per cell charge controller

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  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭

    I'm probably missing something, but isn't the point of the exercise to get all cells to the same (high, fully charged) voltage?

    The problem, if I'm understanding correctly, is if some cells are nearly empty and others nearly full, it may be possible for a parallel connection to allow a discharging full cell to charge an empty cell at a too-high rate (absent some sort of cell-level current limiting)?

    My understanding is in assembling a bank of cells, you would normally charge each cell individually to a set top voltage. The roughly equally charged individual cells would then be assembled in parallel for the desired AH capacity, essentially making one big cell in which small intercell variances will even out in a matter of minutes. A number of paralleled cells are then connected in series for desired bank voltage.

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  • jonr
    jonr Solar Expert Posts: 1,386 ✭✭✭✭

    At $3.50 each, ATTiny85 boards are so low cost that it can be cost effective to use one per cell. A "each cell controls itself" rather than a central controller design.

    I am available for custom hardware/firmware development

  • mcgivor
    mcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭


    Digging through and re-reading all the previous posts I found the paragraph below which clears up some details I may have misconstrued as all cells being in parallel physically.

    While charging they all share the same Negative Return/Common Reference buss, as well as the high-side Positive charge circuit, so I suppose in that sense they are all wired in Parallel, to the extent / while each cell's relay is closed, connecting it to the + / - busses.

    My apologies to @john61ct .So the individual cells are independently charged, commoned at the negative terminals, the positive to each cell is to be controlled via a relay powered by a device Ardunio, PLC or other. Perhaps this is possible, but I do have some concerns with regards to how this will be done, which for now I'll keep to myself unless invited to engage.

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  • john61ct
    john61ct Registered Users Posts: 24 ✭✭

    No apologies needed! I understand readers are not putting in the intensity of effort trying to parse my wording that I am putting in trying to write clearly.

    > Perhaps this is possible, but I do have some concerns with regards to how this will be done, which for now I'll keep to myself unless invited to engage.

    Well "the What" now being clear, "the How" is all that's left, and so of course, do please share any of your concerns.

  • john61ct
    john61ct Registered Users Posts: 24 ✭✭

    > At $3.50 each, ATTiny85 boards are so low cost that it can be cost effective to use one per cell. A "each cell controls itself" rather than a central controller design.

    Well yes, that certainly is a very interesting possibility, thanks.

    However, I will still want (pretty sure) a centralised "place" to easily make adjustments to the various setpoint voltages and delay timers.

    Even for a given bank in production use, different charge source types, current level available, etc will require frequent changes.

    Ideally at some point "restored" from saved profiles.

    But in the early stages no one cycle will be the same, so changing each cells' settings separately would be error prone and inefficient, since they all need to be exactly the same.

    Perhaps an rPi central controller, communicating with the large number of per-cell-regulators?

  • john61ct
    john61ct Registered Users Posts: 24 ✭✭
    edited February 2019 #37

    > isn't the point of the exercise to get all cells to the same (high, fully charged) voltage?

    Yes, perhaps not ever as high as you would consider Full, my definition of that for daily cycling is well below 3.5Vpc, and prior to being put into storage, perhaps SoC of 30-40% whatever At Rest voltage that turns out to be

    but in all cases, certainly all cells ending up as close as possible to the same At Rest finish voltage, without using any CV / Absorb charge stage, CC only, "to a voltage and disconnect" each cell independently.

    > The problem, if I'm understanding correctly, is if some cells are nearly empty and others nearly full, it may be possible for a parallel connection to allow a discharging full cell to charge an empty cell at a too-high rate (absent some sort of cell-level current limiting)?

    No, that may sometimes be an issue, but not the immediate one to be dealt with. For now we can assume most cells are usually pretty close. The goal is as above, to get them precisely to the same restingV, as least as close as possible using this methodology.

    > My understanding is in assembling a bank of cells, you would normally charge each cell individually to a set top voltage.

    Yes, normally, although many prefer bottom balancing, if they anticipate getting dangerously close to single-digit SoC levels.

    However, in this project, the goal is for all the cells to get balanced every time, through this (here normal) independent-cell charging process. Eliminating the need for manual cell balancing or even checking / monitoring individual cell voltages, neither "live", nor as a periodic maintenance routine.

    > The roughly equally charged individual cells would then be assembled in parallel for the desired AH capacity, essentially making one big cell in which small intercell variances will even out in a matter of minutes. A number of paralleled cells are then connected in series for desired bank voltage.

    Again yes conventionally that disassembly / assembly process is a relatively infrequent event. Here it happens every charge cycle. But it would be derailing to the main topic of this thread to discuss that aspect, even if I knew how to do so.

    So for purpose of this discussion, it's not a paralleled nor series'd pack, not assembled in any given xPyS arrangement, just a collection of individual cells, each being independently charged, through the single pair of wires and the relay in between

    (dis)connecting each cell separately to/from the buss, no other gadgets or connections while charging, otherwise each cell isolated, standalone.

    Clear?

  • jonr
    jonr Solar Expert Posts: 1,386 ✭✭✭✭

    For others (not john61ct): no doubt that disassembling a series battery pack, wiring them all in parallel for awhile and then reassembling them is a legitimate way to balance them. Even better if this process can be automated. I looked at this approach once and decided there are better ways (like inductor based charge shuffling).

    A little resistance will limit current flow. Say the maximum voltage difference ever possible is 1V. A .1 ohm resistor with limit this to 10A. But at 10mv difference, the current will drop to .1A.

    I am available for custom hardware/firmware development

  • john61ct
    john61ct Registered Users Posts: 24 ✭✭

    > A little resistance will limit current flow. Say the maximum voltage difference ever possible is 1V. A .1 ohm resistor with limit this to 10A. But at 10mv difference, the current will drop to .1A

    I do not if possible want anything slowing down getting to the finish point in normal operations.

    I think it will not be that hard to get the central logic controller to put per-cell current-checking in place for temp / safety purposes.

    So far IMO the best approach is just reducing the available-from-upstream current level to the buss in proportion to the count of currently closed relays / connected cells.

    I've come across data that is leading me to be a bit more conservative, maybe .3-.6C at least for normal charge cycles, rather than allowing higher rates.

    But the ability to allow up to .8-1C for rare cycles where a fast charge is required, would be nice.

    But for now I want to focus on the core function, identifying components for the central controller, the relay functionality and sensing voltage accurately.

  • jonr
    jonr Solar Expert Posts: 1,386 ✭✭✭✭
    edited February 2019 #40

    > I do not if possible want anything slowing down getting to the finish point in normal operations.

    Well, 1V through .001 ohms will produce 1000 amps (flowing between two parallel connected cells). You aren't going to find a relay to handle that.

    I am available for custom hardware/firmware development

  • jonr
    jonr Solar Expert Posts: 1,386 ✭✭✭✭
    edited February 2019 #41

    I looked into using a per-cell micro-controller a bit more. Giving the pricing and need for communications, I'd use an ESP-12E module on each cell ($1.78 each). Each module can monitor cell voltage and control a passive bleed resistor as instructed (for balancing at say 1A). They can all communicate via wifi to a central CPU (raspberry pi or PC) to handle reporting and control of charging, load and balancing. Could be built for around $5 per cell monitor board.

    I am available for custom hardware/firmware development