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A Better Understanding of Rechargeable and Replaceable Batteries

Which type of battery is best for your device? 
Should you use a replaceable (primary) cell or rechargeable (secondary) cell?

 

At Omnica we are always talking about batteries. We have many requests for developing handheld devices, and most of them require batteries as a power source. But there is always a trade-off between duration of operation and size and weight. For some reason, many of our customers have difficulty understanding the concept. Except in science fiction, there is no way to compactly store the massive amounts of power required for futuristic devices like phasers or hand-held arc welders. Many of those use replaceable alkaline cells, but for rechargeable applications, there are basically three choices: Nickel-cadmium, Nickel-metal hydride, and Lithium-ion.

 

Lithium-ion cells are currently considered to have the best battery chemistry. They are the most widely researched of all portable cell technologies. Development of lithium batteries started in the 1980's, but it wasn't until the mid 90's that they were produced and shipped in commercially available quantities. Early versions of the batteries had a significant drawback; they could explode after repeated recharging!  It was a rare occurrence, but it wasn't until their fourth generation that the safety hazard was eliminated.  Today's Lithium-ion batteries have integral microcircuits (adding cost) that prevent overcharging and over discharging.  They contain no liquid, and are therefore leak-free, making them more environmentally friendly than other types of batteries.

 

Lithium has the highest electrochemical potential of any metal. With a 125Wh/kg energy density, they can provide twice the wattage of a NiM-H and 3 times more than a Ni-Cad. But energy density comes at a price. These powerful cells cost about 4 times more than a comparable Ni-Cad.  Li-ion batteries are the power supply of choice for cell phones and laptops, but they don't provide the electrical current needed for power tools. As the technology improves, this problem should be solved. Nickel-metal hydrides provide high current, so they are the best for high-drain applications. Their significant drawback is a short shelf-life, much shorter than the years-long shelf lives of Li-ion batteries. Ni-Cad batteries will self-discharge 40% of their stored energy in three months. Unused NiM-H cells will discharge completely over a period of weeks.

 

Regarding lithium-ion shelf life, one of our engineers recently met with a battery-house representative. It has come to their attention that repeated charging and re-charging of a less than fully discharged (which you should try to do, but not more frequently than once in about every 30 cycles) Li-ion cell affects the battery "gas gauge" included with many devices. Over a number of cycles there is a resultant loss in accuracy. Frequently, the indicated remaining charge has little relationship to the actual amount of life left in the cell! Fortunately, according to the battery expert, the gauge will reset itself if the battery is allowed to completely discharge from time-to-time.

 

Li-ion cells have now been on the market long enough to establish a good safety record. Prices are still too high for the batteries to sell in all markets, but for medical devices, where reliability and longevity are most important, the coming years are expected to prove Lithium-ions (in their various chemistry configurations) the preferred power source. As prices drop they will begin to replace the next best cell chemistries.

 

Cost aside, when considering which battery is best for your application, there is at least one valid reason for choosing a replaceable battery rather than a rechargeable. Even with all the exotic, powerful batteries and unique shapes available, we still try to design products that use a power source that’s easily replaceable. Common sizes like AA, AAA, and 9 volt types are available everywhere. Have you ever tried to find a special-sized lithium camera battery while on vacation?

 

There is much more information (more than you'll ever want to know) at Battery University