[General Questions and Answers] rosemonkey88:
For Sony Xperia Series Hardware

Try this, i would say it works for most of the Sony Xperia series. only a guess, i have tested only Sony X10, Xperia E, Xperia Z1, Z2. for other models or even poket wifi etc, i would say they are working in the similar way, worth trying. again at your own risk.

do in this way, 3pins on the battery power connector inside the phone, connect the plus to the USB charging port plus(that 5v) - this is because i dont want extra cabling inside the phone, reuse the charging connector on the phone as power supply instead of charging. between the middle pin and the minus , connect a 20-30k resistor, connect the usb charging cable, connect the cable to a 5V power source. the phone will boot without a battery. monitor the fake temperature of the battery shown by a battery monitor app, need to be between a normal working temperature), try and adjust the value of the resistor if necessary.

Pay attention to these :

1. if you think apply 5v power source to a 3.7v battery plus pin, it will damage your phone and make you feel uncomfortable, dont try this at home. you can try 3.7v-4.0 with extra power wiring inside the phone.

2. phone requires very high current(300mA - 1.8A) and varies all the time compared to that a normal charger can provide, dont expect it will boot if u use a "ebay charger" or a PC USB port. i would say , most of the ebay chargers,if not all, will not be able to boot up a phone for current's sake regardless what they claim or be printed on the description. USE A RELIABLE HIGH CURRENT STEP DOWN CONVERTER, get from a powerful and reliable power source (a laptop power charger 19v, up to 3A, for example), keep monitoring the current when you boot the phone. Any car cigarette lighter socket normally can provide enough current, if it fails to boot, check the current your step down converter can provide. The step down converter i m using is just about $6-$7. not expensive one but need to find the right one(i bought more than 10, but only one useful for this purpose, they are all cheap $1-$10).

i came across this issue when i started making my own car onboard GPS system by using an old Sony Xperia X10, then Z1, then later, tested E and Z2 for fun . currently my system evolves to an Android X86 on a normal PC platform bluetooth, wifi and onboard video recording system connected. its all just for fun.

if you are looking for making your onboard phone boot with the engine key, dont need to worry too much about the switch on off, battery charging etc., Another idea is to use or modify a voltage step down converter(12V->5V), make its output voltage vary , say 3v-5.5v by a potentiometer (get it from an old PC speaker), then connect it to the onboard phone as a charger, you dont need to modify you phone, just change the charging voltage so as to the charging current, that means charging the phone while using the phone. if u charge more than you use , then the battery will increase when u drive,if u lower the voltage, the more u drive the more the battery decreases. u never need to take out the phone to charge. of course, you need to automate the phone on off with the engine key, to boot up GPS application automatically etc. i found this very interesting, currently an Xperia Z is running in this way, i found you need to adjust the charger voltage only once for months. theoretically, you have a balance point, always can make when u drive you car, keep the charging equal to your phone using.

happy to share this experience with anyone interested and good luck to everyone coming across this problem.