Determining how long cell phone batteries will last is not an exact science. There are many different factors that can affect the charge, and the overall life, of a battery. The key to understanding them is to first understand what affects the charge and try to conserve power whenever possible.
In most cases, there are two standards by which cell phone batteries are measured: talk time and standby time. Talk time is usually measured in a matter of minutes or hours. Standby time can be measured in a matter of days, perhaps even weeks in long-life batteries. Over the years, the improving technology has made a significant difference in how long a battery lasts. When the first analog cell phones first started becoming popular in the late 1990s, talk time was often limited to 30 minutes with standby time maybe as long as 24 hours. Now, talk time is usually at least two hours, with standby time often lasting seven days or more.
How long a batter lasts can vary widely, however. For example, those using a digital phone in an analog service area may find they have a significantly faster battery drain because the phone is having to work harder to translate signals it does not normally deal with. If there is no service in an area, this may also cause a drain on the battery as the phone continually searches for a signal.
There are also different types of cell phone batteries available. Most are specific to various cell phone models, but in some cases, there may be a battery type, such as lithium ion or nickel metal hydride, that can be used in the same type of phone. Often, the latest battery technology will tend to last the longest in phones.
Another thing for cell phone users to consider is the charge and discharge cycle. Some phones, especially older phones that use nickel cadmium batteries, may suffer from a "memory effect" that will cause the phone to only charge to a a certain level, no matter how much time it stays on the charger. In such cases, the key is to fully discharge the battery and then fully charge it again. Other batteries do not have this type of limitation.
Cell phone batteries can usually be found anywhere mobile phone accessories are sold. Replacement batteries can cost anywhere from approximately $20 US Dollars (USD) to close to $75 USD, depending on the model and type. Generally speaking, the more expensive batteries are made with better technologies and tend to last longer.