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Li-Ion automated charge equaliser

Discussion in 'Renewable Energy' started by Brian Drury, Jan 3, 2018.

  1. Brian Drury

    Brian Drury New Member

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    I have built a simple clockwork logic charge equaliser for a series connected battery of 8 Li-Ion cells. The design is based upon the familiar flying capacitor charge equaliser but this uses flying cells rather than flying capacitors.

    Testing the unit required an automated battery management device to periodically measure the terminal voltage of each cell whilst alternately applying a load and then charging the battery to see how well the equaliser works.

    I have chosen to refer to the device as an equaliser to distinguish it from the more common technique of ‘balancing’ which usually involves the application of dump loads across individual cells should the terminal voltage exceed some pre-defined limit.

    This approach has several advantages:

    1. No wasted power

    2. No software

    3. Low cost

    4. Increases the battery capacity by (2 * cell count)-1

    5. Maximises energy transfer by reducing outliers

    The reason for this post is to see what other engineers think about the basic idea. As far as I can tell this is not something that is commonly done, in fact I have not seen anything published regarding this approach.

    No doubt most people will realise that 2018 will see the start of an explosion in the use of battery technology and currently Li-Ion is one of the leading chemistry types so this is a very topical subject with huge application potential.

    As this is my first post I shall not overload it with schematics and circuit descriptions but simply provide some empirical data obtained by operating the device.

    In the following graphs the cells are reclaimed 18650 cells taken from old laptop batteries. The charge current is 405mA and the discharge rate is 1.3A. The horizontal axis is time with a tick interval of 10 Seconds. The vertical axis is in volts. The top two traces indicate the application of charge current or load.

    The BMS is set to apply a 1.3A load until the lowest cell voltage reaches 3.5V. It then turns off the load and switches on the charger. The charger stays on until one of the cells reaches 4.0V when the charge is switched off and the load is re-applied.

    The first graph is without the equaliser. You can see that cell 7 has more charge than the others and cell 1 is a bit low. As expected, the performance remains the same for multiple cycles because there is no cell balancing.

    The second graph simply carries on where the first graph stops but this time the equaliser is switched on. The initial discharge looks similar to the first but as charging takes place the charge on cell 7 is being equalised with cell 6 which flattens the rate of rise on cell 7.

    You will also notice that without the equaliser charging terminated after 65 minutes but with the equaliser charging took 188 minutes on the second cycle.

    Static cells 1 and 8 have only one flying cell to share power with therefore I would expect them to take longer to stabilise than the other static cells.

    The power required to charge to 3.8V without the equaliser fitted is 13.5Wh. The power drawn is 13.4Wh to reach 3.5V

    The power required to charge to 3.8V with the equaliser fitted is 29.5Wh. The power drawn is 29.3Wh to reach 3.5V

    So, the equaliser provides an additional 15.9Wh or + 118.7% for + 87.5% extra cells.

    My conclusion so far is that the equaliser is highly beneficial. Not only are the cells now working in harmony the energy available from the pack is increased by 118.7% with no wasted power.

    I intend to carry out more tests and will be interested to hear what others think. Also, it would be great if anyone has data they can share using alternative methods.
     

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  2. Diver300

    Diver300 Well-Known Member

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    It's clear that the approach works, but I don't think that it would be as good as balancing resistors that are switched on when needed.

    The test at the start showed that the cells weren't getting better balanced, but nor were they getting worse. There would be quite a lot of energy lost getting the cells balanced using resistors, but once they were balanced, the power loss would be minimal.

    I don't think that you can justify the claim of no wasted power. The wasted power might be a lot less than would be the case with resistors, but using a cell to transfer energy from a higher voltage cell to a lower voltage one will inevitably waste some power, because the energy taken from the higher voltage cell is the higher voltage times the charge, and the energy passed to the lower voltage cell will be the lower voltage times the charge. The charge will be just about the same.

    The percentage by which the capacity is increased depends entirely on how badly the cells were balanced to start with, so there could be a vast percentage increase if the some cells were completely flat and others completely charged, while a balanced pack would see little improvement.

    Having what is in effect a second battery pack just for balancing seems like overkill and a waste of a lot of cells. You could possibly have just one balancing cell, which could be smaller that the others, and more elaborate switches to connect it to each cell of the main pack in turn. That would slow the equalising a lot, but there is no rush.
     
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  3. Brian Drury

    Brian Drury New Member

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    Thank you Mr Diver, that is the most thorough analysis of my circuit that I have so far received and I am very grateful.

    I agree if you only consider the balancing aspect. If you also consider that the flying cells increase the pack capacity then this approach has additional merit.

    I agree with that.
    That is what I expected to happen but the losses due to energy conversion efficiency is extremely small for these cells so a tiny increase in a small loss becomes insignificant.

    I agree with that.

    The flying cells effectively become part of the pack so are not wasted at all. It is common practice for applications such as power tools batteries to be constructed with multiple cells in parallel. This is not so very different.
     
  4. dave

    Dave New Member

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  5. Diver300

    Diver300 Well-Known Member

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    I may not have understood your circuit. I was under the impression that there were 8 main cells and 7 flying cells. If that were the case I couldn't see how you could get more than the AmpHour rating of one cell from the set, while still supplying the voltage from 8 cells.
     
  6. Brian Drury

    Brian Drury New Member

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    A good start will be a simplified diagram plus circuit description:

    S1, S2 & S3 are static cells wired in series.

    F1 & F2 are flying cells.

    Q1 – Q8 are MOSFET’s.

    The MOSFET’s are switched in pairs. Q1 & Q3, Q2 & Q4. (Q1 & Q5 are P type)

    The top 14047 controls Q1, Q2, Q3 & Q4.

    So, if Q1 & Q3 are ON then F1 is in parallel with S1. If S1 has more charge than F1 then current will flow from S1 into F1. Alternatively if F1 has more charge than S1 then current will flow from F1 into S1.

    When Q2 & Q4 are ON then F1 is in parallel with S2. If S2 has more charge than F1 then current will flow from S2 into F1. Alternatively if F1 has more charge than S2 then current will flow from F1 into S2.

    The bottom 14047 controls Q5, Q6, Q7 & Q8.

    The basic action is as above so charge is distributed S2 S3 & F2. Also, because the two 14047 are not synchronised there will be occasions when F2 is in parallel with S2 and F1.

    If we start with S1 charged and S2, S3, F1 & F2 discharged then eventually the charge from S1 will be evenly distributed between all 5 cells.

    The flying cells are switched at about 1Hz and the switch transition time is nanoseconds therefore the flying cells can be considered to be in time division parallel with the static cells. The ‘OFF’ time is a tiny fraction of the ‘ON’ time.

    The 8 static, 7 flying cell version is simply a scaled up version of the example shown.

    Conceptually, we have 8 static cells in parallel with 7 flying cells. Taking this one step further an individual electron may consider the static cells to be in series and in parallel at the same time. This takes a bit of thinking about.
     

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