@derchris, the SparkFun curve you posted isn’t related to the S.O.C curve I posted, at it’s discharging at 6C.
It’s not relevant to what you’re doing at all.
You shouldn’t allow your Li-Po to drop to 3.3V.
You are risking damage for no reason, reducing the expected life of the Li-Po, and it has very little available energy at that level.
I would be suprised that a 2G Electron could connect being powered only by a Li-Po @ 3.3V.
The 2G Electron demand while connecting could be higher than what’s chemically available in the Li-Po @ 3.3V.
Don’t get caught up with Critical Voltages listed in Li-Po technical specs. The Goal is to never operate anywhere near something named “Critical”.
IMHO, your CODE should start to reduce system demand (functionality) prior to reaching 20% SOC, and go to sleep if it hits 20% (see @RWB link/code). Or simply incorporate RWB’s code.
You know everything you need for a simplified Power Budget :
Watts = Amps * Volts
Idle Current ~57 mA (for 3G Electron) = 0.057 amps, not sure about 2G version.
24 hours in a day
Your Solar Panel Rating in Watts.
Assume 50% total efficiency for Solar Recharging (harvest Solar, get it into and back out of Li-Po, dirty panel, etc)
Assume the # of hours of Sunlight for your "Design Day"
Reserve 30% battery capacity, so your 15 Ah becomes 10.5 Ah for the Budget.
You check your Effective Solar Rating (watts) * # hours of expected Sunlight against your average system watt * 24 hours. If this isn’t acceptable, increase Solar Panel Size. For instance, you may decide you want 3 hours of sunlight to replace 24 hours of demand. Don’t forget that Effective Solar Watts = 1/2 of the Labeled Rating on the Panel.
Then check your 10.5 Ah Effective Battery Rating against your system Ah for 24 hours ( 0.057 amps * 24 hr example for Idle 3G only, use your total system’s real numbers) to determine how many cloudy days you can expect to operate without Solar Recharge. If this isn’t acceptable, increase battery size.
There are many other factors such as Li-Po operating temperature, Li-Po self discharge, etc, but this is a good starting point. Once you select the Solar Panel and battery size, test it in the real world to confirm your Power Budget Calculations. Use @RWB’s code for an additional safety factor…that part is free.