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Why is my EV charging so slowly?
#1
As a new (or even experienced) EV driver, you may pull up to a DC fast charger and expect to get the charge rate that your car's specification sheet listed, but you are experiencing a far slower charge rate than expected.  Is there a problem with your car?

In most cases, the answer is no.  Charging an EV is not like pumping fuel into a tank--not exactly anyway, but there are some similarities.  The charge rate you will get will vary based on several factors, which I will cover below.  By becoming familiar with how DC fast charging works, you will have a far better experience with your EV and charge it more optimally.

Things that affect charge rate


  1. Battery State of Charge

    One of the most common determinants of charge rate in an EV is the battery's current state of charge (SOC).  I'll go into the technical details of why in a future post, but for now I will simply say that peak charge rate (the one that your EV manufacturer specs) is only reached at lower battery SOCs.  After a certain point, the charge rate will begin to taper off.  This is not unlike filling a glass of water from the tap and turning down the flow rate from the faucet so as not to spill any water, except that typically the charge rate will start to taper much sooner than at the last few moments.  The following chart illustrates the taper effect (note that different vehicles will start the taper at different SOCs):
       
    Therefore, if you arrive at the charging station with a medium to high state of charge, you will almost certain not experience a peak charge rate.

  2. Battery Temperature

    The temperature of the battery will also affect charge rate.  A cold battery will not accept charge as quickly as one that is warmed up (for this reason, some EVs pre-condition the battery en route to charging stations to get it to the optimal charging temperature).  Likewise, a vehicle with a hot battery may limit the charge rate to avoid overheating the battery.  This is more common in vehicles little to no active thermal management (i.e. Nissan LEAF), but even cars with good active cooling will use some of the charging power to run the battery cooler, which could impact the charge rate to the battery.

    Keep in mind the battery is a very large thermal mass and it does take time for it to reach optimal temperature.  Do not expect a 15 minute drive to do the trick in cold temperatures.

  3. Charge Sharing

    At some fastchargers, you may be sharing charge power with one or more other charging stalls.  This may not always be a problem because one or more of the vehicles sharing the charge may be in their taper down period anyway and not using the full power of the charger, but be aware that this could be a contributing factor.  Pre-V3 Tesla Superchargers (not Urban Superchargers though) have paired charging pedestals (they are numbered 1A/1B, 2A/2B, etc.)  If you plug in to 2A for example, you will be sharing a maximum of 150kW of charging power with the car plugged in to 2B.  It may or may not be obvious which charging stalls are paired with which at all sites, but if you see multiple connectors coming out of the same cabinet, and the station indicates that simultaneous charging is possible, it's more than likely that you will be sharing power with other vehicles plugged into the same unit.  For this reason, it is always best (and good etiquette) to seek out a charging station without a car plugged into the paired connector (if it's obvious which one that is).

  4. Charging Station Voltage

    This one is a bit tricky.  ChargePoint has a charger (ChargePoint Express 250, or CPE250) that can be paired with a partner station, and when this is done, it may be advertised on Plugshare as a 125kW station.  However, it is really only a 62.5kW station with the ability to share power with its partner to bring the total output power up to 125kW (see Charge Sharing above).  However, this is only the case for vehicles with 800V battery architectures (of which there are only a few examples today).  400V EVs (which are the most common) will only be able to get the 62.5kW max charge rate, even if the paired station is not in use.  If you pull up in your EV that advertises 150kW max charge rate, but it's only a 400V architecture, don't expect to get more than 62.5 kW from the charging station.

  5. Charging Site Power Limits

    For larger charging sites, there may be a limit to how much power can be pulled from the grid at any one time.  If a charging site is full, and there is no supplementary power or storage (solar/battery), the entire site may have to curtail power, which will lower the charge rate for everyone.

  6. Charging Station Faults

    Finally, there may be faults with the charging station itself.  This typically happens in the hot summer months and the charging station components themselves overheat, which may take internal power modules offline.  Charging stations are usually constructed of multiple charging units, each of which supply a given amount of power.  If some of these go offline (due to overheating or some other failure), you will still get power from the charging station, but it will not be at the max rate.  One indication of this kind of problem is that the charge rate may go up and down repeatedly as the internal units reset and come back online, only to fail again.
With all of these possible causes of slow(er) charge rates, it may not be possible to pinpoint the exact cause for any slow charging that you are experiencing.  I have seen at least one charging station that does display the power being requested by the car versus what is being delivered, which does help narrow down the reasons, although I will also say that that display did not appear to be accurate!

The best thing is to become familiar with your vehicle under a variety of conditions.  Learn at what SOC it starts to taper off the charge rate (and by how much).  Learn how colder temperatures affect your charge rate.  Be aware of potential charge sharing and site power limitations at busy sites.  Check plugshare.com for reports of damaged or slow stalls (although keep in mind that they may be from other drivers that don't understand all of the reasons stated above).  Once you get used to charging your EV, you will likely be able to more accurately diagnose slow charge rates.
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