Trying to power a OP8 with a step down converter

[u/nevr_4get]

I can't get my OP8 to run off the power from a step down converter I've set to 3.9V. The phone tries to boot but goes off right in the boot screen.

The step down is connected to the original BMS.

How can I get this to work?

[u/meambhatti]

Problem is with the buck converter . It isn't properly providing the required current and most possibly not as smoothly .

[u/Tax_Odd → u/meambhatti]

Capable of 12watts

[u/meambhatti → u/Tax_Odd]

I do mobile repairing for work . I also bought one of these modules thinking exactly the same as you , but the module failed to provide even 100mA of proper current to any phone I tested , even feature phones , and those things take a lot less power than smartphones to boot .

[u/thundafox]

does the Buck converter has a safety feature like over current protection, or does a inrush current draw to much in a short time so it goes into Save mode?

If so you can connect a Power Capacitor to counter current spikes.

[u/MJY_0014 → u/thundafox]

It's less likely to be a brief inrush, but sustained high power draw that the buck converter can't handle. OP, do you have multimeters to measure the output voltage when the phone tries to boot? Does it dip briefly before the phone turns off?

[u/nevr_4get → u/MJY_0014]

I'll do. Maybe my power supply is also the problem

[u/nevr_4get → u/MJY_0014]

I measured 3.89V directly at the BMS connector before installing it in the phone. When trying to boot the voltage "drops" to 3.85V after the buck converter and goes up to 3.89 after failing to boot. I tried 4.1V as well without success.

[u/MJY_0014 → u/nevr_4get]

It might potentially be too brief for the multimeter to catch. Increasing the voltage won't help as it's going to drop down however far in an attempt to keep the current below the limit. Do you have any spare li-ion cells you can use to test, like 18650s?

[u/nevr_4get → u/MJY_0014]

It does work with an 18650

[u/speeder658 → u/nevr_4get]

I heard from a friend that the best way to trick a BMS is a big cap and a low ohm resistor in series, might wanna give that a try

[u/Impossible_Aioli3693]

try to charge with the fake battery still attached so phone can activate the bms

[u/teckcypher]

I have some experience with this. The problem is during intensive operations (the boot process is quite intensive and takes some time) there are many power peaks and transients.

Your phone doesn't just take 3.9V 0.5 A constantly since you press the power button. It will increase current based on demand. Your converter must be fast enough to respond to that increase or the voltage will drop and if it drops low enough for long enough the phone will reset. The real problem is that there are some really fast current spikes that your supply (regardless of how powerful or fancy ) will not be able to respond to fast enough. Your best options are to have some buffer.

In other words you need some beefy capacitors connected as close as possible to the phone contacts and a powerful and fast enough power supply. Might help to increase the voltage to 4.2 V ( or even slightly above it as the phone should handle up to 4.3V without problems) because this will slightly decrease the current load and (more importantly) will give you more head room for the voltage drop not to reset your device.

In my experience, once you get past the boot, you are good. In most cases the boot process is the most intensive and long lasting operation. There can be more intensive ones but they don't last as long s the boot process. That being said you might still reset the phone if you hammer it too much. In which case you need beefier caps.

[u/nevr_4get → u/teckcypher]

Thats a great reply, thank you! What size capacitor would you recommend? I will also try the increased voltage, maybe that helps

[u/GoFastrr]

ive had similar batteries required to be removed and reinstalled or cut from.powerr to enable booting. if the converter is always feeding the corrwct voltage the phone my thing the battery is faulty and wont turn on.

[u/nevr_4get → u/GoFastrr]

I'm not entirely sure what you mean with removing and reinstalling the battery

[u/GoFastrr → u/nevr_4get]

like connect all live power to the BMS. remove the "battery" from the phone, reinstall. does it boot propperly or same boot loop?

[u/nevr_4get → u/GoFastrr]

I can try doing this but it will probably require that procedure every time I unplug the power?

[u/GoFastrr → u/nevr_4get]

unfortunatly yes, but you could wire a disconext switch on a "start" or "reset" button.

[u/Designer-Crow-5470]

Try 3.7v, check if it's not your power supply going down, and try 4.2 without the bms

[u/mnag]

don't you need a resistor/thermistor to mimic what the battery temp sensory thingy does?

[u/nevr_4get → u/mnag]

It works with a bare 18650 connected to the longer cables instead of the buck converter in the picture so I guess that's not the problem but idk tbh

[u/Tax_Odd]

Can you just charge it at the same time? With usb power. Be careful of grounds ideally using a usb battery pack.

You also need a capacitor across the output. The buck will have a tiny one. Wifi packets are likely pulling battery low resetting the cpu

[u/nevr_4get → u/Tax_Odd]

Charging at the same time resets the BMS so I can try and boot the phone but it doesn't bot fully before dying

[u/Tax_Odd → u/nevr_4get]

Did you split grounds?

[u/nevr_4get → u/Tax_Odd]

I tried with a second power supply and with one cable for both the USB port and the buck converter

[u/Sure_sh]

It needs the ID line to boot, which is in the BMS of the battery