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Solar charging module for LiFePO4 batteries

Discussion in 'Power Electronics' started by Ian, Sep 21, 2017.

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  1. Ian

    Ian Administrator

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    I've recently started tinkering with ESP8266 boards, and one of the projects I'm going to build will involve having the board sitting outside for years recording data (this pond water level meter).

    It would be ideal if I could use a LiFePO4 battery, which is trickle charged via a small solar panel throughout the day. I'll then tune the sleep cycles of the ESP8266 so that it should work even when there is minimal sunlight throughout winter.

    Is anyone aware of any modules that will allow me to simultaneously charge (using a solar panel) and use a LiFePO4 battery? It looks like I could create my own using a MCP73123, but I'd really like to use an off the shelf module for prototyping - there are plenty of modules for this on AliExpress for normal Li-ion batteries, but none that I can find for LiFePO4 batteries.
     
  2. Ian

    Ian Administrator

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  3. kellys_eye

    kellys_eye

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    Why?

    Apart from the 'exotic' ness of such, what advantages does it offer over the ubiquitous 18650? which has a plethora of charging solutions.

    Have you forgotten the KISS principal or is there a particular reason for the LiFePO4 route?
     
  4. Terry01

    Terry01

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    Hi Ian. One of the wheelchair forums I use is the place to find out about lithium. The guys there are lithium daft! They know literally everything there is to know about it. Some clever guys! Wheelchairdriver.com is the name of the site. Have fun! :)
     
  5. Ian

    Ian Administrator

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    Good question! The simple answer is that I wanted to try it out as part of the learning process, as I've not used a LiFePO4 battery before. There are some advantages which would be a nice bonus (no need for a step-down/regulator for the ESP8266, plus it'll last longer for a project where it's constantly charged/discharged).

    The more I look in to it, the more I think I should perhaps stick to a 18650 and use a solar charging board like this one:

    https://www.aliexpress.com/item/MCP...ymer-Charger-Board-3-7V-4-2V/32779475903.html

    It would certainly make things easier.

    Cheers Terry, I'll check it out now :).
     
  6. Externet

    Externet

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    Why using a module ? What is the voltage generated by the 'small' panel ?
    If between 4 and 6 volts, connect the lithium cell directly. If more than 6V, a series diode(s) will step the panel voltage down.
    If more than one lithium cell in series, use a 'small' panel (4-6V) for each cell.
     
  7. Ian

    Ian Administrator

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    Yep, it's a 6V (1W) panel - so very small but I'd guess it'd be enough for the short amount of power that the device will require out of sleep mode.

    I thought that connecting a solar panel directly to a charger was inefficient (partly because of the constantly varying current, at least in UK sunlight). Here are some design notes I was reading, based on the same MCP73871 chip as the module above (https://learn.adafruit.com/usb-dc-and-solar-lipoly-charger/design-notes).

    I think I'll go back to basics and just stick to a 18650 after all :).
     
    kellys_eye likes this.
  8. kellys_eye

    kellys_eye

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    Ian likes this.
  9. Ian

    Ian Administrator

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    Please do consider making a project log for your project, as I'd be really interested in seeing how it works :). That module may do the trick for me too (and at £2, a bargain). It also does LiFePO4, should I want to tinker in future :cool:.
     
  10. kellys_eye

    kellys_eye

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    I didn't read that far......:D

    Yes, I'll keep a project log as the system (remote water system monitoring) could be useful to a lot of places round where I live - many villages have their own ground water supplies (into tanks, filters etc).

    Because this is a 'community' project I'm hoping to get money from the local wind farm slush fund (their bribe to the locals to allow those bl00dy things to be put up - fortunately not visible from where I live though).

    My system is based on the 868MHz LORA system with a minimum of two remote monitoring points.
     
    Ian likes this.
  11. Ian

    Ian Administrator

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    Looking forward to reading about it :). I've been considering using LoRa for high altitude balloon tracking, so it'll be interesting seeing how it works.
     
  12. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    LiFePO4 is at the cheaper end of the lithium battery technologies. It suffers from slightly lower capacity, and a fairly significantly lower voltage. However it has a much more benign failure mode (smoke vs fire).

    I have mentioned my colleagues sprinkler valve controllers before. These have a small solar panel, a single lithium ion battery, a microcontroller, and a few assorted other assorted parts. A shunt regulator is placed across the battery to limit the voltage.

    The shunt regulator has been discussed in several places here, the last one in a thread about balance charging capacitors. (Actually, although that circuit is similar, it's not the same). Here is the original link. Sadly, it's still not been used for the intended purpose, but it has been used in a stack of other things!!! Remember that you need to change some component values too get the voltage in the right range and choose a suitable mosfet (or use the bipolar design from the capacitor charge limiter).

    Remember that LiFePO4 batteries have a lower maximum voltage, but otherwise treat them pretty much the same as other rechargable batteries.
     
    Last edited: Sep 23, 2017
    Ian and bushtech like this.
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