Why are some batteries rechargeable but others aren’t?

July 25, 2010 - PRLog -- Rechargeable batteries abound—they’re in your laptop, cell phone, mp3 player and your car. But you may not have thought of using rechargeable batteries in your household gadgets, like your wireless keyboard and mouse, or Wii and XBox 360 wireless controllers. Every time you hear the thunk of dead batteries on the bottom of the trash can, you’re hurting both your wallet and the environment.

There are lots of reasons to use rechargable batteries in your everyday devices, but if you’re not convinced, we’ll be outlining them here over the next few days. First off, why can’t you just stick your Duracells in a charger and be done with it?

1. Why are some batteries rechargeable but others aren’t?

To answer this question, it’s necessary to review what batteries actually are. At its most simple, a battery is a unit that converts chemical energy into electrical energy. A battery holds a chemical reaction that causes electrons to flow from one of its terminals, either negative or positive, through the wiring of a device. The electrons are seeking the battery’s other terminal by traveling through the wiring and creating electricity. Northwestern University has a neat diagram and explanation here.

When any battery is placed between positive and negative terminals—that is, when it’s placed in your device—the reaction begins. Once all of the chemicals inside a non-rechargeable battery have reacted, the battery is out of power. It’s dead. Let go. Begin the grieving process.

 

However, rechargeable batteries contain chemicals that react reversibly. When rechargeable batteries receive electricity—are put into the charger—the chemical reaction inside is reversed (for the most part). This means the batteries can power your device again. Rechargeable batteries wear out only when the chemical reaction inside them can no longer be reversed by the charger.

Placing a non-rechargeable battery like an alkaline battery into a charger may give a short burst of extended use—we’re talking seconds or maybe a minute—but it may also cause the battery to leak or even explode. We are talking from experience and we don’t recommend it.


Rechargeable battery types: NiCd vs. NiMH, and why there are no Li-ion AAs

There are two types of rechargeable batteries in standard sizes: NiCd and NiMH. NiCd, which are nickel-cadmium based, were the first rechargeable batteries available to the public. They’re often referred to as “NiCads,” but that’s actually a brand name.



NiCds were first invented in 1899 in a non-portable form (kind of defeating the purpose), and didn’t enter widespread production in the U.S. until the 1960s. Your first RC car likely had NiCd batteries.



One disadvantages of NiCd batteries is that they suffer from the memory effect. This means that a NiCd that has been charged and discharged a number of times loses its charge more quickly than a newer NiCd battery. This is caused by the buildup of cadmium crystals inside the battery. To prevent this, NiCd batteries should only be placed in their chargers after they are fully discharged.

Another disadvantage is that NiCds also contain toxic materials like cadmium and mercury. For this reason, the sale of NiCds has been banned in the European Union except for select purposes.



 Pros Cons
NiCd -Lower Price -Memory Effect
-Self discharge quickly, but not as quickly as NiMH

-Contains toxic materials
 
NiMH -Higher total capacity than NiCd
-No memory effect
 -Self-discharge more quickly than NiCd
-Can generate more heat than NiCds

The most common everyday use rechargeable batteries are nickel metal hydride batteries, or NiMHs. They were developed in 1989 as an alternative to clunky nickel-hydrogen batteries, which are mostly used in satellites today. NiMHs are much less toxic than NiCds. Due to their low price and relative eco-friendliness, NiMHs are currently the most popular rechargeable option.

You may know that lithium-ion (Li-ion) rechargeable batteries incased in battery packs are widely used in electronic devices like cell phones, laptops and mp3 players. Why aren’t there any lithium-ion standard-size batteries to substitute AA size alkalines? Li-ion batteries run at a higher 3.6 voltage than standard aa size batteries usually do 1.2 volts; that makes them incompatible with both your devices and battery chargers. Plus, due to the high energy and high heat potential, they need a PCB board to control current and over voltage for them to be used safely and effectively.

What about rechargeable alkaline batteries? Generally speaking, rechargeable alkalines cost as much as superior NiMH rechargeables, and have a much shorter lifespan—dozens of recharges rather than hundreds.
 
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