I am looking to make my Solar powered Electron (using a Voltaic 6V/6W panel) stay alive a bit longer on cloudy days, so I was looking at bigger batteries. Adafruit offers this 3.7V 4400mAh Lithium Ion Battery Pack with integrated protection circuitry.
The Electron will be used during Spring / Summer so there should be quite some daylight (and hopefully good sunlight) to help keep the battery charged. It will enter 15 minute sleep cycles after sunset to take some sensor readings, but it will be continuously active during day / sunlight hours.
I assume this battery works fine with the Electron. Does anybody have any experience with it? Does it require any software / PMIC tweaks?
I am not sure if your trying to find something that will plug in directly to the Electron, and have it charge the battery. Or you have you own onboard solar charger / battery setup which is then powering the Electron.
If the prior, just confirm the polarity is correct and your good to go nothing else required.
Just so you (and others) are aware, There is nothing special about the adafruit battery. It's a typical protected 18650 battery. You can make the same much cheaper doing a little online searching for these two products as a reference. #1) Protected 18650 battery. Needs to say "protected" because you can buy with protection or without so pay attention to that detail. Protected means there is a circuit board added to the battery under its shrink wrap to keep it from going to low of a voltage, and in some cases pulling to much current from it. Example of 18650 protected battery
For your reference, the Electron needs a JST-PH2 connector which judging by the image this battery appears to have, but there’s a lot of varieties of JST-connectors so make sure you get one that fits
You can indeed get much more affordable protected 18650 batteries 
which judging by the image this battery appears to have, but there’s a lot of varieties of JST-connectors so make sure you get one that fits