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I may be running a microcontroller for an extended period of time in a portable manner under battery, but do not want its life to be reduced by a silly load such as an LCD screen (5v + backlight) which may easily be drawn from my solar cell.

It is not enough to run the Microcontroller unfortunately (nor would I trust it, if it is intermittent) however wonder in what scenarios I can use the solar cell's power instead.

The LCD example is simple, I can regulate it and power it, only the data pins touching the Microcontroller, however there may be other components. Can I pair together both battery and that @ regulated 5V? Would it prefer one over the other since the solar cell may be closer if connected near or in between?

It would just be nice to get my money out of the solar cell.

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  • \$\begingroup\$ What kind of batteries? (rechargeable? chemistry?) How much power and current does your microcontroller draw? What is the output voltage of your solar cell? \$\endgroup\$ – W5VO Oct 21 '11 at 5:17
  • \$\begingroup\$ @W5VO Probably an alkaline 9V (Duracell), my Microcontroller uses possibly below 50-100mA (ATMega board) with most pins in use, which makes it not much of a candidate for the solar cell. The solar cell and uC both run/output 5V actually. \$\endgroup\$ – Hobbyist Oct 21 '11 at 5:24
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Where on earth will this be used?

A 100 mA x 5V PV (= photovoltaic = solar) cell is not a small item compared to many items of portable equipment.

A modern highish efficiency crystalline PV panel will give you about 10 mW/cm^2 actually delivered in full sun.

So for 5V x 100 mA = 500 mW output you will need about 50 cm^2.

You can connect this via a diode directly across a suitable battery, or via a regulator to a load of your choice. If you connect directly to a battery then you may wish to provide overvoltage regulation. This is easy and cheap and can be discussed if your application sounds like that approach may be useful.

A 9V Alkaline "PP3"size (transistor radio type) battery is NOT intended to be recharged. You could arrange the panel so that it sup[plies load current when it is able to do so, but does not do so, or you could arrange it so it can power the equipment directly when illuminated adequately and the load is active BUT can also charge the battery. Alkaline batteries are NOT intended by their manufacturers to be recharged in most cases BUT a degree of recharging is in fact possible. Charging efficiency is limited, lifetime cycles are poor compared to eg NimH, and overcharge control would be needed. This is a nonstandard application so you would need to be aware of possible problems.

Use of a small rechargeable battery in combination with the main battery could be useful. Charge control is easy and cheap.

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  • \$\begingroup\$ Will I be able to place this before the regulator, with a diode protecting the battery before it, so that it will take precedence over the battery when in sunlight? Will the voltage source that is "closer" be used instead? I may end up using it as a recharging source, possibly turning off the battery if there is sunlight, if my circuit will allow the behaviour. \$\endgroup\$ – Hobbyist Oct 24 '11 at 9:43
  • \$\begingroup\$ Yes, you can place the panel with diode before the regulator in parallel with the battery. If the battery does not have a diode of its own this will charge the battery when the load draws less than the PV panel output. As above, the PV panel may want to be clamped to stop overcharging the battery. | If Vpv > Vbat it will take precedence. If Ipv is < Iload needed then PV will provide what it can and the battery will provide the rest. \$\endgroup\$ – Russell McMahon Oct 24 '11 at 13:28

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