of an energy-efficient headlamp)
• the frequency of discharges
• the temperatures in which it operates (extreme cold or heat are no friends of batteries)
• the pattern of recharging it experiences.

Q: Are all chargers created equally?

A: Not quite. Some guidelines:

• Do not attempt to recharge a NiMH battery in an older, timer-based NiCd charger, which could cause overcharging.
• Likewise, do not attempt to recharge an older NiCd battery in a charger designed for NiMH batteries.
• So-called "smart" chargers are preferred. Smart chargers are equipped with microprocessors that automatically shut off the flow of energy when batteries reach a full charge. Some can also supply a "trickle charge" to keep energy levels topped off until the batteries are needed for use. In some cases, chargers sold with rechargeable batteries are very basic, or "dumb" chargers that lack auto shutoff and other features. This requires the user to be a vigilant observer during a charging cycle to ensure batteries do not overheat.
• For NiMH batteries, "quick" chargers are preferred over slow "overnight" chargers. Quick chargers often get the job done in less than 4 hours. (On average, 3 to 6 hours are needed.) It is generally believed rechargeable batteries last longer if charged with higher currents.
• Older overnight chargers take as long as 12+ hours and may not automatically shut off the flow of energy when a battery's charge is fully restored. This puts batteries at risk of overcharging and damage. Signs of overcharging include excessive heat in the batteries and leakage.
• Some newer "rapid" chargers (using technology called In-Cell Charge Control, or I-C3) refuel NiMH batteries in as little as 15 minutes.
• If a set of rechargeable batteries has been idle only days or a few weeks, it is wise to top off its charge just before putting it into use.
• One clever type of NiMH battery can be recharged using the USB slot of a computer.
• Never attempt to recharge a single-use battery (alkaline, lithium, heavy duty). Note: Some specialized alkaline batteries have been designed to be rechargeable, but they accept only a limited number of recharges and are not recommended.
• If a NiMH has been stored unused for years, it may require more 2 or 3 recharge-discharge cycles (conditioning) to fully activate the electrolyte. If possible, use a quick charger rather than a rapid charger in this situation. Most smart chargers offer a conditioning cycle.
• Conditioning may also help revive a rechargeable battery that is exhibiting sluggish performance or abbreviated charges. If your charger lacks a conditioning feature, a local electronics shop may be able to assist you.
• The terms used here to describe chargers—slow or overnight (12+ hours), quick (3 to 6 hours), rapid (15 minutes), smart (microprocessor-equipped)—are not universally found in manufacturer product descriptions. Look for time claims of charging cycles to determine a charger's capabilities.

Q: Can batteries be left in a charger for long stretches of time?

A: Refer to the instructions that accompany your specific charger. With newer chargers, particularly those with an automatic shutoff feature, the answer may be yes. Most battery experts, however, caution against leaving any battery in any charger for longer than a day.

Q: Should batteries feel warm during recharging?

A: All NiMH and NiCd batteries will grow warm during recharging. (Interestingly, lithium-ion batteries do not.) Note: Batteries may feel very warm in rapid (15-minute) chargers. In slower chargers, though, excessive heat is a sign of potential trouble. If you notice leakage, terminate the charging process.

Q: What are ideal battery-charging conditions?

A: Room temperature; somewhere close to 68°F (20°C). Temperature extremes, particularly cold conditions, are tough on batteries. (It makes the electrolyte sludge-like.) And operating rechargeable batteries in very warm conditions will reduce the duration of their performance.

Battery Capacity Ratings

Brace yourself: This section will get a little technical.

Capacity ratings (which, admittedly, are sometimes difficult to locate) are presented as milliamp hours (mAh). An ampere (amp) is the basic unit for measuring electrical current. A milliamp equals 1/1000th of an hour.

The mAh number indicates how many amps (how much current) a battery can supply over a given period of time. It's sort of the gas gauge of a battery's fuel tank. The higher the number, the more likely the battery will deliver prolonged performance.

Typical AA NiMH Ratings:
Standard NiMH: 2,500 mAh
Precharged NiMH: 2,100 mAh

This means the standard NiMH battery offers greater overall capacity than the precharged NiMH, and so it may be able to perform longer.

If both sets of batteries are charged at the identical time, the higher-rated standard NiMH will contain more energy than the precharged NiMH. So, for example, if you have hundreds of photos to shoot at a wedding and you want all the battery power you can get, the standard NiMH is probably your better choice. The precharged NiMH, meanwhile, will perform better when the period of use extends over weeks or months—say, for a backpacker on a hike lasting a week or longer.

Is a battery with a higher capacity always the better choice? Not necessarily, says Patricia Bennett, a Senior Engineer of Rechargeable Technologies at Rayovac. Since consumers typically equate higher numbers with higher performance expectations, battery-makers are eager to market batteries with ever-higher capacity ratings—up to 1,000 mAh for AAA batteries and 2,900 mAh for AAs. But as capacity increases, cycle life (the number of times you can recharge batteries) tends to decline.

"Depending on type of use, a 2,500 mAh cell without charge-retention technology [i.e., not a precharged battery] may only be good for 150 to 200 cycles," says Bennett. "To achieve capacity higher, changes have to be made that affect the cycle life. A 2,100 mAh battery used the same way may last for 500 or more cycles."

Note: Single-use batteries such as alkalines also are given mAh ratings, though manufacturers rarely include these ratings on product packaging. For a variety of technical factors, mAh ratings for single-use and rechargeable batteries are usually incompatible, sort of an apples-and-oranges thing. Thus comparing ratings of contrasting battery types is rarely helpful.

