In various scientific calculators, we can see that they have "twin" power, basically a mixture of a solar cell and a battery. I would like someone to tell me what kind of circuit usually takes both power sources to power efficiently a small low power MCU for example.

I would like to replicate this kind of circuit. I suppose there are ICs for that, as that would make sense in order to lower the quiescent current to the maximum, but I'm not much knowledgeable in this area. Take my Sharp EL-546w (picture here for example. I can put two LR44 or SR44 button batteries inside, and they last a few years even with frequent usage. What I don't understand as well is that these batteries are technically not rechargeable (is it only a trickle charging solution found on these calculators then?).

I would really be interested in a solution that always takes the solar panel current first and somewhat "mixes" it with the battery current, while also making the voltage somewhat constant without the need to sink the excess current with a diode (as it draws power uselessly).

  • \$\begingroup\$ You don’t tell us what battery chemistry you plan to use, but assuming something rechargeable, please read this: electronics.stackexchange.com/questions/280949/… \$\endgroup\$ – winny Aug 31 '19 at 6:36
  • \$\begingroup\$ I was rooting for a CR2032 or a LIR2032, or two SR44.I plan on using a MCU like a STM32L5, around 1.7V is needed for those. \$\endgroup\$ – Yannick Aug 31 '19 at 20:55
  • \$\begingroup\$ You should make that clear in your question. If you have non-rechargeable batteries you need an additional diode. \$\endgroup\$ – winny Aug 31 '19 at 21:05

It is generally a diode OR circuit, a diode from the battery and a diode from the solar panel, when ever the solar panel has enough light, its voltage is higher than the batteries, so it stops it discharging and runs off only the panel, the earlier devices would also have a shunt regulator to keep the maximum solar panel voltage from getting too high,

A more modern approach would be something like a power ORing IC, it monitors the voltages of both sources and switches to whichever is higher, and some of them can be quite low power,

These calculators generally run on nano-amps, so the power lost through diodes is practically 0, as the voltage drop is much lower at these currents, you still get the same mAh capacity, just at a lower voltage that the device was already designed to run on, most IC's current does not change significantly with supply voltage.

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  • \$\begingroup\$ I suppose you mean two (diodes+source) in parallel? Isn't that a bit too simple? I plan using something somewhat more "advanced" than the ASICs these calculators have, thus drawing more power, like a low power ARM MCU such as the STM32L5 line. The thing with the diodes or the ORings is that the solar panel won't be used if it doesn't provide enough power/voltage. Except I was hoping for something that would always use whatever the solar panel can provide. Would a solution with an energy harvesting chip along a rechargeable coin cell LIR2032 be reasonable or overkill? \$\endgroup\$ – Yannick Aug 31 '19 at 21:00
  • \$\begingroup\$ This will in part come down to power budgeting. How much power can you extract from the solar panel e.g. indoors under office lighting. Vs how much power your device consumes. Those rechargeable coin cells have a much lower capacity than the non rechargables and a higher esr. Which means you get into having to plan serious supply capacitance if you need over 1mA. And at a point you will find a perfect ceramic capacitor of the kind if size you might need leaks over a uA constantly \$\endgroup\$ – Reroute Aug 31 '19 at 21:23

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