There are lots of charger circuits for LiPo but not for LiFePO4 and the ones for LiFePO4 allow only 6 volts maximum. The Seeed Studio panel for outdoors sounds excellent but can hit 10 volts. Does anyone have a circuit for charging LiFePO4 from voltages higher than 6 volts?
The power output is not big. The battery will probably be a 1600mAh 18650 size. I need reliability over peak power and cool running. Our outdoor temperature yesterday was 42 in the shade.
Hi Peter
For starters 0.5W @ 5V is only 100mA without allowing for conversion losses. That will not provide much in the way of charging anything I don’t think.
Don’t forget that what you use in 24 hrs has to be replaced during solar charging time, about 4 or 5 hours on a good day. Unless of course you are tracking the sun.
There are ways of keeping the temperature down without power. Involves an enclosure and an overhanging roof with an air gap between the enclosure and roof. Similar to a Fly on a tent at a camp site.
Cheers Bob
There are people using the Pico W with WiFi reports back to base using about 10 seconds then the device sleeps for a minute or ten. Low average usage. One report an hour could work. I can probably get away with a 600 mAh 14500 LiFePO4 battery. The only current problem blocking a working prototype is a charge circuit for the LiFePO4.
Hi Peter
You are right. There are plenty of mails powered chargers (and solar) to charge 12V high capacity batteries but not so for a single cell.
At a last resort you may have to change chemistries and drop the “Fe” bit.
Cheers Bob
Hey there @Peter18516,
May I ask why your project requires LiFePO4 batteries instead of LiPo?
If you’re getting 42C in the shade is the heat the issue?
Hello Jane,
Two reasons for LiFePO4. The current generation are supposed to recharge twice as many times. LiFePO4 do not burst into flames.
Some of my projects will be indoors and some will be among flammable material. A few weeks ago, 12 houses were lost in a fire near us. Last year, a friend lost part of his house to a professionally installed solar system bursting into flames when the installers first switched it on.
Any of the above would be good reasons for using LiFePO4 instead of LiPo.
As an experiment for use with mains power, I bought a ready made charge circuit that accepts a maximum 6 volts for use with a single cell. It can charge from USB 5 volt. It may need a heatsink. Reports vary. The chip inside is mentioned in this article.
https://www.hackster.io/sainisagar7294/lifepo4-battery-charger-with-protection-a04ddb
I may build my own to limit the max current to reduce heat while charging.
I started looking at circuits to produce 5 volts from 12 volt sources. The common ones require 7.5 volts minimum as input. One circuit uses transistors to get 5 volts from as little as 6 volts but I did not save the link as some of the detail and explanation was missing from the article.
The ideal would be a circuit from someone who has used it long term without heatsinks or fans.
Hi Peter
That would probably be the 78XX series linear regulators, In your case the LM7805. These require a typical minimum input of 2.5V above required output. They are good for 1A. Being linear in your case it would have to drop 7V and @ 1A this would be 7W which could het a bit hot at that current. TO220 package so heat sinking is fairly easy..
Something like this ??
This is a handy circuit to remember for when you want a quick and cheap (not nasty) regulator and you don’t have anything on hand.
And it works. Even cleans up most bumps and variations in the supply voltage. (as will the 78XX series)
The 317 is a handy one to have in the box as it is adjustable from 1.2V to about 35V with same min requirement as the 78XX. I think the modern variants of this 317 have a lower input voltage requirement.
Cheers Bob
Cheers Bob
With the above simple circuit a 5.6V Zener would be used for 5V output. This is a readily obtainable preferred value.
EDITED: 5.6V Zener was 5.1V Zener (brain snap)
While I am jumping in here… a little blind…
What is the max current, min/max voltage on the input side.
I was thinking for low current needs… e.g. upto 300mA a bit higher if needed, and LDO Voltage Regulator should work ? Or has this been discussed and ruled out ?
Hi Michael
I don’t think there has been any real discussion. The original request was for a charger to handle LiFePO4 cells which was proving to be a bit difficult.
The last post above indicated that Peter has found a charge that works from 5V (Max 6V).
The current quest I believe is how to easily get 5V from 12V. He found a couple of solutions looks like and my post above is an attempt to clear up exactly he found and suggest a little circuit he could lash up in the interim until a permanent solution is decided.
Cheers Bob
Hi Peter,
Can you design a custom PCB? there are many single cell multi-chemistry chargers available from OEMs like TI and the like.
Unfortunately it seems like Core doesnt have any dedicated breakout boards.
No. That is way beyond my skill range. If you wanted the perfect caffè latte, I might attempt that. I am still learning back at two resistors to divide a voltage.
Thank you for the explanation. A 6 volt max LiPoPO4 charging circuit is on the way and I will test charging my 18650 battery from there using USB 5 volt input. That will get me started working out the power requirement after I optimise the software.
The Seeed panel is also on the way and I can test output voltage in my location. Next week, I should have a better idea of the bit in between the panel and the 6 volt max charger circuit.
I might buy the ready made 7.5 volt minimum input circuit for testing and work on building a transistor based circuit as an alternative. I will then have the freedom to experiment with different panels. Along the way I might learn something, like how to make a transistor light up like an LED. 
Hi Peter
Another quick 5V regulator using commonly available bits.
By changing the value of the 390Ω resistor you can get any value you like between 1.2V and 37V. The input criteria is it has to be about 3V above the output.
Handy IC is the 317 when you wand a quick regulator, particularly for an odd voltage
Data sheet will recommend any caps required but depending on circumstances probably none are needed.
Cheers Bob
To charge a LiFePO4 battery from a 0.5 W Seeed Studio solar panel (55 × 70 mm), a dedicated LiFePO4 solar charging circuit is required. The panel typically provides around 5–6 V open-circuit with limited current, so the charger must handle low power efficiently. A common and reliable solution is to use a LiFePO4-specific charge controller such as the CN3065, MCP73123, or TP5000 (set to LiFePO4 mode). These ICs regulate the charging process using a CC/CV profile with a final charge voltage of 3.6–3.65 V, which is essential for LiFePO4 battery safety and longevity.
The solar panel output is connected to the charger’s input through a blocking diode or internal reverse-current protection to prevent battery discharge at night. A small input capacitor helps stabilize voltage under fluctuating sunlight. The charger output connects directly to the battery, with optional protection circuitry for over-discharge. This simple circuit ensures safe, efficient charging even under low-light conditions.
Hi Unikeyic
I think Peter was originally after a ready made plug and go device. this diverted a bit and finished up wanting to get 5V from a 12V supply. I am a bit lost as to exactly is required and as there is no diagram presented as to what the ultimate goal is it has become a guessing game.
I originally expressed a degree of nervousness about his solar panel size of 0.5W. That is not much of a charger. Especially when you consider he only has a few hours of good sunlight to replace what is used in 24 hours.
Cheers Bob
Hello Robert93820.I will test your circuit in about a week when I have the parts.
Hello Unikeyic311228,
I found a small ready made circuit to handle the LiFePO4 charging from up to 6 volts but the Seeed panel can put out up to 10 volts so I need to convert the higher voltage down to 6 volts or less.
My approach is now to test the LiFePO4 charger from a 5 volt USB source then find a circuit to convert higher voltages down to 5 volts. Converting a higher voltage lets me use other panels if the Seeed panel is not enough.
I am starting with the cheapest local panel as I will need a few devices. I will experiment with optimising the Pico power usage as I do not need the frequent measurements used in other projects. This is my first Pico experiment so I am ready to test options.
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