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Higher voltage for charging?

Discussion in 'Battery Discussion' started by Tiger, Jan 25, 2019.

  1. Tiger

    Tiger Active Member

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    Aston Martin says they use 18650 format cylindrical cells in a 800V configuration for significantly quicker fast charging.

    If you couple similar batteries in parallel you retain voltage, and in series you double voltage.

    If high voltage is the key to charging, wouldn't it be as simple as to dynamically change the coupling/wiring to higher voltage for the duration of charging?

    Or am I missing something here? If you understand why dynamically switching the battery pack voltage wouldn't work for faster charging, please educate me. And also please explain why a fixed higher voltage setup would give faster charging (how is it any different from dynamically coupled)?

    If dynamic coupling would work, Tesla could match any charging voltage 800V or 400V on demand.
     
  2. ggr

    ggr Roadster R80 537, SigS P85 29, M3P 80k

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    The maximum speed of charging depends on only the C rate that the batteries can take. Which configuration and voltage makes no difference. What it does effect is stuff like how heavy the cable is, how much energy is lost to resistance, whether cooling is needed or not. So charging at higher voltage definitely has advantages, and may enable the use of higher C rate batteries, but does not automatically increase charging rate.
     
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  3. Tiger

    Tiger Active Member

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  4. dgatwood

    dgatwood Member

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    This does seems kind of counterintuitive, unless perhaps they are putting in a pack that is big enough that you couldn't hit the C rate through a cable that's thin enough to be usable.

    Also, I would think the series resistance of that many batteries in a row would start to become a problem in terms of unbalanced packs and capacity loss, wouldn't it?
     
  5. arg

    arg Supporting Member

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    That's precisely the point. Existing Tesla cars are charging at currents which are close to the limit of what can be handled with reasonable cables - they are helped in comparison to other EVs that Tesla constrains the chargeport location and so Superchargers only need short cables on them.

    There's no point in re-organizing the cells in the existing packs, as they are already charging at maximum rate. But if they further increased the number of cells (unlikely due to the constraints of size and weight) then they would either need to go to a higher voltage or come up with some solution for increasing the current (like liquid-cooled cables).

    Series resistance is not a concern since the key point here is that the individual cells are being treated exactly the same whether you put all of them in series or all of them in parallel or some arrangement in between.

    So if you shuffle the cells around such that they have twice the pack voltage (and so half the number of cells in parallel in each block), then the current is also halved to achieve the same power, and the current in any given cell is the same as it was before.

    Balance is an issue, but it becomes an issue as soon as you have even two cells in series; techniques exist for dealing with it and don't get materially more difficult with larger numbers in series.
     
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  6. mongo

    mongo Well-Known Member

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    The volume/ quantity of super caps you would need dwarfs the pack size (and cost). You're better off to just wait 1.5 minutes longer at the supercharger (3kWh capacitor bank vs 120kWh charge rate).
     
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