How To Design With Solar Cells To Replace Button Battery - Page 1
Forum section: electronics / projects

author: EPAIII
author_rank: Super Contributor
timestamp: December 06, 2023, 01:17:11 am
content: I have one of the imported digital calipers, probably a no-name one, definitely made in China. It is installed on my milling machine as a vertical axis DRO (Digital Read Out). And it is a frequent offender in the dead battery department. There was some work and time involved in installing it there, so I would prefer not to exchange it for one with lower battery drain, as some have already suggested. Besides, finding one that has lower battery drain is a hit and miss proposition. What I want to do, what I intend to do is convert it to operation with a solar cell or two for power. But I have never designed with solar cells before now. And looking at some data sheets for them does not seem to be a big help.https://www.mouser.com/datasheet/2/1009/Electronic_Component_Spec_Sheet_Ind_DD7C8ED311483-2487468.pdfhttps://www.mouser.com/datasheet/2/1009/LL2002_917F03A83826B-2487571.pdfhttps://panasonic.net/electricworks/amorton/assets/pdf/Brochures_Amorton_E_2.pdfJust some sample data sheets, nothing that I have selected for an actual design.This caliper and virtually every digital caliper uses a 1.55 V, coin cell (357, SR44, etc). I have measured the current with battery power at 18 uA.What I need is a design approach for converting this battery powered caliper to one that uses a solar cell in an indoor setting. As a possible, inexpensive source of a small solar cell, I have purchased three calculators that use a combination of battery and solar power. They also use the 1.55 V coin cells. But when I removed the solar cell from one of them and measured it the Voltage was around 5V. And that single solar cell does not provide enough current to activate the display of the caliper. I measured the current it does provide and it was only 9 uA, which seems to be only half of what the caliper actually needs. And that is not a mistake or misprint: I actually measured only 9 uAs; less than 1/100 of a milli Amp.Looking at the available information on those semi-randomly selected solar cells (above) I see that the current that they supply remains fairly constant over a broad Voltage range before abruptly falling to zero above a certain point. That current seems to depend mostly on the level of light falling on the cell. But that is perhaps the maximum current available with the actual current being determined by the load. What is puzzling is the wide range the Voltage can take, from 0 to around 5 V, all at the approximate same current and the fact that current goes down as Voltage goes up so the graphs seem to indicate a negative resistance characteristic.I need to explore how an actual solar cell reacts to changes in both the light input and the load current. What actual Voltage can be expected under different circumstances? I am concerned with the Voltage level. If I power the caliper with a solar cell and no battery, will the Voltage rise above the 1.55 V level which the battery supplies? And then, would that harm the circuitry of the caliper, either short term or long term?Do I need something to act as a Voltage regulator, keeping it at that 1.55 V level or below some other safe level? Is that why we see products (calculators, calipers, and more) that have dual power instead of just solar cells for power - the solar cells alone would produce a Voltage that is too high and therefore harmful to the circuitry, perhaps only in the long term? Would some other Voltage regulating device be used? Any such device would require some quiescent or operating current and there is not much available here for that use.Or do I need a solar cell with a lower maximum output Voltage? Just 1.5 or 2 Volts?So does anyone have actual experience in designing solar power for low Voltage and very low current devices like a digital caliper? What is the approach taken to such a design?

author: uer166
author_rank: Super Contributor
timestamp: December 06, 2023, 02:46:28 am
content: Quote from: EPAIII on December 06, 2023, 01:17:11 amAt this power level, trying to use a solar cell is pretty insane. Just get a bigger or better battery. A single AAA cell here would last8 yearsand is much smaller than a solar cell that can deliver the power reliably indoors..

author: liaifat85
author_rank: Regular Contributor
timestamp: December 07, 2023, 05:55:39 pm
content: I agree with uer166. Still, if you wanna stick to solar panel, I think you'll have to do some trial and error to find out the proper combination of solar cells.

author: BrokenYugo
author_rank: Super Contributor
timestamp: December 07, 2023, 08:04:35 pm
content: Use a somewhat oversize panel, shunt regulate with some rectifier diodes in series to get ~1.5V. At this power level you could also easily do a similar shunt regulated supply from a 5V USB plug.Or do like uer166 suggests, cheap AA holder and done, maybe use one of those 1.8V lithium AAs? Battery would probably outlive the caliper.