Battery Innards

Standard 500mAh as supplied with phone

Extended 1800mAh

Standard Battery
Extended battery Innards
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Thermistor (Thermal Sensor)

Battery capacity identification resistor

Line Line
Ident resistor PCB

Thermal cutout

Thermal cutout
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Temperature sensor and battery  capacity identification resistor

The standard battery includes a thermal cutout as well as a temperature sensor because the compact flat cell construction (prismatic cell) is less tolerant of overcharging than the larger capacity cells

The standard NiMH cell typically has a capacity of about 500 to 600mAh.  Extended batteries are two to three times this capacity but are usually cheaper than standard batteries.  The reason is simply that the standard battery uses the quite expensive button or prismatic cells.  The extended battery uses the much more widely available cylindrical cells.