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Topic: Advice needed: 3.2v LiFePo4 solar charger circuit. (Read 507 times) previous topic - next topic


Please excuse me if I have inappropriately categorized my post.

I'm in the process of searching for information as self-titled. I am working on a project that features a pair of solar charged lithium phosphate batteries, specifically, 850mAh 3.2v LiFePo, two cells connected in parallel. I started out the project with a solar powered garden light, but at this point, I would like to move away from the OEM circuit board that came with the light. I'll be using these batteries, in tandem of course with a solar charging circuit, and a voltage step-up circuit to power a very very generic ESP8266-12E. The purpose is data collection. I'll be taking measurements of soil moisture, temperature and humidity periodically and recording this data with timestamp to a CSV file that will be stored in SPIFFS and later offloaded to a micro-sd card. I'll also be displaying graphs on a web interface hosted by the ESP.

So back to the point, the issue I'm running into now is finding an acceptable circuit or schematic that is purpose-designed to charge this type of battery using a PV. I know from testing that in direct sunlight, the PV that came with this light outputs ~3.8 volts.

As for the voltage step-up circuit, I imagine I could use one of a number of off-the-shelf solutions that will give me 5v and around 300-500mA. I'm also looking at several very similar schematics that will do this. so really, my only stumbling block currently is the charging circuit. I'll be attempting to logically deconstruct the solar light circuit to a functional schematic, but I have a feeling this schematic or one similar has already been published and I'm just using the wrong syntax when asking Google for help finding it.

If anybody has an idea, recommendation or suggestion for this dilemma, please share your insights here. I would greatly appreciate it and it's bound to be useful to some other folks as well.

Please let me know if I have left out any critical details.

Thank you.


You want a PV panel with about 6V open circuit voltage, not 3.8 - otherwise it just won't charge.

The voltage overhead is needed to allow charging in overcast conditions when the voltage will drop

full sun: 6V,
likely voltage overcast 4.5V
loss due to schottky reverse blocking diode = -0.4V
max charging voltage LiFePO4 ~ 3.9V
(so even 6V is a little low, as the regulator circuit needs a bit of headroom too)

Any charging circuit has to avoid overvoltage and over current, note.  The schottky is required
to prevent the battery discharging heavily through the PV panel in darkness.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]


Hi MarkT,

Thank you very much for the advise. I had purchased what I did assuming it used one or more 3.7v LiPo cells as they were being sold in four packs right next to the solar light that I purchased, only to open it up and home and find something that I have never worked with, and the more research I'm doing, I'm finding that these lithium iron phosphate cells are an entirely different thing all together.

Can I posit an alternate "what if" to your suggestion of a PV with higher output though? I will look around and see what PVs I can find with a higher output, but I was hoping not to have to modify my garden light too much from what I bought and adding a larger solar panel would probably require that. What if instead, I built a voltage step up circuit for battery charging using the PV I currently have? I suspect that the PV won't provide enough current to make that work, but I have the components on hand to fabri-cobble something together to that effect. Do you think it would work to step the 3.8v up to 6v or so and then feed that into the charging circuit?

The whole solar battery thing is pretty new to me and so is this type of battery, so I'm not to the point of ruling out scrapping the whole kit and going with a solar/LiPo arrangement, but it would be a shame to have to start over now.

Thanks again for your response!

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