This article explains all of the factors affecting the charging rate to help explain why you are getting slower charging than you expect, or to help with choosing between charging options.
The car always charges as fast as it can in the circumstances, so you can normally just plug in and leave the car to work out what to do. However, if you want to work out the charging rate (eg. to pick the best time and place to charge from a number of options) then you will need to consider a number of factors.
The factors are different between Supercharging and normal AC charging but it is always the most limiting (i.e. slowest) factor that matters: If you connect to a chargepoint offering more power than the car can use, it will still work as the car will simply draw less than the maximum available.
Car using 32A from a 62A chargepoint, with the 3-phase symbol.
The display in the car can be misleading as to charging speed: To know how fast the car is charging right now you need to look at the power, either by setting the display to show in kW or by multiplying the numbers shown for amps and volts. If you have the charging rate shown in miles (or km) per hour, that is an average across the whole charging session and so can show a much higher or lower figure than reality if the rate of charge has changed (especially at Superchargers). The photo shows the right-hand side of the main charging display with the car drawing 32A at 248V and the 3-phase symbol shown: 32 * 248 * 3 = 23808 = 23.8kW. It also shows that this chargepoint is offering up to 62A.
If you are not getting the charge rate you think you should and want to seek advice (either posting a message on the forums or asking Tesla) please make a note of the volts/amps shown on the right-hand side of the charging screen. For Supercharging there will be two numbers: Volts and Amps. For other charging there will be: Volts, two figures for Amps (actual/max-possible) and perhaps the 3-phase symbol. Often, the easiest thing is to snap a quick photo of the charging screen with your phone.
This section discusses normal charging using an ‘EVSE’ (Electric Vehicle Supply Equipment); either using a permanently-installed EVSE, such as the wall-mounted charge point you may have at home (Tesla HPWC, ‘Chargemaster’, ‘Rolec’ etc.) and public charge points with a standard type1 or type2 plug, or a portable EVSE (the Tesla UMC or third-party alternatives) that you plug in to a normal power socket.
When charging from the AC mains the voltage is largely fixed by the supply company: It will vary slightly from place to place depending on national customs and factors like the distance from the nearest supply point. The voltage should not change significantly during a charging session. Higher voltage gives faster charging, but there is little you can do to affect it at a given location. In North America the voltage deserves slightly more attention, as charging points may be supplied with 240V, 208V or 120V and you may have a choice among nearby locations where the voltage is higher.
With the voltage fixed, it is the current (amps) which varies to give you different charging rates. This is controlled by the charger in the car, based on the factors listed below. Most of these are fixed so the rate of charge will normally be fairly constant during the charge.
- The rated current of the EVSE. This is fixed by the design of the EVSE and the size of its internal cables etc. The Tesla UMC is rated at 40A (North America), or 16A three-phase / 32A single phase (Europe). In North America public charge points are often 30A, with the Tesla HPWC (and some third-party units) at 80A. In Europe 16A, 32A or 62A are the common ratings: Tesla does not supply their own EVSE for fixed installation here.
- Current advertised by the EVSE to the car. For a permanently-installed charge point this will normally be the same as the rated current but, if the building’s electrical supply can’t deliver that much current, some models of EVSE can be adjusted internally to a lower value. Portable EVSEs often have a means to adjust the advertised current depending on the type of socket it has been plugged into – either manually or, in the case of Tesla’s UMC, automatically based on which plug adapter is being used. Some specialised EVSEs can adjust the advertised current during charging; perhaps to maximise use of local solar power. In each case, the advertised value cannot exceed the EVSE’s rating. This is the second number shown on the car’s display – if it says 32/63A, the EVSE is advertising 63A but the car is currently using 32A.
- Single or three-phase (not applicable in N. America or Japan). Connecting to a three-phase supply gives three times the rate of charge compared to single phase at the same number of amps. There’s no immediate way to tell whether or not a public chargepoint is 3-phase: the connector is the same and signage is often unclear, so you may need to plug in and see. Note that if you are using a detachable cable between the chargepoint and the car you need to make sure it is a three-phase cable – otherwise the car will fall back to single phase and charge much more slowly than need be.Cables supplied by Tesla (including the one supplied ‘free’ with the car in the UK) support three-phase but other suppliers often offer the choice of single or three-phase cables. The single-phase version is typically cheaper and lighter and so is a sensible choice for home use if you don’t have three-phase there, but not to carry for on-the-road use.
