LiFePo4 Batteries for Omega2/+
Something I've been using for my ESP8266 ESP-12F projects lately (which I only mention because it's a common module and also runs off a native 3.3v) is LiFePo4 batteries.
They're the same size as AA batteries (so mounting holders are easy to come by) but they're about 3.2-3.3v typical output, comparatively cheap, and rechargeable! It works beautifully in the ESP8266's 3.0-3.6v input range, with no need for voltage regulators (details) so less lost energy.
My question is -- what is the acceptable voltage range for the Omega2/Omega2+? I know typical is 3.3v, but what's the acceptable margin and dropout voltage and such?
Note that a LiFePO4 cell when fully charged, its terminal voltage is 3.6V.
@ccs-hello Yup! It is, but very briefly, then it drops down and stabilizes.
"briefly" is irrelevant if it is outside the specs of components while it occurs.
You can check the MT7688 data sheet yourself, and decide if you want to take the risky edge of what you find.
Typically Lithium-powered systems using 3v3 circuitry require either a full buck-boost, or else deciding that the system can reliably operate at the lowest expected "still has usable energy" loaded cell voltage, potentially with a brown-out circuit to inhibit operations that would be risky if they fail.
@Chris-Stratton Aha! That's the datasheet I was looking for. According to that datasheet, section 4.1, maximum voltage is 3.63 (3.3 +/- 10%), so a peak at 3.6 is close but should be okay. Worst case I'll throw a voltage regulator on if I burn anything out in testing, but I don't want a buck due to possible interference.
There is already, inescapably, a buck regulator involved to generate the yet lower core voltage.
The issue with adding a regulator to your battery is that regulators have overhead - the output voltage will always be somewhat lower than the input - so it's hard to have one that merely limits the voltage when it is too high, but doesn't also reduce the voltage when it is already marginally too low.
Hence the trend to either using a buck/boost, or designing a system that can operate a regulator drop below the lowest expected loaded voltage of the cell.
re: LiFePO4 or Li-Ion rechargable battery technology
It's always advisable to add bettery charge/discharge protection circuitry, instead of just cell only.
With that in mind, the commonly seen Li-Ion 3.7-4.2v Li-Ion is more attractive, just add a buck (step-down) converter to drop the battery voltage to 3.3v. Most of the design works when cell drops in between 3.3 - 3.0v (output voltage = input.) When cell drops below 3.0v, protection circuit cuts out to protect the battery.
A TP4056 based charger/1 cell protection based combo serves as the first part of the design, then add a 1A or 2A step down to complete the design.
It's my understanding that some people (in the ESP8266 community) have simply put a LiFePo4 cell across 3.45 V as a sort of minimalist UPS scheme. The battery won't fully charge so there's no risk of going over voltage. There's also no protection against fully discharging the cell, but my understanding is that LiFePo4 chemistry is stable under deep discharge, whereas deep discharge of LiPo cells makes subsequently recharging them problematic.
Note that I've not done in depth investigation/circuit analysis on this topic and would recommend against trying this out on the strength of my current understanding.
Edit to add: Here's a link to one such discussion: stm32 forum. I'll have to give this a try (starting with a cheaper ESP-01) at some point.
Info seems to be inconsistent and probably due to different designs from various manufacturers.
Also LiFePO4 is so new thus no long term stability report.
One thing that seems to be true is it does not have the same problem as Li-Ion/Li-Po (Li-Cobalt) as it has gas swelling problem -- and dead, when fully depleted.
I've seen one factory recommendation:
do not deplete below 2V and never go below 1.5v (as has potential to shorten the number of recharge cycles/usable life.)