Why Tesla Is Limiting Supercharging Rates On Some Frequent Users

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Originally published on EV Annex.
By Charles Morris

Tesla recently caused a fuss with its decision to limit the Supercharging rate for vehicles that have racked up numerous DC fast-charging events. A similar stir erupted a few months ago when the company used an over-the-air update to limit the use of Launch Mode (in response to owner outcry, Tesla later removed the software limitation). In both of these cases, however, the company is not trying to spoil anyone’s fun — on the contrary, its goal in controlling Supercharger speed is “to ensure that our customers have the best experience at Superchargers and preserve as much vehicle range as possible.”

Fast charging stresses a battery, and doing it too often or at too high a rate can shorten a battery’s useful life. Tesla explained in a statement: “The peak charging rate possible in a Li-ion cell will slightly decline after a very large number of high-rate charging sessions. This is due to physical and chemical changes inside of the cells. Our fast-charge control technology is designed to keep the battery safe and to preserve the maximum amount of cell capacity (range capability) in all conditions. To maintain safety and retain maximum range, we need to slow down the charge rate when the cells are too cold, when the state of charge is nearly full, and also when the conditions of the cell change gradually with age and usage.”

Some vocal owners become incensed at any limitations on the way they use their Teslas. However, everyone wants to maximize their battery’s lifespan, and most will accept Tesla’s engineers’ explanation that, if a particular pack meets certain conditions, it’s prudent to limit the Supercharging rate. The inconvenience is minimal. “This change due to age and usage may increase total Supercharge time by about 5 minutes and less than 1% of our customers experience this,” says Tesla.

In any case, the controversy has inspired many owners to learn a bit more about how fast charging events affect a battery over time. Jeffrey Jenkins, writing in Charged, recently offered a detailed discussion of the issue, which we’ll briefly summarize here.

As Tesla explained, many things can reduce the lifespan of a battery pack, including charging at a high C rate. The C rate is the rate at which a battery is charged or discharged, relative to its maximum capacity. For example, if a battery is being charged at a rate of 1C, it will reach a full charge in 1 hour. Increasing the current increases the C rate, and decreases time to full charge. Charging at high C rates causes chemical changes to the battery cells that can eventually reduce their lifespans. If you frequently charge at a high C rate, after a few years, the battery can’t take as much current as it used to, at least not safely and efficiently. It’s analogous to the effect alcohol has on a human body — steady moderate use may not hurt you, and even an occasional binge won’t cause permanent damage, but frequent benders over a period of years certainly will.

Another factor that affects battery life is temperature. For maximum life, batteries need to be operated within an optimal temperature range. Excessive heat can shorten life (as some Arizona LEAF owners learned), but so can excessive cold. As the 19th-century Swedish scientist Arrhenius discovered, the rate of a chemical reaction is dramatically affected by changes in temperature. At lower temperatures, a battery’s internal resistance increases, and lithium ions in the electrolyte have a greater tendency to come out of solution and deposit onto the electrodes – a process called plating out. This is bad news, because not only does it permanently decrease capacity, but it can eventually cause a short circuit, with disastrous results.

The tendency for lithium to plate out is more pronounced at higher charge/discharge rates, so one potential solution is to limit current at low temperatures, as Tesla alluded to in the statement quoted above. Automakers are secretive about their battery pack designs, but it seems likely that there are numerous temperature sensors throughout a Tesla pack, and that those sensors communicate with the charger during Supercharging.

Maximizing battery life isn’t the only reason for Superchargers to monitor the charging process — safety is another. Tesla’s Superchargers aren’t just power outlets that passively deliver electricity — they keep tabs on many different parameters as a vehicle charges, in order to keep your battery safe, durable, and working as intended.

Editor’s Note: Aside from the important technical points above, there’s also a popular hypothesis that Tesla is limiting the Supercharging rates of frequent users to try to prevent Supercharging stations from getting clogged as more and more Teslas get onto the roads and need them for convenient long-distance travel.

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