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Solar Alkaline Battery Charger
By: Dave Johnson

I received an email the other day from a guy who was tired of replacing alkaline batteries every 2 months in his wireless weather station.  He wanted some way to draw energy from the sun, so he could keep the station running for a much longer time before the batteries had to be replaced. 

I wonder if he could extend the operating time to years by pumping a small amount of current into the two AAA 1.5v alkaline cells inside the weather station during the day.  The idea is to put a bit more average current into the battery than is taken out.  In theory, this should keep the battery capacity at some constant level, extending its life to many years.  But, would this idea work?
Weather Station   Alkaline AAA Cells Alkaline AAA Cells
If you divide a 1 Amp-hour AAA alkaline cell by 60 days, then divide that by 24 hours, you get a figure of 0.7 millamps. This would be the average current drawn from the two cells by the weather station circuit.  This is a pretty small amount of current but it is high enough to deplete the AAA cells in just 60 days.  Increasing the battery size to a pair of AA cells might increase the time to 4 months.  Going all the way to two big D cells might result in an operating time of 2 years.  That still may not be long enough for a guy who has to climb up on a roof to replace those batteries.
How about NiMH
rechargeable cells, charged by a solar panel?  Yes, this might work but since the weather station wants to see 3 volts from the battery pack and two NiMH
cells would only deliver 2.4 volts, three cells would need to be used, producing 3.6v.  That is certainly possible. But, could the weather station tolerate a voltage of 4 volts during battery charging cycles? Maybe!  I just donít know.  I suppose a good 3v voltage regulator and three NiMH
cells could be made to work.
Could a super capacitor work?  Letís see.  Letís assume that the weather station needs to operate for two full days under dark conditions, drawing power only from the capacitor.48 hours times 3600 seconds per hour is 172,800 seconds.  If two 2.5v super capacitors were wired in series they could be charged up to 5v.  By using a very good
3v voltage regulator, the capacitor could be discharged down to maybe 2.7v before the weather station would no longer function properly.  This leaves a voltage change of about 2 volts. The equation for the rate of voltage drop in a capacitor with a fixed current is dV/dt = I/C where dV/dt is the voltage change with respect to time, I is the current in Amps and C is the capacitance in farads.  If we plug in the 2 volts change into the equation and 172,800 seconds, with a current draw of 0.0007 Amps, we get a figure of 60 farads.  Since two capacitors would have to be wired in series, each would have to have a value of at least 120 farads.  Although such capacitors do exist, they would be bulky and expensive.  The charging circuit would also have to have some kind of balancing circuit across each cap to insure the voltage was not exceeded.  The real advantage of this approach would be a long 10 year life or more.

I still like the trickle charge option.  I would like to see someone test the concept of using the existing alkaline AAA cells installed in the weather station and use a small solar panel to push a bit of current into the cells.  Could this allow the battery to last 5 years?  I think it would be worth a try.

There is not much data on trickle charging alkaline batteries.  Some commercial alkaline battery chargers exist but they dump a lot more current into the cells than needed in this application.  Battery manufactures say that their batteries could be explode if charged but again, by limiting the charge current to just a few milliamps, I donít see that happening.

I ran a quick test on a fresh alkaline cell.  The open circuit measured 1.5v.  With 3ma of current flowing into the device for several hours, the voltage across the cell during charging increased to 1.65 volts.  With two cells under charge, the voltage would be 3.3v.  I think the weather station would be able to handle that voltage just fine. 

140 Farad Super Capacitors

A typical day has about 6 hours of useable sunlight.  If the station needs 0.7 milliamps for 24 hours, then that means about 17 milliamp hours would be drawn from the battery each day.  To make up for that battery discharge, the solar panel would need to pump about 3 milliamps into the battery and to the weather station load during the day.  In theory, this current would average out to a slightly positive value, going into the battery every day.

A 9 or 12 cell solar panel should work well, if the current from the solar panel to the battery was limited to 3 milliamps.  I have drawn up a simple current regulator which will work with solar panels up to 18v or so.
4.5v Solar Panel  5.0v Solar Panel
Even cheap solar cells have an efficiency of about 10%.  During a bright sunny day about 0.7 watts of solar power strikes every square inch of a solar cell.  So, with an efficiency of 10% about 0.07 watts of power should be available for each square inch.  Each solar cell produces a voltage of about 0.5v so with 0.07 watts, each square inch of solar cell area should be able to crank out 140ma of current.  This is a lot more current than needed for this application.  In fact, if the goal is a current of only 3ma, with a voltage of about 4.5v, the solar panel area would only need to be about 0.5 inches by 0.5 inches.  But, such solar panels are rare.  To produce a voltage of about 4.5v to charge the 3v battery, 9 solar cells in series would be needed.  A 5v panel would require 10 solar cells.   The circuit below will certainly work with panels with a higher voltage if that is all you have.  If someone tries this trickle charge method, let me know how it works out.

3ma Current Limiting Circuit

Please send comments to me

May 2011     Issue 17

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