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If I were to look at "normal" batteries online, they often have charging C-rates of less than 1. However, with a car that uses regenerative braking, obviously such a low charging rate would not be acceptable, as this would mean very little energy can be recovered by this process.

For example, F1 cars can deploy 160 horsepower for approximately 33 seconds per lap from the battery system (4MJ). Given Lithium-Ion batteries have a specific energy of up to 0.875 MJ/kg, this means the 20kg battery can only have a maximum possible capacity of 17.5 MJ. Therefore, a deployment of 4MJ in 33 seconds would mean a discharge C-rate of around 25. This is without even considering the extreme charging C-rate that would be required to recharge the battery during such heavy braking (little time) as in F1.

Therefore, my question is how electric/hybrid car batteries can handle the charging/discharging demands of regenerative braking?

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  • $\begingroup$ What have you found so far? Which manufacturer(s) did you consider? $\endgroup$ – Solar Mike Apr 18 at 10:25
  • $\begingroup$ @SolarMike I have done a few google searches for various batteries and I have looked at several different battery suppliers. I guess most are designed for other applications that don't require fast charging and discharging. However, I don't quite understand what makes these rates vary so much between different batteries and how electric car manufacturers can produce batteries that charge much more quickly than most batteries. $\endgroup$ – PhysicsGuy123 Apr 18 at 10:28
  • $\begingroup$ Which battery chemistries are you considering or have you considered? $\endgroup$ – Solar Mike Apr 18 at 10:50
  • $\begingroup$ As a specific example of what I think is being asked here, I drive a Prius, whose battery indicator can move from nearly full to nearly empty, and then back again, each within a very short time. Even if the full/empty shown is actually 80%/20% or some such thing, that's still means a 60% change in charge in less than 5 minutes. Or is the full/empty range actually much smaller, say more like 55%/45%? $\endgroup$ – Ray Butterworth Apr 19 at 0:56
  • $\begingroup$ Horsepower! How quaint. $\endgroup$ – Transistor Apr 22 at 21:48
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The "batteries" you find online are probably lithium cells.

The batteries in electric cars are not single cells. Instead they combine thousands of cells in a configuration with some of them parallel to each other (for higher current) and others in series (for higher voltage).

This means that each single cell is only providing a fraction of the total power and each cell only gets a fraction of the total power input when charging (either by being plugged in of regenerative braking).

As a side note: to keep the batteries viable for the expected lifetime of the car the charge controller only uses a small part of the total capacity of the batteries because fully charging or fully discharging the battery will decrease the lifetime of the battery. That longevity range is around 50% charged.

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  • $\begingroup$ Thanks for your answer. One question I do have is how having multiple cells makes it possible to charge faster? Surely, regardless of the number of cells, if the C-rate of the battery is X, then the C-rate of each individual cell would have to be X too? $\endgroup$ – PhysicsGuy123 Apr 18 at 13:54
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    $\begingroup$ C-rate is the percentage of energy that you can get into/out of a cell in an hour. Bigger battery means that C*total capacity is still larger. $\endgroup$ – ratchet freak Apr 18 at 14:01
  • $\begingroup$ I get that, but surely it makes no difference whether it is made up of a single 5kWh cell or 500, 10Wh cells, as the C-rate would still mean the same percentage of energy can go in/out per hour. Is there any reason why the C-rate would be higher for a smaller cell? $\endgroup$ – PhysicsGuy123 Apr 18 at 16:29
  • $\begingroup$ @PhysicsGuy123 If one cells C rate is 1, then 1000 cells would be 1000. This is also why Teslas with bigger batteries accelerate faster despite being heavier. More cells can absorb and deliver more current. $\endgroup$ – Eric Shain Apr 19 at 21:15
  • $\begingroup$ @EricShain - Unless I've missed something, I can't understand how that would be the case? If one cell has a c-rate of 1, then it can theoretically recharge fully in 1 hour. If you have a battery with a thousand cells and want this charged fully, you still have to fully charge each individual cell too, therefore surely it would still take 1 hour to charge the entire battery and hence the c-rate would still be 1 for this larger battery? Why would the c-rate increase to 1000 simply because you have 1000 cells? $\endgroup$ – PhysicsGuy123 Apr 20 at 14:07
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What matters is the stability and longevity of the electrolyte system in the batteries. Even wet lead batts can be dump charged at a fantastic rate (c 100 or so) if their state of charge is low and they are manufactured to high tolerances. But the rate of charge acceptance that doesn't cause damage drops of sharply starting around 80% charge. From a design point of view, Charge acceptance is controlled by surface area of the elecrtolyte/anode/cathode interface. It is also limited by internal heat generation and heat dissipation. Most electric vehicles use fan-cooled batteries with sophisticate cooling. So the first item is physical design - surface area, heat dissipation, and eletrolyte stability enhancements such as gels and glass mats in the case of lead batts.

The second item is the controller or BMS. A sophisticated BMS will monitor each cell individually and regulate the charge rate of each individually. This can be several hundred cells in an electric car. This greatly enhances the overall battery lifespan when pushing it through hard charge/discharge cycles.

The third item is the choice of battery chemistry. There are dozens in production, and hundreds that have been tried. Some are just better at taking a high charge rate or work better in the configurations used in high charge rate batteries. Lion is well known for taking a high charge rate up to 99% of fully charged.

I'd also like to comment of the regenerative braking ability of cars. Most can't and don't need to regenerate a very large proportion of their capacity. The average stop will store about 1/2 mile's worth of cruise power. Some cars (and trains) use a special section of the battery just to handle small amounts of high rate regeneration. These supercapacitors then bleed the power into the rest of the battery at a slower rate. These tend to be found in lead-acid traction batteries, but are being researched for other chemistries as well.

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