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Supercharger even faster

Discussion in 'Model S: Battery & Charging' started by Andy Lap, Dec 17, 2014.

  1. Andy Lap

    Andy Lap Member

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    I found this statement from Tesla;

    Optimal Charging
    The fastest way to replenish your Model S is to charge to 80% state of charge, which is enough for travel between many Supercharger stations. Charging the final 20% takes approximately the same amount of time as the first 80% due to a necessary decrease in charging current to help top-off cells. It's somewhat like turning down a faucet in order to fill a glass of water to the top without spilling.

    I am not a battery engineer, but I have a question;

    Possible to make it charge battery faster with bypass or return to power storage without spilling??
     
  2. rdrcrmatt

    rdrcrmatt Member

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    #2 rdrcrmatt, Dec 17, 2014
    Last edited: Dec 17, 2014

    It is, by upping the voltage as the charge current "tapers".. but the trade off is EXTREME premature degradation of the battery.


    I wish there was a post or article I could reference each time this topic comes up.. About how it just isn't possible without doing damage. The whole supercharger taper thing isn't because tesla slows the charge down, it is just how to safely, properly charge the battery chemistry in our cars for longevity.
     
  3. Andy Lap

    Andy Lap Member

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    Can I only rely on supercharge all the time?
     
  4. David99

    David99 Active Member

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    Yes absolutely. It won't damage your battery. There have been tests with a two Nissan Leaf, where one is charged normally and the other one is charged only at DC quick charging. The difference in battery degradation is minimal. Now the Leaf has no temperature management on the battery side and it charges at a higher relative rate. The Model S charges fast in terms of miles per hour, but since the battery is 4 times larger than the Leaf the c-rate is actually lower. On top of it the battery is actively cooled. In other words, the Leaf battery is holding up pretty well under quick charge conditions that are much worse to the battery than the Tesla Supercharger do. So it is safe to say it is absolutely fine to charge the Model S on a Supercharger all the time.
     
  5. Chris TX

    Chris TX Active Member

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    It would be nice if we had some kind of super capacitor bank that could charge quickly, then slowly charge that extra 20% on the road. My old Citizen Ecodrive watch had a solar cell that charged a capacitor quickly, then that capacitor would slowly charge the battery. It was a great way to charge it up quickly. Super cap tech would need to improve, though.
     
  6. David99

    David99 Active Member

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    That kind of super capacitor would be massive in size. Probably not easy to fit into the car. But I would love to have a home version of a Supercharger that uses super capacitors as storage. It could be charged over night with cheap off-peak electricity and then when you need a quick recharge during the day it could charge the Tesla at max speed (120 kW).
     
  7. arg

    arg Member

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    There would be no point: if you had such a device, you would just use the current out of it for driving and not bother to transfer it into the main battery.

    In fact such a device exists today: just make the existing battery 20% bigger and never bother to charge it above 80% full - giving you the same capacity as today's battery but faster charging.

    Or another way to look at it: your S85 doesn't have an 85kWh battery, it has a 68kWh fast-charge-capable battery and a small additional battery that can be charged slowly if you really want to use it.
     
  8. Saghost

    Saghost Active Member

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    Possible with a bypass, in theory, yes - but the bypass would have to be at the cell level (hence permanently installed as part of the car battery pack,) it'd add a bunch of complexity to the system, and it's not entirely clear how you'd know when to trigger the bypass.

    The taper is to prevent overcharging and related battery damage as you surmised - with lithium cells, that is usually defined as an individual cell voltage over 4-4.2 Volts - which has to be measured with the cell disconnected from any load or voltage source.

    To change an individual cell with a higher than safe voltage, you'd have to monitor both voltage and current flow to figure out the moment it reaches the correct charge level and set up the bypass - possible if you have lots of electronics handy and a complete understanding of the performance of the cell. A group of cells in parallel creates its own problems I expect, since some cells perform differently and you're deliberately driving into the danger zone. Sets of cells in series are undoubtedly worse, though...
    In the current system, they charge the pack as a stack of 96 groups of cells in series, giving around 400V fully charged. Putting bypasses into that would be especially dangerous - when the first group became full and the bypass switched in, the remaining groups suddenly get a higher voltage, meaning they charge faster/overcharge faster - leading to either a cascade failure or a cascade of bypassing if the bypass is fast and reliable enough.

    Overall, it is possible, but I don't really expect to see anyone implement it - the costs and risks don't seem justified by the benefit to me.
    Walter
     
  9. Andy Lap

    Andy Lap Member

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    Can we treat our battery pack as a buffer tank to a very big reservoir, easily charge up at very fast rate without spilling
     
  10. scottm

    scottm Active Member

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    Maybe think of the battery as a V shaped tank holding fluid that is fast to fill at the bottom but as the level rises (and the filling pump remains constant) it takes longer and longer to reach the top.
     
  11. David99

    David99 Active Member

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    The analogy Tesla is using to explain that charging needs to slow down towards 100% to not 'spill' is misleading. It makes people think in the logic of liquid and filling a tank that actually can be filled fast to 100%. A battery can't be charged as fast close to 100% as it can be charged when it's almost empty. It's a chemical process that needs to slow down or else the battery is permanently damaged. A battery is not 'filled', it is charged. If properly cooled and monitored, a battery can be charged much faster than 1.4C. But it requires a very big and powerful cooling system. Way too big to fit in the car. But even then, you have to slow it down significantly as you approach 100%.

