going from LiPo battery to wall power?

Hi Ehab,

I suspect that the circuit that maintains and monitors the battery is shutting the unit down when it detects that the battery is missing. Where are you attaching the 3.7 volts? To the charger Mini USB port or directly to the battery contacts? As long as your power supply has enough current capacity the voltage is all that matters. The unit will draw the current that it needs from the 3.7 volts. If you have a meter you can monitor the voltage when you turn the unit on. You may need a good capacitor on the output of your buck supply so and surge demand of the unit doesn't cause it to pull the power down momentarily. Some batteries have more connections than just the + and - power. If your battery has more than 2 connections the unit may need the battery in the circuit in order to power up.

John

Thank you for the response!
I have connected the buck converter's output directly to the battery pins (there are only 2 pins). if it makes any difference, I'm providing the converter with an input of 5v. You mentioned that I need to connect a capacitor to the output of the buck converter. How much exactly? I don't really know the formula to calculate that
oh, and the battery that came with the tablet is 3.7V 7800mAh

Hi Ehab,

5 volts input to the 3.3V converter might be a little low. Usually they need at least 2 volts higher input than output. I would start with a 470 uF capacitor at any voltage 10 volts or higher. If you have a way to measure the voltage of the converter when you turn on the unit you will be able to see if it can maintain the 3.3 volts required. If it drops below 3 volts you can try a 6 or a 7 volt supply to your converter. If you battery had only 2 terminals then we should be able to get this to work. Connecting to the battery terminal is what I would do too. Also on hooking up the capacitor make sure the plus lead of the capacitor goes to the plus lead of the converter and the negative lead of the capacitor goes to the negative lead of the converter.

John

hello there!
sorry for the late response, I was busy with finals, and the capacitor took its sweet time to arrive. I did what you told me to do, but it did not change anything, however. I added the capacitor, and increased the input voltage, but it's still having the issue.

I also measured the voltage as it was booting up, and it was dropping, but only by a little bit. It goes from 3.80v to 3.72v, and it does not go lower than that. from what I understand, a completely charged lipo battery should have a voltage of around 4v, and a discharged one should have a voltage of 3.0v. The measurements I'm getting are in between, so they should be ok?

John

Many of the tablets use 15v to charge the battery (they detect the device and up the output voltage), so its possible the battery management electronics is deciding the battery is not there or is depleted based on use and charging.

Maybe some info on the actual tablet model might help in overcoming the issue.

Mark

Hi Mark,

Thanks for your insight. I personally have tried to circumvent an integrated battery in a piece of equipment. You are undoubtedly right as it apparently isn't working.

John

Hi John,

I think this is a likely issue. There are often three or more connections, one being a thermistor in the battery pack. With that absent, the power management IC won't start up the device.

Tablets have fairly big batteries so there may be a third connection somewhere. We need to know the tablet model. According to youtube teardowns, even supermarket-branded tablets can have more than two connections to the battery pack.

thanks for the reply!
the tablet is a chinese one made by a company called PIPO. And the model is the Pipo M9. here's the battery for that tablet:

Original 3.7V 7800mAh Rechargeable Lithium Battery For PIPO M9 Tablet - US$37.26

Sorry I mistyped  - I meant to say I have never personally tried to circumvent an integrated battery so I am not the best person to be answering this question.

John

Are you sure the tablet actually works? Perhaps there is a fault causing it to immediately power off.