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How to select a battery
BY BRION MUNSEY
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.
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
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.
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.
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.
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).
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.
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.
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.
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.
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.
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.
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.
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.