I have my Raspberry Pi3B running from a Lithium Polymer battery which is regulated to 5.25v with an MP2636 Boost / charge module.
After a lot of fiddling and testing, I've managed to get this system stable all the way to ~75% CPU usage with no throttling no matter if I power via the battery or the vin which is fed from an official Raspberry Pi power supply.
Where I do have issues, is when plugging in an external power source to charge the battery, at the same time as using the Pi (or disconnecting it)
For a moment, the undervoltage indicator pops up then very shortly after, disappears.
To fix this, I've considered adding a capacitor that could pick up the slack in the split second it takes the boost circuit on the MP2636 to kick in / switch back to passthrough.
Would this help at all? And if so, would I put it in series with the 5v out from the MP2636, or bridge it across the 5v out and 0v?
I don't really understand capacitors well enough to work this out on my own.
Schematics for reference:
