Hi all battery gurus. Hoping to get some insite on this problem. Scenerio is thus:
I have an ESP 8266 and a ultrasonic sensor configured as a tank level monitor. This is remoted and as such needs to be run on batterys. In order to save battery longevity, when the ESP boots , it takes the reading, then reads the internal voltage using the ADC, reports those to an mqtt broker, then goes to deep sleep mode, and awakes ~1 hour later to repeat. Within that process, I turn on and off power via a mosfet to the sensor so that it doesn't draw any power while the ESP is in deep sleep. I felt that using a 3.2v LiFepo4 cell similar to ifr14500 would provide the needed voltage with out the overhead of regulation etc. This yields a run time of about 3 weeks, before the battery discharges to the point where the ESP won't boot.
So I thought the easiest solution was to buy a readily available AA adapter, where 3 batteries can be placed in a holder the size of a "D" cell. To my suprise and dismay, this yielded a run time of about 3-4 days. Can someone explain what's happening here?
Thanks all.
It comes down to battery capacity. The Duracell AA battery can hold up to 2,600mAh of charge. This is one of the highest capacities for an AA battery, making it a great choice for high-drain devices. It would appear your LiFepo4 battery has a lot more capacity. Post a link to the batteries you are using. Also there is a discharge curve where the voltage gets lower as the battery is used.
Probably something like this
Are the batteries in series in your battery holder ?
Yes, those are similar, and they are in parallel. Series would obviously provide too much voltage. I should also mention that when recharging, they're charged individually, by removing them from the holder, charged then placed back in the holder.
That's the problem. Never put batteries in parallel
There are many instances of barriers in parallel in the world. For example, battery operated tools use a series-parallel configuration to derive the needed voltages (12,14,18,etc) with those strings paralleled to provide the needed current output. Can you give an example of why you think that shouldn't be?
If the battery impedances are not well matched.
Gotcha Jim, thanks. So it would appear to me there is no simple solution to this without getting into bms (more parasitic power consumption), essentially leaving me stuck with a one cell system
This should triple your run time.
Thanks Jim, So I take it that these are not the same as the 18650 lithium-ion batteries commonly used in laptop battery packs, with a slightly lower voltage?
I had considered using the lithium-ion 18650 but had some concerns regarding the higher (charged) voltage on the 3.3v ESP8266. When I saw the LiFepo4, with the 3.6v charge, I felt those to be safer, and steered toward those.
That is correct it's a LiFePO4 like the one you are already using (3.2V) except the mAh rating is much greater.
When I saw the LiFepo4, with the 3.6v charge, I felt those to be safer, and steered toward those.
And they can be recharged about twice as many times as a Li-ion
Truly appreciate your help Jim, 
Glad I could help.
Paralleling batteries is OK but they need to be matched. Using several in parallel when purchased in the same lot should work OK without testing. Look at what they do in electrical vehicles. Yes they have a sophisticated charging system. When recharging they need to be charged the same preferably fully charged. If one is lower then the other it will drain the higher one until they match. You can always add more batteries in parallel if you want. I would suggest a charger that would charge all of them concurrently.
Where can you buy two LiFePO4 matched impedance batteries?
Don't assume.
Connect the batteries in parallel and don't connect the load.
Measure the voltage every day for a week or so.
LiFePo4 could go down to 3.3volt, but should stay there for many months.
I just measured one that I haven't charged for more than a year. Still 3.3volt.
Connecting batteries in parallel should not be a problem, unless you have a dud one with high internal leakage. Then it of course also drags the other good one down.
Leo..
If a battery is defective and doesn't work in parallel, it will not work in series.
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