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).
Best Answer
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.