I need to create a interruption-proof power for a project of mine, very similar to the design of this link.

My problem is that I need to adjust the current delivered of the solar panel (21V, 1.2A) to 5V and simultaneously charge a battery 12V, 18Ah. If the solar panel does not supply power, the battery takes its place.

Does anyone know any circuit to do something? Or can someone give me a light as do something?


Rough block diagrams below show two ways this can be achieved - "Easiest" costs least but is inefficient. "Better" uses PV panel output more efficiently. Details of either can be discussed if of interest.

'Schottky diodes' should be used to minimise losses.

(1) "Easiest"

Connect PV panel to battery with diode.
Panel is clamped to battery voltage (+ diode drop).
This will not hurt the PV panel but wastes some of the available power.
[What is the formal PV panel specification. Is it 21V O/C or loaded or .... Do you have a datasheet link for it?].

Use 12V (nominal) to 5V converter to power load. This could be

  • A linear regulator - only about 40% efficient - very wasteful.

  • A buck converter (build or buy from eg ebay)

enter image description here

(2) "Better"

Use a buck regulator to charge the battery - ideally an MPPT (maximum power point transfer) controller. Details can be discussed if you are interested.

Use a second buck regulator to power the load. This must be capable of (at least) 11V to 22V input. Power this from a Schottky diode from the PV panel AND a Schottky diode from the battery. When the panel voltage is > battery voltage the panel will power the load.

enter image description here

  • As a supplement, there are pre-made buck regulators specifically designed for all of your requirements; i.e. micro-power (solar/wind/vib), charge the battery when it's able, use the battery at night, etc. An example MPPT (for a single-cell lipo) sparkfun.com/products/12885
    – Jon
    Feb 12 '15 at 0:02
  • @Jon The LT3652 that Sparkfun use is quite a good match to this application. It is able to be adapted to a 12V SLA battery by a resistor change and Vin of 32V is high enough. | The MPPT method used is proxy one - they hold V_PV at some fraction of V_PV_OC BUT they claim to get within 2% o true MPPT. (I've meant to try these for some while but not yet done so). Feb 12 '15 at 9:58
  • I wish Sparkfun would give me free samples like Linear does :)
    – Jon
    Feb 12 '15 at 10:24

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