I have opened up an old siemens A60 mobile phone and soldered wires onto the contacts which connect to the (+) and (-) of the battery (with battery removed) so I can use an external 3.7v lipo battery. It works fine. Hower I am controlling the mobile using an Arduino uno and I want to power everything using a single power supply - a 12v, 3Ah Lead Acid battery including the phone (the arduino should be ok at 12v I think).
Does anyone have any hints or tips on how to step down the voltage on the 12v battery to get 3.7v for the phone? I believe it will need to deliver about 0.4A for the mobile to work properly - not sure - got info off mobile phone charger. Oddly (to me a novice) the phone dosn't work off it's own charger when connected direct to the power wires I soldered in, but I note it reads 5v not 3.7v
PS the project is to build a GSM alarm system for my shed which has no mains power - hence the 12v lead-acid battery, which I will hook up to an off-the-shelf soral trickle charger.