Battery charging circuitry - applying power to a phone when using a battery eliminator

Thread Starter

bobMuk

Joined Aug 29, 2018
3
Hi
I'm planning on using an old cellphone as a screen in a DIY project (ie. the phone will be functioning as normal with some video playback running)

I don't want to leave the battery in the phone, I'm assuming it'll eventually burst from constant charging cycles, but this phone won't boot unless it detects a battery. So I built a battery eliminator using a 4.3V supply attached to the header/connector which I've carefully removed from the existing battery.
It works great - the phone thinks it has a battery at 80-90% charge and runs fine.

But... to make the phone boot on demand I need to provide power to the USB port (I can just turn it on for a few seconds then off).
I know nothing about battery/charging technology - is there any kind of risk to doing this? The 4.3V supply will presumably have current coming into it from the phone as it thinks it needs to charge the battery.

I'd assume that including a diode in the circuit should prevent this but, being more a software guy, I wouldn't know which type to use.

Any help would be appreciated!
 

Thread Starter

bobMuk

Joined Aug 29, 2018
3
Yes. I've unwrapped the phone's battery and removed the header connector circuit which has the 4 pads which connect with the 4 pins on the phone. The bottom of the header has 2 contacts which are simply attached to the 2 pins of the physical battery cell. In wiring my power supply directly to those contacts in place of the battery, the phone works fine - as it thinks that it does indeed have a battery inserted. Any data that the phone expects on the extra pins is still being provided.

That doesn't really help with my question though...
 

-live wire-

Joined Dec 22, 2017
897
What type of phone is it? Android or iPhone? Let's start there. Also, the typical max voltage of a lithium battery is 4.2V, so maybe add a diode or something to that 4.3V supply. Additionally, you probably don't want the USB pumping an amp or two into your power supply.
 

Thread Starter

bobMuk

Joined Aug 29, 2018
3
Hi
It's a Galaxy S5. Google suggested the real battery would reach 4.3v at 100% charge so I was kinda hoping the 4.3 supply would make the phone think the battery was 100% and not attempt charging.

In reality (using a battery monitor app) the supply seems to be providing about 3.9-4.1v so the phone thinks it's 80ish percent.
The USB input will be 500mA coming from a USB hub.

So that was really my question - if the phone attempts to charge a non-existent battery, will it kill the supply or damage the phone charging circuit? It sounds like you're saying yes.
Providing power via USB is the only way I can remotely boot the phone - it's been rooted and configured to boot when it sees USB power.

So I did think about including diode(s) which (for any conventional circuit) would only allow current to flow in the one direction, but I know zero about charging circuitry and wondered if there was anything I hadn't considered.
 

Hymie

Joined Mar 30, 2018
713
I would guess that it would be very unlikely that what you are proposing would cause damage to either supply - based on them both supplying around 4.3V to the phone battery.

When you power the usb port, the voltage supplied to the battery terminals might increase by 0.1-0.2V. Although for this short period all power will be from the usb port, your battery eliminator circuit should not be damaged as a result of applying 4.5Vdc to its output (when powered).
 

-live wire-

Joined Dec 22, 2017
897
Hi
It's a Galaxy S5. Google suggested the real battery would reach 4.3v at 100% charge so I was kinda hoping the 4.3 supply would make the phone think the battery was 100% and not attempt charging.

In reality (using a battery monitor app) the supply seems to be providing about 3.9-4.1v so the phone thinks it's 80ish percent.
The USB input will be 500mA coming from a USB hub.

So that was really my question - if the phone attempts to charge a non-existent battery, will it kill the supply or damage the phone charging circuit? It sounds like you're saying yes.
Providing power via USB is the only way I can remotely boot the phone - it's been rooted and configured to boot when it sees USB power.

So I did think about including diode(s) which (for any conventional circuit) would only allow current to flow in the one direction, but I know zero about charging circuitry and wondered if there was anything I hadn't considered.
The main reason I suggested the diode was because it's a very simple way to reduce the voltage. I have heard of lithium batteries with slightly higher voltages, but it's always better to play it safe.
 
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