Based on this question I created the following circuit:
Since I made some (component) changes, first I would like to have a validation of the circuit for long-term usage (and some improvements).
Used components (different from linked circuit):
- 1N5822 diode
- AO3401 P-channel mosfet
- 3.7 V 2600 mAh 9.62 Wh (protected) Li-Po battery (from another project)
- Added a ceramic capacitor and a toggle switch (I hope to the right position)
I made this circuit as a battery backup solution, so 99.9% of the time it will be powered externally, but I don't want the Pi to shut down when there is a power outage.
Here comes my question #1: do Li-Po or Li-Ion batteries last a long time if I connect them to a TP4056 charger constantly?
Currently I'm testing with a Pi 3b+, but I want to upgrade to a Pi 4 later. The circuit works fine when there's external power, but I have issues when I switch to battery powered mode; during boot, the Pi shuts down. If I'm right, the battery protection in the charger module kicks in and saves the battery from over-discharge. When this happens, I get no voltage from the out pins, so I made this modification:
So I connected the battery directly to the MOSFET, because the battery itself has a protection circuit. With this modification I almost get to the desktop, but just before that I get a reboot. If I turn off the external power when the Pi is in idle state, the circuit works until I open an app and then it reboots.
I don't know the C rating of my battery unfortunately; I got it from China. But I have a linear bench power supply.
Question #2: Is it safe to connect it to the TP4056 as a battery replacement to test how much current the boost converter needs in battery mode?
Question #3: I'm thinking about buying a reliable 18650 Li-Ion battery with a high current rate. Do you think it will solve my problem?
As an alternative solution I just connected another charger module in parallel like this (with a second battery), but I saw weird things happen, like they randomly showed the 'charging-done' state when they weren't full. After reconnecting the external power they started charging again. So I would like to stick to one charger/battery if I can.
My question #4: is there a DC/DC boost converter that has a higher efficiency than the MT3608 when using batteries? Also, I don't really need the adjustable output, and actually, that potentiometer looks like a cheap component which is not reliable at all. I just need a stable 5 V output.
Thank you for your help and answers!