Rechargeable Batteries, Which One Is Best for Cordless Phones?

by Jim Hanks

Cordless phones are so loaded with features these days, that when you shop around it's easy to lose track of what's truly important. Conveniences such as telephone books, Caller ID, and pager notification make you forget that if your handset can't hold a charge, you might as well use a corded phone.

So which rechargeable battery is best? Nickel-cadmium (NiCd), nickel-metal hydride (NiMH), or lithium-ion (Li-Ion)? Which has the longest life? After reading this overview, you should be able to distinguish between each technology and know which best meets your needs.

Battery types
Nickel-cadmium (NiCd)
NiCd batteries were one of the first ones used in cordless phones. They were (and still are) known for quick charging rates and even better discharging rates, making them ideal for sporadically used products that require bursts of energy (such as power drills). Unfortunately, they do not have high capacities and they do contain cadmium—a toxic metal that is quite heavy—so they are not particularly well suited for cordless phones.

Another drawback of NiCd batteries is that they are subject to "memory effect." This phenomenon occurs when you repeatedly recharge batteries before they are completely depleted. Doing so reduces the talk time of the phone, often permanently altering a battery to 50% of its original capacity.¹ Memory effect can be avoided by allowing your battery to completely discharge before recharging.

Nickel-metal hydride (NiMH)
Often interchangeable with NiCd batteries, NiMH batteries provide 20% to 30% more capacity, are lighter in weight, and do not contain heavy, toxic metals. NiMH batteries are also much less susceptible to memory effect, but completely draining them every 8 to 10 uses is advisable.

Lithium-ion (Li-Ion)
Li-Ion batteries are the best all-around rechargeable batteries on the market today and, as you would expect, they're also the most expensive. They have 50% more capacity than NiCd batteries and are much lighter in weight than either NiCd or NiMH, making them ideal for laptop computers and cell phones. They are not susceptible to memory effect and do not pose a significant environmental problem for landfills.

One of the best attributes of Li-Ion batteries is the way they deliver power. While NiCd and NiMH batteries lose overall strength as they are used, Li-Ion batteries hold and deliver full power during most of their use cycle (and then quickly deplete to zero charge). This facilitates reliable cordless performance throughout a battery's use. The cost of these batteries, however, prevents them from being used in most low- to middle-end cordless phones.

How you can help your battery
If you're unhappy with the battery capacity of your present cordless phone but don't feel like upgrading, here are a few things you can do to improve your battery's performance:

  • After your initial cell phone purchase, make sure you give your handset a full, uninterrupted charge. Often this will take as long as 12 hours.
  • Regularly clean the contacts of your battery with a cotton swab and alcohol to prevent corrosion.
  • If you have a NiCd battery, drain it regularly until it can no longer function. Beware, though, because completely discharging the battery every time you use it can also diminish capacity. It's best to "cycle" the phone every 3 to 5 times you use it.
  • If you believe your battery is a victim of memory effect, you can have it reconditioned by a battery analyzer. These machines typically cost a few hundred dollars or you can pay a one-time reconditioning fee at an electronics store.
  • Retire your present battery and get a replacement from the manufacturer, a reconditioner, or a battery dealer.

¹When you recharge a NiCd battery before it is completely drained, bubbles form on the battery's plates thereby significantly reducing the surface area needed to produce electricity. This process is commonly termed "memory effect," but is often incorrectly applied to problems that should be attributed to "voltage depression." Voltage depression is caused by, among other things, overcharging. It can be remedied by depleting the battery completely and then recharging it while making sure it does not overcharge, i.e., don't leave the handset in the cradle for more than 12 hours.