    I think the biggest factor in the future to speed up the charging process is larger batteries that will be able to take the full 120 kW for a much longer time. Right now it only accepts the full 120 for the first 60 miles, then it already starts to reduce the power. The other big potential is have each stall with a dedicated charger, not shared as it is right now.
     
  12. tga

    tga Active Member

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    Look at any consumer device using a Li-Ion battery. Almost always the time to charge from 0-80% is similar to the 80%-100% time. A Tesla is no different.
     
  13. rdrcrmatt

    rdrcrmatt Member

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    The superchager doesn't slow down the charging rate. The internal resistance of the batteries comes up as they become more charged. As the resistance comes up, the amps go down. The only way to increase amps at that point would be to increase voltage (which the supercharger does but only as appropriate as to not damage the battery), but increasing voltage to maintain KW output would be extremely damaging to the battery (not considering cooling but that's another topic).
     
  14. David99

    David99 Active Member

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    Um, no, the charger adjusts down, it's not a function of the higher internal resistance. The internal resistance changes little with the state of charge.
     
  15. tga

    tga Active Member

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    Can you explain that? During the initial constant current phase, cell voltage rises while current is held constant, until it reaches the cell's max. Then the voltage is held constant while current trails off.

    Both imply in increase in internal resistance during the charge cycle.
     
  16. sandpiper

    sandpiper Active Member

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    I'm reaching a bit out of my comfort zone here, but I believe that modern LiOn batteries are charged in a more actively controlled way, where the state of charge (output voltage) of the battery is periodically measured and the charging current is actively adjusted. The current for any given state of charge is selected to achieve best trade-off between battery life and rate of charge.

    If I'm all wet here, please feel free to shoot me down.
     
  17. Danal

    Danal electricmotorglider.com

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    If the battery were a resistive load, your statement would be true. The battery is not purely a resistive load.

    The power in the charge goes into shifting the ionic chemistry of the battery to a different state. That changes the "resting" voltage of the cell. Which is quite different than changing its resistance. You can measure the resistance of the cell during charge; many chargers do this. A typical 18650 style cell has a DC resistance in the 110 Micro-Ohm ballpark when healthy. Tesla doesn't publish specs... it is going to be in this order of magnitude area. 110 Micro Ohms is 0.00011 Ohm.

    The pack is arranged in a serial/parallel arrangement that I can't quite track down as I type this... the parallel part reduced the effective resistance and the serial part increases it. Any reasonable assumptions still end up at tiny fractions of an ohm.

    Charge current is not going into resistance (in fact, any charge current that DOES go into resistance exits the pack as heat). Charge current is going toward changing the status of ions in the pack.



    So... why taper? As stated with many different phrasings above, pushing too much current during the latter portions of the charge cycle raises the cell voltage to the point that PERMANENT DAMAGE occurs to the ionic chemistry of the pack.

    If you want a physical metaphor, think of charging as being like pushing trillions of tiny needle points against a stretchy plastic film. If you push too hard, you will push some of the needles through the film, and this can never be undone. In the initial part of the charge, when the film is quite slack, you can push all the needles fairly hard (lots of current). As the film begins to get tighter and tighter (and you can measure this tension, it is called voltage), you need to start backing off the push (current) when the film gets near break strength (voltage gets to 4.135 per cell at 20C).




    TL;DR - Resistance has nothing to do with tapering charge. It tapers because exceeding the critical voltage of the ionic chemistry of the cell, even a little, damages the chemicals in the pack, permanently.
     
  18. mgboyes

    mgboyes Member

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    ^^ +1 Danal - excellent analogy.

    The voltage and current provided by the SC are entirely under the control of the car's systems, based on their knowledge of how to most rapidly charge the battery without causing it any damage.
     
  19. scaesare

    scaesare Active Member

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    Your points regarding the need for the chargers to actively modulate the current for charging Li-ion cells are spot on. Nice explanation.

    Modules of 74 parallel cells. There are 96 of those modules in series.
     
  20. Johan

    Johan Took a TSLA bear test. Came back negative.

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    I like to think of it like this: as the voltage is low (low state of charge) the cathode has many "open spots" for electrons. As you charge these gradually get filled up: as you push energy in electrons move from anode to cathode and occupy these "open spots". The voltage increases because the electrons increase the negative charge of the cathode relative to the anode. As your near the end of the charge fewer spots are free at the cathode and hence current (flow if electrons) must decrease. Voltage is always held slightly higher by the charger than voltage of the battery - otherwise no current would flow in to the battery (actually from anode to cathode).

    So what if the SC just applied 500V at the anode to avoid tapering? Well, as others have stated this would increase current (Amps) but at the cost of chemical/physical damage to the cathode (micro fractures) that cause irreversible damage and together with degradation of the electrolyte is the main reason for battery degradation. Local heat build up in the battery and at the cathode is partly responsible for this.

    The simplest way to improve SC is to have a 100kWh battery where only 85kWh are used, but seems wasteful.
     

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