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By Rural
#34571 Any folk out there who have found a solar charge controller that, combined with a LiPo battery, is appropriate for ESP8266 sensor projects? Or could avoid the solar charge controller by using a battery with built-in over/under charge protection and a buck converter to protect the ESP8266? A third alternative would be just using a battery charger as if it were a solar charge controller. Does that seem insane?

To clarify, I've got some projects that consume too much power to run for a sufficient period of time without a prohibitively large battery, but a modest solar panel to charge the battery during the day is perfect. If I could count on the battery's over/under protection, this would end up being a really simple, cheap, and compact project.
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By Rural
#34576 So my third option, using a battery charging module as if it were a solar charge controller, is doable. This Instructable does exactly that as part of a solar powered weather station. The battery charge controllers are easy to find and inexpensive. A complicating factor is that they take 4.5-5.5V input, so one has to choose appropriate solar panels or buck/boost the solar panel's output to 5V for the battery charge module. The battery charge controller assumes a single Lithium ion cell, meaning that one has to choose an appropriately sized battery.

A buck module will be required to bring the batteries 3.7v or more down to 3.3 for the ESP8266.

I should add that my option two, counting on the over/under charge protection in the battery is troublesome. The problem is that the batteries need to be charged within a certain Amperage range. I have my doubts that the over/under charge protection in the battery protects for charging outside that rate. If a solar panel sized for your application happens to charge the batteries at the right rate too, great! Otherwise, you need a battery charging module to control for that.
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By Barnabybear
#34582 Hi, I've looked at Li-x batteries with regard to solar charging & it's a bit of a minefield. These batteries don’t easily lend themselves to charging from solar (where the full charge my not be completed, not enough sunlight) or when they are being discharged during charging.
I came to the conclusion that if space isn’t an issue, use 8 NiCd & a 12V solar panel.
12Vsolar ->
buck stepdown (buck / boost to max potential)->
charge regulator ->
NiCd’s ->
Discharge limiter ->
Buck stepdown ->
ESP module.
This doesn’t benefit from the latest technology, but should work. As we are looking at 500 charges from a Li-x battery if you nurse it, as opposed to 1000 from NiCd which are far more tolerant to low or incomplete charge cycles.
The other complication is that Li-x charge at about 4.2V, which is above the recommended supply for an ESP. Whilst an ESP can be powered (not recommended) from 4.2V if you do the current consumption increases & defeats the point. That means you will need a voltage regulator, unfortunately most have a +1V dropout, which means that you start voltage is 3.2V (4.2 - 1.0 + 3.2V) & will drop from there, as it’s reported that the ESP will operate with voltages as low as 2.7V this sort of works.
I have a project running at the moment & rather than using a voltage regulator I’ve used a 0.7V drop diode in series with the supply, this delivers 4.2 – 0.7 = 3.3V -> 3.7 – 0.7 = 3.0V, which is about the bottom limit for Li-x under light loads (yes 3.0v if your discharging at C10, but we are not) and if you sleep this has implications.
Anyway just my thoughts, Li-x great for aircraft and cars if you want 15 minutes of max power and 2 to 4 hours of controlled charging. NiCd use and abuse then bin and replace.
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By Rural
#34721 Good point about the NiCd battery option. They would yield at least four years of use in a solar-charged system, which would be really nice for at least one of my intended applications. I'll look deeper into it, but expect that I will apply it.