Cellphone care and feeding
Cellphones are frequently used as gateways for communication with computers via SMS messages. For that purpose they tend to be tethered to the host computer by a serial cable (usually FBUS) and controlled by a suitable host software, eg. Gnokii. They are also powered from a mains adapter.
However, when transmitting, the phones demand a huge pulse current (sometimes 2 amps and more), even if its average current consumption stays in low hunderds milliamps at worst. The wall wart adapter is inexpensive and anemic, therefore even with mains feed, the phone stays woefully dependent on its internal battery (usually NiMH or Li-ion) to serve the energy when the mains feed is not enough.
Late in its life cycle, the battery becomes old, its capacity drops, and its internal resistance soars. The result is growing voltage drop during the pulse load. If the battery voltage falls below a threshold, the phone switches itself off. The symptom is a phone dying during handling a phonecall, and later even during sending/receiving a message.Read more..