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Im not exactly from the EE field and I would happy for guidance. long question a head:

Im trying to build a circuit for keep running Raspberry Pi using solar panel + rechargeable battery.

I came across this nice article. on the bottom of it, there is section of Power Sharing which seems like good and working design. but the thing is that this design talking about two scenarios:

1. what happens when power source connected (both load and battery getting current)
2. what happens when power source disconnected (load getting current only from battery)

Now, I thought to use that design and add the solar panel so it will feed both the load and the battery.

On sunny day, we have no problem, both the load and the battery getting current: enter image description here

The issue is what happens on bit cloudy day. the solar panel could output much smaller voltage than 6v so it will be necessary to detach the solar panel from the P-Channel-Mosfet, so the load will get its current only from the battery.

If I keep the wiring like this:

enter image description here

The voltage could be not low enough for influence the Gate and feed the load only from battery + the DC-DC will not react good to 3v\2v\1v because it minimum voltage is ~6v.

So I understand that there is a need to disconnect the solar panel , so the gate will 'turn on' the mosfet and current will flow from Drain to Source and feed the load + disconnect the charge controller. so we have two options:

option 1:
enter image description here

option 2:
enter image description here

So, I guess my questions are:

  1. The main question- How can I disconnect the solar panel (option 1 \ option 2) when the voltage gets under threshold voltage (for example 4v)? there is a need with another power source of that threshold for comparison (less preferred option)? if there is a way implementing it without chips \ minimum waist of current from solar panel (I guess with mosfets) it will be very helpful.

  2. The values on this example are not set yet (solar panel + DC-DC conv + pull down resistor), Im looking for DC-DC that can get as input range of ~3v to >15v and output 5v but couldnt find one, minimum input is about 6v.. is there unit that can implement that? (the rest of the values will be set according to DC-DC input volatage)

Sorry for the long question :)

Thank you all!

user1673206
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  • Thank you for the help, it doesnt work so much for me because they talk about source that connected \ disconnected. my question is about how to disconnect one source (solar panel) when getting to low threshold. – user1673206 Aug 20 '20 at 08:43
  • Very close to what you are doing. Two diodes and let the solar panel win when there is sun? – winny Aug 20 '20 at 08:49
  • you right, its very close, but what would you connect to the second diode? I guess the threshold voltage.. I prefer to avoid connecting another power source, if there is a away implement it without another power source, it would be great.. – user1673206 Aug 20 '20 at 09:06
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    Take a look at the LTC4414 ic or similar. – Swedgin Aug 20 '20 at 09:52
  • The battery is your second power source. – winny Aug 20 '20 at 10:40
  • the battery is rechargeable, it means that it's voltage can change. if I set lets say 3v as reference (with resistor), it could (will) be changed.. – user1673206 Aug 20 '20 at 10:47
  • 1st you MUST define the problem. There are various useful answers on this site - once you know the task. || You show a 7.2V battery BUT seem to suggest it's voltage can be as low as 3V. No sensible battery chemistry qualiifies. 6 x NimH = 6S gives 7.2V nominal and 6V ~= flat and about 8.5 to 9V under charge at full charge. ||. 2 x LiIon cells = 2S gives 7.2V nominal, 8.4V fully charged and about 6 to 6.4V fully discharged. | Lead acid - none of these. | IF you can specify the battery chemistry and number of cells - and ANYTHING else of relevance, we have a better chance of helping uou. – Russell McMahon Aug 31 '20 at 10:09
  • Two diodes will probably do what you want. People (not me) will vote to close this question if you do not better define your task. – Russell McMahon Aug 31 '20 at 10:10

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