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Power ESP32 with batteries - Voltage Converter

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Nikos A.:
Hi everyone, I'm designing a project where I intend to power an ESP32 with 18650 batteries ( two batteries in parallel). This is the datasheet

https://cdn.shopify.com/s/files/1/0674/3651/files/lg-hg2-spec-sheet.pdf

From the datasheet, I can see that the batterie if fully charged at 4.2V and the cut off voltage is 2.5V. I need 3.3V for the ESP32. Firstly, I was considering an LDO because of low quiescent current and dropout voltage. This looks fine to me

https://eu.mouser.com/datasheet/2/268/MCP1700-Low-Quiescent-Current-LDO-20001826E-737536.pdf

For 3.3V out the input voltage according to the datasheet should be VIN >= (VOUT + 3.0%) + VDROPOUT which is around (3.3V +3%)+ 110mV = 3.5V

The 110mv dropout is at 150mA load (FIGURE 2-12 of the datasheet).

My problem is 3.5V is quite high, and I would like to drain the battery at least 2.8 - 3V. But for such low voltage the output voltage of LDO is going to be under 3V and the ESP32 is going to malfunction

Next, I considered a buck/boost converter like this one below, but the quiescent current is much higher.

https://www.pololu.com/product/2122/specs

Do you have any suggestions?


Peabody:
Based on my memory of the discharge curve for an 18650, at 3.5V the battery is about 80% discharged, and voltage drops rapidly after that.  It's possible the buck/boost converter would give you a shorter battery life than an LDO.  I think that's particularly likely if your ESP32 spends any significant time sleeping (switching regulators are typically less efficient than LDOs at very low currents).

But you may find that you need an LDO that will provide more current than the MCP1700.  Nominally, 250ma should be plenty, but wifi devices can generate large current spikes when they transmit, and more headroom in the LDO helps deal with those.  Also, there's a Youtube video by Andreas Spiess on powering the ESP8266, and he ends up recommending a large (1000µF) electrolytic cap on the 3.3V line to prevent voltage drops caused by the spikes.

Hopefully those with lots of experience with the ESP32 can give you some good advice on this.

Kasper:
LDOs generally have lower quiescent current than switchers but you might find a buck that has better quiesc than the buck/boosts.

If you can find a buck with low quiesc and low drop out then youre set.  Else maybe a buck with low quiesc and put the batteries in series.

Or have a button or other MCU switch power to the ESP and its regulator.

jbb:
As Peabody said, the LiIon will be mostly empty by the time you get below 3.5V.  I wouldn't recommend drawing them down to 2.5V.

I thoroughly recommend a buck converter with low quiescent current.  You'll need to hunt through some data sheets.  It's a common feature nowadays, but you have to be careful because many data sheets show off the quiescent current when the buck is disabled.

However, the cells have some resistance.  If you apply heavy (Amps) current loads, the cell terminal voltages will dip.  What other loads will you have on the battery?

I looked at the ESP32 data sheet.  It says the ESP32 works over a range of 2.3 - 3.6V (absolute maximum) and uses LDO regulators inside.  If the other parts in your circuit allow, you could drive it with a 2.5V or 2.7V buck regulator and should get the same performance.  This will save (3.3-2.7)/(3.3) = approx. 18% battery power (more or less, depends on buck regulator efficiency curves).

mariush:
Just use a switching regulator. For example

diodes-incorporated/AP65111
https://www.digikey.com/product-detail/en/diodes-incorporated/AP65111AWU-7/AP65111AWU-7DICT-ND/7652826
Datasheet: https://www.diodes.com/assets/Datasheets/AP65111A.pdf
See example circuit in datasheet, very easy to reproduce.

diodes-incorporated/PAM2305
https://www.digikey.com/product-detail/en/diodes-incorporated/PAM2305AABADJ/PAM2305AABADJDICT-ND/4505341
datasheet: https://www.diodes.com/assets/Datasheets/PAM2305.pdf

on-semiconductor/NCP1521
https://www.digikey.com/product-detail/en/on-semiconductor/NCP1521BSNT1G/NCP1521BSNT1GOSCT-ND/1560581
datasheet: https://www.onsemi.com/pub/Collateral/NCP1521B-D.PDF

If ESP32 works at 3.0v, I'd set the output voltage of the regulator to that... when the battery is at 3v, it's already pretty much depleted so it doesn't matter if input is too low.

With LDO you'd waste a lot of energy, ex 3.6v in , 3.0v out ... ~83% ... the regulators above can be >95% efficient.

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