Reducing the LiIon charge termination voltage

[Stereodude]

Vusb is 5.193V with the Rpi turned off and just the MightyHat on.  The voltage seems to rise slightly with it turned on to 5.205V.

Also, the LCD seems permanently garbled now. 

Edit:  The LCD went back to normal after cycling through the backlight brightness settings.  It previously stayed garbled through power cycles, resets, etc. 

[Felix]

Vusb is 5.193V with the Rpi turned off and just the MightyHat on.  The voltage seems to rise slightly with it turned on to 4.205V.

Also, the LCD seems permanently garbled now. 

Edit:  The LCD went back to normal after cycling through the backlight brightness settings.  It previously stayed garbled through power cycles, resets, etc. 

You might be right on the edge. If the schottky drops 0.2V and the PMOs1 another 0.8V through the body diode, then we're around 4.2 which can explain the slightly higher 4.205 in your battery.
The solutions were already mentioned, but for your trouble, I can offer to fix it for you if you want to return it.

[Stereodude]

You might be right on the edge. If the schottky drops 0.2V and the PMOs1 another 0.8V through the body diode, then we're around 4.2 which can explain the slightly higher 4.205 in your battery.
The solutions were already mentioned, but for your trouble, I can offer to fix it for you if you want to return it.

I had a typo there that I've fixed.  However, I will see how the MightyHat behaves with a 5V different power supply (closer to 5V) before I do anything else.

[Felix]

Ok no problem. Let me know..

[Stereodude]

Ok no problem. Let me know..

After connecting a more tightly regulated 5V supply connected to the barrel jack of the MightHat the behavior of the board changes noticeably.  The wall wart I'm using measures 5.06V open circuit / 5.03V when connected to the MightyHat with the RPi3 turned off / 4.89V with the RPi3 on.  The open circuit voltage of the charging circuitry with the RPi3 off is only 4.13V.  It drops even lower and bounces around with the RPi3 turned on.  It settles around 4.03-4.04V.

What is the Red Charging LED actually supposed to do?  With no battery connected it's now off (with the RPi3 off) instead of flickering like it did before.  However, once I turn the RPi3 on, the charge LED turns back on (with no battery connected).

I will charge a Li-Ion cell and report the results back later.

[Uncle Buzz]

What is the Red Charging LED actually supposed to do?  With no battery connected it's now off (with the RPi3 off) instead of flickering like it did before.  However, once I turn the RPi3 on, the charge LED turns back on (with no battery connected).

While your RPI is off, the MCP73831 "see" a "battery" voltage through the body diode of PMOS1 at about 4.13V which is almost the value of a full charged battery and there is no current flowing into the battery, since there is no battery and PMOS1 is off. So the charging led is off, but when you powered your RPI, your USB voltage drop and the voltage seen by the MCP73831 follow, so it could try to charge this lower battery voltage and  lights the charging led

[Stereodude]

I will charge a Li-Ion cell and report the results back later.

So, with the RPi3 off the battery charges to about 4.13V.  With the RPi3 on the battery charges to 4.205V (the protection cell is not limiting the voltage as this battery will charge to 4.25V before the protection circuit opens).

Edit:  Removed incorrect conclusion

[Stereodude]

It's what I meant when I said it wouldn't be the case with common values, but is a lower body diode voltage possible on this board in particular ? higher VUSB ? It's just a possible scenario you didn't check even if it's unlikely.

0.8V (minimum body diode voltage) + 0.3V (forward shotky diode voltage, don't know the real value of yours) + 4.2V = 5.3V, so any VUSB > 5.3V will flow current into a full charged battery through PMOS1 (with those values)

But it could be less because of the forward shotky diode voltage which is not a constant value but depends of the current flowing through it, and with some mA or even µA, this value could be lower (but needs lot of time to overcharge the battery)

EDIT : for the low current through the shotky, it's possible only if you can shutdown or sleep anything powered by the mightyHat, but maybe on the same way, the body diode voltage of the PMOS could be lower with very low current ?

So after thinking about this more...  Is the already charged battery the worse case scenario with this circuit design?

What happens after a power failure when the battery is depleted (from powering the Rpi), the Atmel chip has turned off the RPi (due to battery depletion), and then power is restored?  You'd now have a battery at something like 3.0V and 5V on the input.  Since there's a voltage differential dependent current path from the input to the battery around the MCP73831 couldn't there be quite the current surge into the discharged battery?