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I would like to run my esp32 on coin cell batteries, tho it needs at minimum 3v, that means if there just is too much resistance in the vires or anything else, stuff like Bluetooth will make it brownout,

So what is the best way to step up 3v to 3.5v?

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To be honest you are better off stepping down rather than stepping up. Simply because step down converters tend to be a higher efficiency and can deliver more current than a step up converter.

With step up converters you effectively convert current into voltage. With step down it's the other way around - voltage into current. Since button cells have very very little current capacity stepping up will reduce it even more and starve the ESP32 of power. By stepping down you increase the current.

Two 3V button cells in series will provide nominal 6V and about 40mA current. By adding a step down converter to 3.3V you are almost halving the voltage, but at the same time almost doubling the current. Adding a third cell in series will give you even more current output.

It is possible to get some incredibly tiny and high efficiency step-down modules. I myself make and sell one based around the TPS82130SILT which has the ability to run at as low as 3.7V with a 3.3V output (it has a "pass through" mode which allows a zero dropout voltage, but I include a protection diode with 0.4v drop). That means you get the entire 6V to 3.7V range out of your batteries before the voltage starts to droop (assuming the batteries still operate at 1.85V).

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Generally, to step up voltages you can use a boost converter. But I doubt that using coin cells to supply an esp32 is a good idea, because they probably won't last very long and are relatively expensive. Even if you use sleep modes, the voltage margin will be small (of course, that depends on the boost converter).

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  • well the esp will only be active 4-6 times a day, and only in a few seconds for sending information to a database, the rest of the time it will be in deep sleep – Marius Wanscher Oct 10 '19 at 17:56
  • it's going to run on 2 3v 260mah coin cell batteries, I calculated it to be about 5 months of run time – Marius Wanscher Oct 10 '19 at 17:56
  • i could be wrong tho – Marius Wanscher Oct 10 '19 at 17:57
  • Alright, I personally just don't like using coin cells and till now I always had enough space for a bigger LiPo. The converter will have some losses, but maybe that's fine... – Sim Son Oct 10 '19 at 18:00
  • well i want the final to be as slim as possible, since there already needs to be place for pcb and chip, 2 coin cell batteries on the back of the pcb would be perfect – Marius Wanscher Oct 10 '19 at 18:07
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Why not use three button cells and a buck converter to lower the voltage from 4.5V down to 3V?

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  • haven't read on buck converters, is it just a kind of step down? – Marius Wanscher Oct 10 '19 at 18:33
  • + I want to only use 2 coin cell batteries, for the size, it wouldn't be the biggest disaster to make it a bit bigger to fit 1 more battery, but the buck converters I found online looked pretty big, Too big – Marius Wanscher Oct 10 '19 at 18:34
  • @MariusWanscher in general, both buck and boost converters consist of the same components and they both exist in very small dimensions – Sim Son Oct 10 '19 at 19:00
  • @MariusWanscher I make a really really really tiny buck converter. You can't get smaller. It will run from 2x 3V cells in series (or 3x if you can) and down to 3.3V on the input for 3.3V output. ebay.co.uk/itm/183933078163 - however I can't guarantee that the ESP8266 will like the low current output of button cells. – Majenko Oct 10 '19 at 19:22

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