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Firstly, I'm new here so don't be too harsh if I get something wrong, please just inform me of the right thing to do. I'm a second-year engineering student in New Zealand.

I am using a DS3231 Real Time Clock module on a custom PCB with an ATMega328P to keep the time in a system that may be powered up a handful of times per year. Most of the time, when the microprocessor and board do not have power, the RTC will keep its time with a 3V 2032 coin cell battery. Other people state this battery lasts ~2 years, but I want mine to last much longer. I, therefore, want to charge the backup battery when the board is powered. I am thinking of using the TP4056 charger, but I want to charge the battery in place on the board. When the battery is being charged, the RTC is powered by the board. I have attached my thoughts on a schematic with a Schottky diode (D1) between the charger and the battery. Is this diode necessary or the best option? I have also attached the datasheets. 5V and 3.3V are both from regulated supplies powered by a 24V plug. This 5V is the same as that powers the ATMega.

TP4056 datasheet | DS3231 datasheet

schematic

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Matt Hadler is a new contributor to this site. Take care in asking for clarification, commenting, and answering. Check out our Code of Conduct.
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    \$\begingroup\$ No need to write thanks or sign your question. Show thanks by up-voting and accepting answers. Your signature is always included automatically. \$\endgroup\$ – JYelton Mar 26 at 7:04
  • \$\begingroup\$ is the cell rechargeable? ... how long will it last between charges? \$\endgroup\$ – jsotola Mar 26 at 9:02
  • \$\begingroup\$ There are both rechargeable and non of the 2032 size battery. The non-rechargeable tend to have about ~4 times the mAh as the rechargeable. The non-rechargeable should last about 2 years and the rechargeable about 5-6 months between charges. \$\endgroup\$ – Matt Hadler 2 days ago
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I read the datasheet again for the TP4056 charger and it states

No blocking diode is required due to the internal PMOSFET architecture and have prevent to negative Charge Current Circuit.

I think this means that I don't need the diode I placed at all, and the internals of the chip should keep everything in line.

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Matt Hadler is a new contributor to this site. Take care in asking for clarification, commenting, and answering. Check out our Code of Conduct.
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  • \$\begingroup\$ Hi, You don't seem certain of this answer. Either (a) You are writing it as an answer, because it really does answer your original question. In that case, please "accept" this answer (green "tick") to close the whole topic / question. Or (b) You are mistakenly writing this as an answer, when in fact you still want replies / comments about it. In that case, this is an update and not an answer, and must be "edited into" your original question i.e. click "edit" under the question, add this update (e.g. at the bottom of the question) then delete this "answer". Which applies here, (a) or (b)? \$\endgroup\$ – SamGibson yesterday
  • \$\begingroup\$ FYI, there are other problems / areas of concern on your schematic - I just haven't had time to think them all through (e.g. C9 can't be there; not sure what you meant). Hopefully someone with more time might get to it. But we can't discuss it here because this is an "answer". Please read the tour and help center for more information about how Stack Exchange sites are different from typical internet forums e.g. different places for answers and for questions, and why questions get updated inline with new information, rather than adding new information (wrongly) as "answers". Thanks. \$\endgroup\$ – SamGibson yesterday

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