All about cordless phone batteries
All cordless phones use rechargeable batteries in the handset. The most common rechargeable battery, nickel cadmium (NiCad), requires a short explanation below on how to use it and recharge it properly.
Early cordless phones had to be returned to the factory if the batteries wore out after being recharged many times. Today, all cordless phones come with easy-to-replace batteries, available at your local retailer or here on eBay.
While these batteries have a reasonably long life, you can do several things to improve the life of your NiCad battery as well as improve the performance of your phone.
When you first take your phone home, and before you plug it into the phone line, turn on the phone and let the battery drain for about 12 hours. This removes any static charge that may have built up during shipping. Removing the static charge can greatly improve your phone's reception.
After draining the phone, be sure to charge the phone for at least 8 hours. For best long-term performance, most manufacturers recommend that you completely drain the phone and recharge it once every three months.
Some extended range cordless phones use the newer nickel metal hydride (NiMH) and lithium ion, which have none of the recharging issues associated with NiCad rechargeable batteries.
If you do have a phone with a NiCad battery, be sure to always let it drain completely before recharging. Only recharge your battery when the phone's low battery indicator light is on. Why? NiCad batteries have a problem called "battery memory syndrome." If a battery is repeatedly drained a small amount, then recharged a small amount, eventually, that smaller amount becomes the battery's new capacity. For example, if a phone was taken off the recharger, used for one hour then immediately recharged, the battery will start to "remember" only being used for one hour. Even if the original charge capacity of the battery was four hours, it eventually will only contain 60 minutes of charge.
If you are buying a new cordless phone, you may want to consider phones with the newer NiMH batteries that have greater capacity and do not have "battery memory syndrome."
Nickel metal hydride (NiMH) batteries
These batteries take about as many recharges as nickel cadmium, last longer, and do not develop "memory" problems.
Even though newer battery types may not have the memory problems, all batteries will wear out eventually after repeated recharging. As you may have guessed, you can usually find a complete line of replacement batteries here on eBay.