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How to select a battery

BY BRION MUNSEY
House of Batteries
Huntington Beach, CA

Although the battery is the lifeblood of today's portable electronic devices, it is amazing how often this critical component has been left out by engineers until the last minute in the design process. Because battery technology has not evolved at the rapid rate that semiconductors or some other electronic components have over the last 20 years, the battery can be one of the more limiting factors in an electronic design.

There are more batteries to choose from now than ever, and each type of battery chemistry offers distinct advantages and disadvantages. A number of critical questions need to be addressed when selecting a battery for a new application.

It is important to review every detail of a battery's specification before implementing it into your new design.

Cost
Low cost is often thought of as the primary factor when selecting a battery type. We would all like to see a battery the size of a pinhead with thousands of ampere-hours of capacity that costs only pennies.

The reality is, however, that on many occasions a higher initial battery cost pays for itself in long-term benefits. Cost vs. specific performance needs should be addressed early in the design process. Set a reasonable target price, but expect it to move up or down based on your specific needs.

Primary or secondary usage
Primary cells are "one-time-use" batteries. Once the chemical reaction takes place, the cell loses its ability to produce energy; it is then discarded and replaced with a fresh cell or battery. Applications that see infrequent use or long-term use at very low drain rates are good candidates for primary cells.

Secondary batteries are "rechargeable," and the chemical reaction that takes place within the cell is reversible and repeatable for perhaps thousands of times depending on the chemistry and application. Any application that sees a lot of daily use, such as cellular phones and laptop computers, are good choices for rechargeable batteries.

Voltage
Most cells operate over a specific voltage range. For example, an alkaline cell has a nominal voltage of 1.5 V, an open-circuit voltage of 1.6 V if it is fresh, and is not completely discharged until it reaches 0.8 V or less under load.

NiCd cells have a nominal voltage of 1.2 V, an open-circuit voltage of 1.3 V or more for a freshly charged cell, and are not completely discharged until they reach 0.8 V or less under load. Multiply the number of cells in series, and you can have a fairly broad voltage range that your circuit has to deal with.

Discharge current
If the discharge current is intermittent, determine the amplitude and duration of minimum and peak current drains. Most electronic devices use varying amounts of current as they are used. A cellular phone is probably the best example: During transmission a cell phone can consume 1 to 2 A in short pulses, while its standby current is something like 100 mA or less.

Some batteries might do well at the lower drain rate, but fail to perform under peak current demand. Be sure to select a cell that can maintain a high voltage relative to the nominal voltage under peak loads.

Drain rate
How fast will your battery be used up? Will the load be a few microamperes, or will it be 10 to 50 A or more?

Lithium coin cells do well at low loads over long periods. Alkaline cells perform better under more demanding loads than carbon zinc or zinc chloride and even most lithium cells.

In rechargeables, lead-acid and NiCd are good choices for high-rate applications. Newer NiMH cells will perform well at higher drain rates also.

A battery may be discharged under different modes depending on the equipment load. The type of discharge mode selected will have a significant impact on the service life delivered by a battery in a specified application. Three typical modes under which a battery may be discharged are the following:

Constant resistance (R): The resistance of the equipment load remains constant throughout the discharge.

Constant current (C): The current drawn by the device remains constant during the discharge.

Constant power (P): The current during the discharge increases as the battery voltage decreases, thus discharging the battery at a constant power level (Power = Current x Voltage).

Operation
Will the application be used often or will it sit in storage most of the time? For example, a flashlight used only on an occasional camping trip is probably better suited to a nonrechargeable alkaline battery, which stores well and has plenty of power.

A flashlight that's used everyday would be better off with some kind of rechargeable battery, because the cost of constantly replacing primary batteries would become prohibitive. Some rechargeable chemistries like NiCd and NiMH are better suited to continuous discharges, while lead-acid batteries perform better in standby applications such as uninterruptible power supplies.

