There are several things to know and consider when buying a replacement battery for your phone.  Here is some quick information to help with your buying decision.

  Capacity    Size    Weight    Battery Type   Battery Life

Capacity is measured in mAh (milliamp-hour).  The higher the capacity, the longer the standby and talk time.  The first impulse is to get the battery with the highest capacity.  However, keep in mind that the capacity has to fit somewhere.  The highest capacity battery is generally heavier and bulkier than lower capacity batteries.  You may still need the highest capacity cell if you want to use your phone longer between recharges.  Most people are happy with a standard capacity battery.

Size, or more generally, thickness, is determined by the battery type and capacity (see above).  Extra capacity will make a battery larger.  You can offset this by selecting a battery made from lithium ion or lithium polymer since these newer type cells have a higher power density (they pack more power in the same amount of space).  You will see terms like slim, ultra slim, main, and extended used by different vendors.  This is not a good indicator of the size as everyone's idea of slim is different.  Size is usually measured in mm (millimeter).  

Weight is closely related to the size of the battery.  The higher the capacity, the thicker the battery, the heavier it will be.  Weight can be offset by selecting lithium ion or lithium polymer batteries as they have a higher power density (they pack more power in the same amount of weight).  Keep in mind that a few extra grams or ounces is not very noticeable.  You will generally not notice a difference until the difference is 50-100%.

Battery type is one the most misrepresented characteristics.  Most vendors seem to claim that each type is the latest and greatest.  In general, there are four battery types used in cell phone batteries.  They are listed below from newest to oldest.  Whenever possible, you DO want the newest type. 

Lithium Polymer - This the newest battery type being used in cell phones.  They have a higher power density than the other types.  This allows manufacturers to provide either a thinner or lighter battery, or some combination of both.  They are often hard to find.   Factory Direct Cellular is one of the few stores that carry lithium polymer batteries.  We currently have them for Nokia 5100/6100/7100 and several Samsung phones.  Lithium Polymer batteries do not suffer from the dreaded memory effect and can be recharged anytime.

Lithium Ion (Li-Ion) - This was the latest battery technology before lithium polymer.  They have a higher power density than NiMh and NiCd batteries. Li-Ion batteries generally come standard with most phones.  For most people, this battery technology represents the best combination of size, capacity, and value.  Lithium Ion batteries do not suffer from the dreaded memory effect and can be recharged anytime.

Nickel Metal Hydride (NiMh) - This was the most popular standard battery just 12-24 months ago.  They are lighter than Nickel Cadmium batteries, but heavier than Lithium batteries.  While NiMh batteries are not as susceptible to the dreaded memory effect as NiCd batteries, they must still be cycled once in a while for optimum performance.

Nickel Cadmium (NiCd) - This was the oldest battery type used in cell phones.  They are the heaviest and lowest capacity battery type.  NiCd batteries pretty much started the notion of the memory effect.  We don't carry this type of battery.

Battery life is a factor of battery capacity and battery drain.  Any quotes that you see for standby and talk time assume ideal conditions.  Most of us will never use a battery full under ideal conditions the whole time.  Using a phone in analog mode drains a battery really quick.  So does leaving a phone on when there is no coverage.  This can happen when you are in a building, tunnel, underground, or in a plane.  In these cases, the phone will spend a lot of power trying to reestablish a connection to the tower.