Some thoughts on solar charging…
I recently talked to a salesperson in the solar industry regarding another project, and inquired about MPPT chargers for a single panel mobile device. I was told it was pointless for single panel, as MPPT requires much higher voltage (i.e. serial connected panels) to reap efficiency benefits over regular PWM chargers. (As far as I knew, it is the amps that change with the light, not the voltage, so I was a bit dubious). In any case that was out of the question for this particular project.
I would prefer to have any panels connected in paralell, as shading a single cell in a serial connected array drops the total power output by 75%. It is also wise to have a diode for each panel. Yes it lowers efficiency, but the panels tend to suck power from the battery at night if directly connected (at least the cheap, unregulated ones do). I don’t know if the TTgo charging circuit takes this into account.
If your panel has any circuitry for the USB output this may not apply, but for cheap unregulated panels it does, most definetly.
(I was on a hike and plugged in my phone at 50%, it went up to 60% before dark, but when I woke up the phone was completely drained.)
I see sugestions on using solar gardenlights as a possible powersource. An interesting and apealing idea, but… these devices charge all day, and discharge at night. We still want to power the device during day, which introduces a paracitic load on the charge circuit. depending on the size of the paracitic load vs charge load, this may lead to incorrect charging and premature battery failure.
The simplest way I can think of to avoid all that math, is to use 2 batteries and charge 1 while the other one runs the load, then switch at a predetermined discharge voltage. That keeps the charge circuit isolated from the load at all times.
If the TTgo charge circuit+a somewhat intelligent panel (a regulated, possibly of the type used to run wireless security cameras) is paired I think that would be the best solution, unless there are issues with TTgo’s charger I don’t know about. Anyone?