Questions about the application for a backup power supply

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    I have an application for a backup power supply, which requires guaranteed energy available, but very infrequently, think burglar alarm. I would naturally gravitate towards lead acid as it has a well defined float charge protocol. However my client wants to use lithium, probably for cost in the small size that’s required.

    My first port of call was, and the various battery manufacturers. The BU papers 409, 808 and 808b cover charging and longevity of lithium cells.

    The classic cyclic charge protocol is to charge to 4.2V, terminating when the current is around 0.04C, then let the cell decline to a lower voltage, before repeating. Keeping the cell at 4.2v would accelerate the cathode oxidation wear mechanism.

    This protocol makes the cell’s full capacity available at the time of the charge, but when it drops to a lower voltage, less capacity is available. The guarranteed capacity is therefore with respect to the lower voltage.

    Improved lifetime can be obtained by using lower end-point voltages, and various sources mention voltages of (end_point/recharge_point) of 4.20/4.05, 4.10/3.90, 4.05/4.00. BU808( plots the highest lifetime as a cycle between 75% and 65% SoC, which equates roughly to 4.00/3.90. I’m not sure that’s the highest possible, only the highest of the options they considered.

    Where hi-rel is required, satellites for instance, NASA apparently charge to 3.92V, which is a compromise between a higher voltage which induces cathode oxidation, and a lower voltage, which loses capacity via SEI (solid electrolyte interface). However, this is still cyclic use. Dr Dahn (who’s he?) suggests that below 4.1v, the oxidation mechanism is not significant.

    Getting near to the question.

    Nowhere have I seen (apart from a few questions on AllAboutCricuits or Instructables, which were not answered satisfactorily) a discussion about whether float charging to a low voltage is ever permissible.

    Consider a 4.05/4.00 top-up protocol. Now compare and contrast to a 4.00 float charge.

    1.the minimum voltage, so the guaranteed energy, is the same
    2.the cell reaches a higher voltage with the top-up protocol
    3.the cell is cycling with a top-up, but not with a float

    That seems to me to be a draw, then two wins for floating, IF floating is permissible.

    In one case, I’m supplying just enough charge over a long period in bursts to keep the voltage at 4v or above, in the other case I’m supplying just enough continuous current to keep the cell at 4v or above. Given that cell leakage is higher at higher voltages, and charge efficiency is less than 1, I would expect to be supplying less total charge over time when floating than with topping up.

    Question – When you are well below the maximum voltage, say in the 3.9V to 4.0V range, is there still the prohibition about applying a small continuous current that there quite rightly is at 4.2v?

    Question – different wording – When at a low voltage, does applying a lower charge continuously actually damage the cell more than bursting the charge, which involves a higher peak voltage and cycling?

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