Electronics > Power & Renewable Energy

Solar power for remote sensors


I set up an ESP8266 outside to report temperature/humidity about once a minute.  It is powered by a 12V 14AH SLA battery through an MP1584 buck converter module.

The battery is good for a couple of months before recharging.  But it would be more elegant to have a solar cell and charge controller recharging a battery and powering the ESP8266.  A different battery chemistry might be better suited to the temperature range, which can be -20F (-29C) in the winter to 100F (38C) in the summer.  The ESP8266 can draw 300mA at 5V when transmitting, but is only on for a few seconds every minute.

In concept, this is pretty much what those solar walkway lights do.  I have not taken any of those apart, but my impression is that most are junk.  There are solar "power bank" units, but they do not look to have weatherproof construction.

With enough continuous power, the ESP8266 could act as a charge controller.  But that will involve charging algorithms and development.

Adafruit has a small LiPo solar charge controller, but that seems like the wrong chemistry for cold weather.

Any suggestions for small solar charge/power modules and battery technology?

Other common battery technologies have similar or worse temperature limitations than SLA.

At its cheapest and dumbest, to keep a SLA happy, you need enough solar panel area (assuming the worst month's average insolation) to cope with the average load  + an extra 5% C/month for self-discharge (approx 10mA), and a temperature compensated linear regulator to maintain the correct temperature compensated float voltage for a SLA battery..

There are various ways to add on a simple charge controller.  Ujless the panel ig greatly over sized a controller will seldom ge beyond bulk charge mode.  fixed voltage charge mode should be sufficient for your application.  Consider harvesting more power by controlling input voltage to a buck and track power point voltage of panel with temperature.

If you insulate the battery well enough, you can use part of its capacity to keep itself warm overnight.

There is probably also a reversible chemical reaction that could do the same thing.


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