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Experience with the Cheap CC-CV chargers from ebay?

Discussion in 'Renewable Energy' started by EternityForest, Mar 28, 2014.

  1. EternityForest

    EternityForest Member

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    Ebay has a ton of these constant current regulator things that they occasionally market as solar charge controllers, which I would imagine you would use with a lead acid or as a CC-CV charger with a lifepo4 single cell. I have a few of the constant voltage only lm2596 ones and they work well enough.

    But has anyone actually used these for solar battery charging? Specifically, I'm curious about the brownout characteristics, and if there is any possibility of long-term damage happening due to undervolting the converter for long periods(Which will no doubt occur during sunrise and sunset, probably for several hours a day)

    Nowhere in the datasheet is anything about brownouts mentioned, except a note on how to make a power up delay because "some circuits" need to avoid turning on until power is stable, implying that the regulator itself is just fine or at least not seriously angry about brownouts.

    In the testing I've done(with the non-constant current version), it seems that they have a fairly "linear regulatorish" dropout, where they will continue to provide whatever voltage they can even when the input is below the output setpoint, slowly tapering to 0V output at about 2.0v input, and I assume most SW regs are like this, because I haven't seen any(Except really fancy ones) that aren't, so no issue there, the reg will regulate whatever the panel can provide.

    In this shutdown state(Unsure if caused by the IC or by the module design), if a 3.2v battery is connected to the output, they will draw ~12mA in backpower from the battery, meaning at least with the modules I have, a diode(Probably a FET active diode if charging a 3.2v cell) is needed to prevent discharging the cells(And possibly harming the regulator?)

    The voltage accuracy is pretty good, and I always charge lifepo4 to less than full anyway, and lifepo4 is pretty safe especially with a protection PCB.

    A lot of the cheap boards seem to have what appear to be shoddy looking capacitors that are rated to only 35v, giving you absolutely no safety margin at the full 35v the regulators claim to be able to handle, which shouldn't be an issue with ~17v solar panels.

    What do you guys think? Anyone ever used these? I'm interested in building a small portable solar system, with about 15-50W maximum capacity, and maybe 100Wh battery capacity, just for basic stuff like charging cellphones and flashlights and powering low power radio gear if I ever get off my butt and get my license and a rig...

    http://www.ebay.com/itm/1pc-DC-DC-L...628?pt=LH_DefaultDomain_0&hash=item1e84f79ba4

    http://www.ebay.com/itm/5A-DC-to-DC...232?pt=LH_DefaultDomain_0&hash=item484c99fbf0
     
    Last edited: Mar 28, 2014
  2. NorthGuy

    NorthGuy Well-Known Member

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    Properly sized solar panel cannot overcurrent the battery, so you don't need CC, only CV. You need two things from simple solar charger - limit battery voltage and disconnect when it's dark.

    There's a chance that your "protection PCB" already does voltage limiting.

    Whether or not the chargers will conduct in reverse in the dark can be found by experimenting, but a diode would be cheaper for this purpose anyway.
     
  3. EternityForest

    EternityForest Member

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    Most of the lifepo4 protection PCBs I've seen don't stop an overcharge until 3.9v or more. I think they are designed to prevent immediate damage with one or two accidental.overcharged but not regulate voltage in normal operation.

    I'm somewhat concerned about the diode voltage drop messing things up, but I suppose I can just set the voltage output for 3.6v and consider the 0.2v drop as an extra safety margin that gives me better cycle life anyway. Woulda been nice if they didn't draw back current though. I wonder if there is a reverse conduction diode in switching regulators that is like they have in 7805s?

    In general this looks to be a pretty easy project. Solar panel, regulator, boost converter to charge cellphones from the battery, cheap LED panel mount volt meter and amps in/out meter, and a few switches.
     
  4. dave

    Dave New Member

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  5. NorthGuy

    NorthGuy Well-Known Member

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    The diode drop is more like 0.6 to 1V, but it doesn't matter because you regulate on the battery side.

    3.6V is about 50%, if not less, charged.
     
  6. EternityForest

    EternityForest Member

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    Really? Most lifepo4 cells seem to specify 3.6V as fully charged, or maybe I'm just remembering wrong or reading the wrong source.

    If I use shottky diodes(Or a super barrier diode), I should get about 0.25V of drop especially if I use two in parallel.
     
    Last edited: Mar 28, 2014
  7. NorthGuy

    NorthGuy Well-Known Member

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    You're right. They're different from regular Li-ion. But then the management circuit should have a limit of 3.6V to avoid overcharging, shouldn't it?
     
  8. EternityForest

    EternityForest Member

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    Batteryspace has a few lifepo4 specific protection boards and even a few that balance multicell packs.

    The diode voltage drop issue is enough to me want to use a higher voltage pack and a balance module though.
     
    Last edited: Mar 28, 2014
  9. NorthGuy

    NorthGuy Well-Known Member

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    If they provide the voltage limit at the desired level this could be all you need.
     
  10. EternityForest

    EternityForest Member

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    Unfortunately they only voltage limit at 3.9v, just low enough to prevent a fire or something I guess, but way too high for normal use.
    Besides, the eBay modules are switching regulators which might be a bit more efficient.

    I'm slightly surprised that there are so few purpose-built lifepo4 charge controllers for <200$, especially for one cell and 2 cell packs, since a simple CC-CV regulator is less than ideal and doesn't give you any monitoring features.

    I wonder how much interest there would be in a product that actually does it properly, and maybe gives you a nice LCD UI.
     

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