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.
The most obvious solution is taking a big fat and if available low-ESR electrolytic capacitor and connect it in parallel to the battery. This prolongs the useful life of the battery. However, later the battery dies anyway and a more permanent solution is required.
Another option is replacement of the battery cells with three standard NiMH AA cells with a holder. As the standard BMC-3 "extended" battery has the capacity of 900 mAh and "normal" NiMH AA cells commonly reach 2400 mAh or more, this approach has a distinct advantage for mobile applications. The low cost of the commodity AA cells also favors us in comparison with the proprietary models. Additional advantage is the ability to carry arbitrary number of charged batteries which are usable also in your radio or camera, sharing resources as needed. They can also be charged in a high-current ultrafast charger; there are situations in life where shortening the wait for charged batteries to 15 minutes is well-worth the associated loss of lifetime of the cells (as fast charging amounts to pretty rough handling).
However if the phone is used in a tethered application, it does not have to have its own onboard power at all. There is usually a computer around, with its big fat pulse power supply capable of delivering dozens of amps at smoothly stabilized 5V DC. The battery can be easily replaced by an external power feed, if the phone gets tricked to thinking it still runs from a battery. Fortunately this is easy.
Also, the self-switching off can be cured by an additional circuit, the PhoneOn device.
The Nokia 3300 series phones use the BLC-2 or BMC-3 batteries. The BMC-3 battery consists of three NiMH cells connected in series, yielding 3.6 V. The batteries have four terminals:
The phone does not care if it gets juice from battery or from something else, as long as it thinks it has the battery; when the BID and BSI signals aren't connected, it refuses to switch on (tested on a Nokia 3310). So we simulate the battery signals with resistors, using a 5.6 kiloohm resistor (R1) between BID and GND and a 47 kiloohm resistor (R2) between BSI and GND, according to the associated schematics. We also use a big capacitor (C1, I chose a 3300 microfarad one assembled from three capacitors of smaller value that were laying around, the value was a wild-ass guess that worked) to make the phone able to draw high current for short times. In this case, we use stabilized 5 volts DC as a power source; to bring its value down to the desired battery value we use two silicon diodes (D1, D2) in series, as there is about 0.65 volts loss on each.
Use of other means to obtain the desired 3.6..3.7V volts is possible as well, eg. if we have unstabilized wall-wart supply, we can employ the cheap and venerable LM317 stabilizer.
Original board, top side
Disassembled BMC-3 battery
BMC-3 circuitboard, other side