Example: An alkaline battery may be rated to 3,000 mAh. But if it is used to power a high-drain device such as a digital camera, its capacity is drained quickly. A NiMH battery rated 2,500 mAh, with its inherent ability to prolong the length of its discharge cycle, will outperform an alkaline in a camera. The exception: If the NiMH battery has not been properly maintained, needs conditioning or is very old.

(Technical note: Voltage pertains to the force that pushes electrons through a battery, not the battery's capacity.)

Battery Tips

• Do not attempt to simultaneously recharge batteries of different capacities, different brands or different ages. Do not use batteries of different brands or different ages together.
• Do not put single-use batteries in a refrigerator or freezer.
• Some people in the battery industry assert that placing charged NiMH batteries in a freezer, if well-packaged to avoid collecting moisture, will enable them to retain a high percentage of stored energy. Frozen batteries should be restored to room temperature before usage. Other experts discourage the use of refrigeration or freezing for any batteries.
• This tip was previously stated, but it's worth repeating: If left unused for long stretches of time, recharge standard NiMHs every 1 to 2 months, precharged NiMHs every 6 to 9 months.
• Remove batteries from devices if they will be left unused for months at a time. This prevents a device from exerting a tiny drain on the batteries even though the device is inactive.
• Remove single-use (non-rechargeable) batteries from a device when they are being powered by household AC current. Doing so spares the batteries from any tiny drain on their power reserves by the device.
• Do not store batteries, particularly single-use batteries, in locations where heat can become intense, such as car trunks, attics or garages.
• Avoid tossing batteries into a drawer, briefcase or bag where they may contact metal objects such as coins or paper clips. Doing so may cause a short or it could negatively affect a battery's polarity.
• Never put batteries into a fire. Doing so could cause them to rupture and spill their contents. Also: Avoid tossing them into a metal container where heat could build up.

Solar Chargers (and Other Alternatives)

This new, quickly evolving product category offers the promise of power in faraway places as long as you have a line of sight to the sun.

These solar-collecting devices are configured as thin scrolls, fold-out panels or some similar space- and weight-saving design. In general, they need 8 to 10 hours (or more) of sun exposure to achieve a full charge. They transport their charge to portable electronic devices via an assortment of small cables and adapters.

They do not provide a huge or inexhaustible supply of power—a frequently cited estimate is one hour of solar collection provides enough power for 10 minutes of mobile phone usage. Nevertheless, they are a clever, clean way of keeping a GPS receiver or other small device powered during an extended backcountry journey. They work for PDAs, music players, game players and other low- to moderate-drain devices, too.

Another newcomer to this portable-power category is a sort of "mini-generator"—a rechargeable block of energy known as a lithium iron phosphate (LIP) battery. This external battery (the smallest model weighs about 5 ounces) uses cables and adaptors to connect to and power smaller portable electronics. Larger LIP batteries can even run a notebook computer.

Wrist-top Altimeters/Fitness Watches

Typically, watch batteries are tiny, button-shaped lithium batteries that in most cases require a professional's touch for proper installation. Often they must be carefully placed into small spaces where misguided probing could damage a device's circuitry.

Many manufacturers of watches, altimeter and heart rate monitors will void the device's warranty if a consumer attempts to replace the batteries. Others tout "consumer-replaceable" batteries.

Usually no guesswork is involved in selecting a battery for these devices, since the devices can only accommodate batteries of a specific size.

Recycling/Disposal

Imagine this: The U.S. Environmental Protection Agency estimates that Americans purchase nearly 3 billion dry cell batteries each year. This means on average every American discards 8 batteries per year. Here's how to lessen your impact:

Rechargeable Batteries

Recycling rechargeable batteries is required in some states. To learn how and where you can recycle them, visit the Rechargeable Battery Recycling Corporation website at www.Call2Recycle.org or call 1-800-8BATTERY.

Single-use Batteries

Many communities allow single-use batteries, made without mercury since 1996, to be placed in household trash. One notable exception is California. The state still regards all batteries as hazardous waste and requires them to be either recycled locally or taken to:

• a household hazardous waste disposal facility
• a universal waste handler (e.g., storage facility or broker)
• an authorized recycling facility.

Are batteries truly recycled—meaning that their materials are reclaimed for use in the manufacture of other products? Paul Schneider, director of sales and marketing for Kinsbursky Brothers, a waste-materials recycling specialist based in Anaheim, Calif., says rechargeable batteries are definitely recycled. It's a different story for single-use batteries, though. While recycling technology is available, he says, it is rarely used because little economic incentive exists to use it.

"Nickel in NiMH batteries and cobalt in lithium-ion batteries have intrinsic value," Schneider says. "There's not much demand for what's in alkaline batteries. We send them to our partner in Canada, Toxco, which recycles them for us. When alkaline batteries are collected in most local communities, they're usually sent to a hazardous-waste landfill." This reality presents another reason to seriously consider using rechargeable batteries.

Your best move for depleted single-use batteries? Contact your community's waste-disposal company for guidance. Some electronics stores may collect batteries. Recycling resources include The Big Green Box, Earth911 (1-800-CLEANUP), Kinsbursky Brothers, Toxco and Battery Solutions.

For a summary of this article, see our Quick-reference Guides.

Technical consultants: Patricia Bennett, Senior Engineer of Rechargeable Technologies at Rayovac; Suzanne Phillips, Director of Operations (Akron, N.Y., facility) of MGA Research Corp., which tests batteries among other industrial products; Todd Sweetland, Technology Manager for Micro Power Electronics, Inc., of Beaverton, Ore., a supplier of custom battery systems for portable devices in the commercial, military, medical and data collection industries.

Quick-reference Guides

• Rechargeable Batteries Quick-reference Guide (PDF Download)
• Single Use Batteries Quick-reference Guide (PDF Download)

Quick-reference Guide: Rechargeable Batteries

Quick-reference Guide: Single-use Batteries