Temperature
An often-overlooked aspect of battery performance, temperature is nevertheless a major consideration when selecting a battery. Batteries do not charge or discharge well at low temperatures.

High temperatures are detrimental to shelf life (self-discharge). Most batteries perform best in the –20° to +60°C range. Lithium primary cells do better than most chemistries at both temperature extremes. Some lead-acid batteries do well at –40°C, and special "high-temperature" NiCds are designed to perform at +70°C.

Size
The semiconductor revolution has led to a dramatic reduction in the size of portable electronic devices over the last decade. Batteries are often the largest single component of a portable electric device.

Batteries have lagged behind silicon in terms of size reduction. Lithium coin, silver oxide, and zinc air batteries are all small primary cells that work well in pocket-sized devices.

In rechargeables, significant improvements have been made in both NiCd and NiMH chemistries giving them much higher ampere-hour ratings than they had just 10 years ago. Further reductions in size and particularly weight have been achieved with Li-ion and lithium-polymer chemistries.

Weight
The recent evolution of electronic devices is also consistently driving their weight down. Batteries are also often the heaviest component in a portable electronic device.

Carbon-based primary batteries are lighter than alkaline, but with less performance and shelf life. Lithium cells are lighter than other primary chemistries and have superior shelf life and performance. Li-ion and lithium-polymer chemistries are far lighter than other rechargeable batteries the same size and have improved the portability of many electronic devices.

Charge rate
If your battery is a secondary type, how fast do you need to recharge it? On average, 140% of the energy taken out of a rechargeable cell needs to be replaced when recharging it.

Lead-acid batteries (at a C/300 charge rate) and some special NiCds (at a C/20 to C/30 charge rate) do well on continuous-float or trickle charges for "standby" applications. A 14-hour charge (at a C/10 charge rate) is considered "standard" for sealed lead-acid, NiCd, and NiMH batteries.

Seven-hour fast charges are possible for most secondary chemistries. If you require a rapid charge of 1 hour or less then NiCd and some NiMH cells are a good choice. Li-ion and lithium-polymer batteries require a special two-stage charge that usually takes 3 hours to complete.

Storage requirements
How much shelf life do you need? A battery is said to be "spent" when it cannot deliver at least 80% of its rated capacity. How quickly will a cell lose its energy just sitting on the shelf?

Most primary batteries will store well for several years at room temperature. Carbon-based primary cells last about two years on the shelf, alkalines five years, and lithium cells 10 years or more.

Rechargeable or secondary batteries lose their charge when put into storage after charging. Lead-acid batteries need a "top charge" about every 6 months. NiCds last about 90 days to 80% capacity, NiMH cells lose their charge in four weeks, and lithium rechargeables can sit for perhaps several months at room temperature. Higher ambient temperatures during storage will reduce the shelf life of all batteries, perhaps significantly.

Cycle life
If your battery is rechargeable, how many recharges do you need? Lead-acid batteries will deliver from 200 to thousands of cycles depending on how deep each discharge cycle is.

NiCds will deliver from 500 to thousands of cycles, depending on how they are used. NiMH cells are good for several hundred cycles or more. Li-ion and lithium polymer will yield several hundred cycles.

Rechargeable batteries are sensitive to the recharge regime given to them. In other words, the better--and generally more expensive--the charger is, the better the long-term performance of the battery.

Recycling
Currently, by law all rechargeable chemistries must be recycled. This includes sealed lead-acid, NiCd, NiMH, and even Li-ion. These batteries must be clearly marked with the "chasing arrows" symbol and a national toll-free phone number that end users may use to locate their local recycling center.
When rechargeable batteries are purchased, a fee covers the cost of recycling. These fees may be invoiced as a separate line item, or they may be added to the overall battery price. Either way, they must be paid.

It is important to review every detail of your battery's specification before you implement it into your new design. That way you will be able to predict the battery's performance with some accuracy. Of course, after selection, careful testing is needed in the application to ensure that no unexpected behaviors prevent your new design from performing at its best.

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