Using a mobile phone without battery, using a capacitor and a resistor

Thread Starter

kelaboy

Joined Mar 3, 2017
20
So, I use this cheap 4G feature phone, permanently USB tethered to one of my computers, as a 4G modem.

I am wary of the constant charging/discharging that the battery has been going through.

I want to keep it tethered to the computer without the battery.

Tried both,
1. Booting the phone without battery, while USB tethered
2. Removing the battery from booted and working state, while USB tethered.

No luck, with the above two methods.

This particular phone/battery has 3 pins, +ve, -ve and thermistor. I get a reading of around 7K ohms between the thermistor and -ve terminals .

Dug, a little on the software side, and theoretically it might be possible to make this device work without battery with a modified boot.img file. But, that would be a very involved process.

Can't we just hook up a capacitor and a resistor of similar value to the pins of the phone and fool it into thinking that it has a battery?

Please note, I don't need any battery back-up from the phone - it is permanently connected to the computer.
 
Last edited:

Externet

Joined Nov 29, 2005
1,518
Forget the capacitor and resistor. Power the phone with the 3.3V computer supply rail to the phone power terminals that now have no battery. Do not power with 5V via the USB charging port, and come back with results.
 

Thread Starter

kelaboy

Joined Mar 3, 2017
20
Forget the capacitor and resistor. Power the phone with the 3.3V computer supply rail to the phone power terminals that now have no battery. Do not power with 5V via the USB charging port, and come back with results.
Thanks for the reply.

Well, I forgot to mention. I did try to power it on using an external Li-ion battery. Didn't work. That made me wonder, whether the phone won't boot untill it senses some value at the thermistor pin. Shall try again. And shall try again with a computer power supply as well.
 

MisterBill2

Joined Jan 23, 2018
6,311
You can keep the phone connected with the battery and supply the external power across the battery connection so that the battery never discharges. That should satisfy the internal phone software. A battery kaept at full charge should last a long time.
 

Irving

Joined Jan 30, 2016
797
I'd feel safer not using a power-supply and battery combo. A 7 - 9v DC wall-wart and an low-cost buck converter from eBay (look for one based on LM2956) adjusted to 3.7v and an 6k8 resistor to replace the thermistor - it won't boot without that.
 

Thread Starter

kelaboy

Joined Mar 3, 2017
20
Forget the capacitor and resistor. Power the phone with the 3.3V computer supply rail to the phone power terminals that now have no battery. Do not power with 5V via the USB charging port, and come back with results.
Tried, did not boot.

You can keep the phone connected with the battery and supply the external power across the battery connection so that the battery never discharges. That should satisfy the internal phone software. A battery kaept at full charge should last a long time.
Should work. But,rigging up a separate power connection to the battery would be a pain. I would need to take the battery out of the phone and make a custom case/holder with good contacts to the battery terminals.

I'd feel safer not using a power-supply and battery combo. A 7 - 9v DC wall-wart and an low-cost buck converter from eBay (look for one based on LM2956) adjusted to 3.7v and an 6k8 resistor to replace the thermistor - it won't boot without that.
I do have a few a buck converters lying around. But, still this would be a 2 wire solution- USB cable and wall-wart cable. And, yes the resistance value is a must. I tried booting up another cheap feature phone ( another model) with an external 18650. It did boot and work, but the home screen always read - Check Battery :) . And, it wouldn't charge. So, resistor is almost a must, I guess.

Theoretically, shouldn't a capacitor work?
 

Irving

Joined Jan 30, 2016
797
Theoretically, shouldn't a capacitor work?
Its unlikely the USB charger module in the phone will provide power to the phone unless there is a battery present that appears to be accepting a charge. You could try a capacitor in series with a small resistor, say 100uF and 1.5ohm - otherwise the charger might see the capacitor as a short initially - and see if it'll fool the charger module but I doubt it.
 

MisterBill2

Joined Jan 23, 2018
6,311
Tried, did not boot.


Should work. But,rigging up a separate power connection to the battery would be a pain. I would need to take the battery out of the phone and make a custom case/holder with good contacts to the battery terminals.


I do have a few a buck converters lying around. But, still this would be a 2 wire solution- USB cable and wall-wart cable. And, yes the resistance value is a must. I tried booting up another cheap feature phone ( another model) with an external 18650. It did boot and work, but the home screen always read - Check Battery :) . And, it wouldn't charge. So, resistor is almost a must, I guess.

Theoretically, shouldn't a capacitor work?
Why not just use a 3.5 or 3.7 volt wall wart supply??? With whatever the resistor needs to be.
 
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