This idea will only work with phones or tablets if the battery is removable. Observing polarity is important, of course. Please be careful not to damage your device through carelessness. If you are unsure of your ability to do this then use an old mobile to practice with first and/or get someone to help you. So far all the phones I have adapted to external battery, or mains power, have worked perfectly.
Please Note. Because of the wires running from the back of the phone when tethered we simply leave the back cover off. The dummy plate is secured at the bottom with a small piece of Prestik to stop it dropping out of the battery compartment. We didn't want to alter the back cover to accommodate the wires but it could be done if you wanted to but I recommend that you don't unnecessarily damage the back cover of an expensive phone, just leave it off.
I started this project because my wife's Samsung S4 was constantly running out of battery power. This phone loses battery power even with the charger plugged in and, also, the micro USB port doesn't work properly. She has three 3 generic batteries which we keep charging with universal chargers, so two are always on charge whilst one is in use. Being online with WIFI these batteries don't last long and she is constantly changing and charging batteries. I decided to make things a bit easier for her by rigging up an external battery and eliminating the onboard battery, unless she goes out, when she just changes back to a Samsung battery we keep charged with a universal charger.
External battery power.
Tools needed, Multi-meter, soldering iron, small flat file, Stanley knife, hand tools. Q'bond.
Parts needed, single or four row 18650 battery holder, twin-flex, plastic plate and contact strips for dummy battery plate.
My first idea was to use a 18650 battery wired up to the + and - terminals in the battery compartment. I Googled this and found one image where someone had twisted two wires to the +/- contacts in the battery compartment. This was not a good way to do the connection so I decided to make a plastic dummy plate about half the thickness of the battery to fit in the battery compartment and make two contact plates cut and shaped to align with the +/- contacts and glue them into position.
The plastic dummy plate must be a close fit in the battery compartment with the top corners filed to engage under the battery locating tabs so that the plate will not just pull out at the contact end. The contact area of the plate should be filed away to clear the contacts. I soldered wires to the plates before gluing them. The ends on the plates must be cut and aligned properly with the phone +/- contacts and protruding a millimeter beyond the edge of the plate, then glued with Q'bond and filler powder or a similar quick drying adhesive.
A 1.5m length of twin-flex soldered to the 18650 battery holder observing polarity finished the job. At first using one 18650 cell was not a big enough increase in battery life so I joined 4 cells in parallel and that was a big improvement. Using four rows of four cells in a 4 x cell holder giving 16 cells would give about 32 amps and would run a phone for a long time. Ideal when on a trip with no way to charge a phone battery..
A mains powered phone (no battery at all).
Parts needed AC/DC power supply 8.5 to 30 volts at 2.5A half wave or full wave rectified, LM2596S buck converter, pill bottle, length twin-flex. same dummy battery plate with contacts as above.
Because this phone is used mostly at home, where there is mains and WIFI available normally one could just plug in the charger whilst on WiFi, but this does not work with this Samsung S4. Battery capacity still drains from the battery whilst it is on charge. So, I thought why not just have the phone tethered to the mains instead of to the charger. The cell phone battery can be charged with a universal charger(pic) and will always be available when we go out. At least there is no damage to the battery that constant charging in the phone can cause, like swelling an overheating, and, also, constant use of the micro USB port is minimized. My first attempt was with a 8.5 volt, 1.27A half wave rectified AC/DC supply that worked well with a Samsung Y but was not enough to power a Samsung Fame. It would start up but then switch off. For the S4 I used a laptop power supply with a 19 volt DC output at 3.5 amps and figured that if it is able to power a laptop with the battery removed it should be able to handle the phone easily. It did.
My latest attempt to power the Fame was a 12 volt, 2.55 amp half wave rectified AC/DC supply which works perfectly. All these different input DC voltages had to be adjusted down to 4 volts so after much searching the web I decided to try a LM2596S buck converter module which can handle 3A max. current draw but the output had to be readjusted for the three different voltages I tested. I used a pill bottle with a hole at each end as an enclosure for the buck converter and just cut and wired it into the power supply output line.
I have also realized that any DC power source like a car, motor cycle, UPS or alarm battery, or DC power supply within the input range of the LM2596S module can be used to power a phone as long as the output voltage is set from about 3.6v to 4.2v and the correct polarity is maintained. Using a car battery whilst in the car can be a problem because a running engine will up the input voltage to the buck converter and change the output voltage so one must be careful.
I will be experimenting with other devices as I get the chance to do so. Also, I don't know what will happen if the polarity is connected wrongly and I don't want to damage any phone by trying it out. I also like being freed from the battery hell of remembering to plug in the charger every day. Not all phones can be adapted to wireless charging (QI) to make things easier, never mind the cost of this new technology. Also, if you have a spare phone lying around it can be connected to mains and be always on, like a landline, if you don't have one. I like to think of my phone as a landline that I can take with me when I go out.
I have tested the Fame whilst running on mains by plugging in the USB data cable to test if it works and the only thing that happened is the battery charge indicator started flashing. This means that the mains connection does not appear to interfere with data transfer at all when tethered to my computer.
An always on cell phone used in a work environment could also be freed from having to always be charged.
I am sure that there are other uses that this idea can be used for that I haven't thought about.
I hope I have explained myself well enough with this Instructable but, if not, I would appreciate any feedback and ideas sent to me.
I am 75 yrs. old and not financially well off so I have added a PayPal donate button so that anyone who may like my idea can help me out by clicking here