- Single or dual chargers. Your Model S may have been built with single or dual chargers – originally this was an option at the time of ordering, now it is an optional upgrade which Tesla Service can fit for you.In North America single charger cars support up to 40A and dual chargers support up to 80A. In most European markets single charger cars support up to 16A and dual chargers up to 32A, whether single or three-phase. In the UK, where three-phase is hard to obtain at home, cars ordered with single charger are actually fitted with dual chargers but limited by software to 16A three-phase or 32A single-phase. This is the position up until early 2015 but it is anticipated that this special treatment in the UK may change in future.
- Current set manually on the car’s touch screen. The charging page on the main screen has a manual dial which you can use to select a maximum rate of charge. Before connecting the charging cable you can select any rate up to the maximum allowed by the car, but once the cable is connected the maximum you can set is limited to the value advertised by the EVSE. Again, this is a maximum setting and does not guarantee that the car will draw this much.Once a setting has been made the car will remember it for that location. This can cause slower than expected charging if, for example, you charged somewhere at a slow speed using a domestic socket then later had a proper chargepoint installed there but forgot to clear the manual setting in the car.
- Battery temperature. If the battery is very cold, the car will need to heat it to allow full-speed charging (or, in extreme cold, any charging at all). If you have plenty of power available you will see the car drawing less than full power until it has warmed up. If you have less power available the car will draw as much power as it is allowed but nearly all of it will go into running the battery heater rather than charging the battery. This is particularly noticeable in North America, when charging with the very small amount of power available from a domestic 120V outlet. Under such conditions it is important to plug in immediately on arriving after a drive so that you can start charging (keeping the battery warm) before it has had a chance to ‘cold soak’.The opposite problem – battery too hot – could theoretically cause reduced charge rates, but at these modest charging rates the car can normally keep the battery cool enough with only minimal energy used for the coolant pump and radiator fan.
- Use of heating/air conditioning. Running the cabin heating (plus seat heaters, window de-frosters etc.) during charging will use up power and hence reduce the amount actually going into the battery. If you are charging from a medium power supply (around 30A) then running all the heaters at full blast will use all the available power and charging will stop. If you are charging from a lower power supply (like a domestic socket) then not only will charging stop, but some of the power will need be taken out of the battery. In each of these cases, as the cabin warms up the power needed for heating will go down and charging will resume. Cooling uses much less power than heating but will still have a similar, though smaller, effect.
- Battery close to full. Charging rates have to be reduced when the battery is close to full in order to prevent damage to the battery. Normal charging is slow enough that this doesn’t usually have any effect: If you are charging at maximum rate on a car with dual chargers you might notice a slight reduction when over 90% full, but otherwise the rate stays the same regardless of the state of charge. A different effect occurs when the display shows 99% full: The charging current falls to a very low level and it can therefore take anything from a few minutes to a couple of hours to actually report 100% full and stop charging. What is happening here is that normal charging has in fact finished but the battery is being ‘balanced’ – evening out the state of charge between the many cells that make up the battery. If you are sitting waiting for the charge to complete so you can drive off then there is no point continuing to wait when it reaches this stage, but otherwise it is useful housekeeping and the car should be allowed to finish in its own time.
- Voltage fluctuations. If the car detects voltage fluctuations which it believes may be caused by a fault in the wiring, it will reduce the current by 25% to reduce the risk of causing a fire. There should be a message on the screen when this happens. Occasionally there can be ‘false alarms’ where normal fluctuations on the grid can trigger the detector, or where the problem is outside your control (perhaps your neighbours are using a lot of power and the local transformer is overloaded).
Supercharging and CHAdeMO (DC Charging)
This section covers DC charging, where the car is connected to a separate charger in a cabinet beside the road rather than using the car’s on-board charger. This allows use of a bigger charger than would be feasible to carry around in the car, and so faster charging rates. There are two types of DC charger that can currently be used with the Model S: Tesla’s own Superchargers and the CHAdeMO system (which was created by some Japanese manufacturers and is most commonly used with the Nissan Leaf). Superchargers are typically between 2 and 4 times faster than CHAdeMO.
These two systems are extremely similar in the way the actual charging works, with the car setting the charge rate by sending instructions down the cable to the charger. In principle, all the same technical factors apply to limit the charge rate in both cases, but the different power rating and minor differences in the way the equipment is installed make a big difference to their actual impact on day-to-day charging. For this reason the list of factors is presented twice, with separate discussion of their impact on Superchargers and CHAdeMO.
The difference which most affects how you should drive and plan your charging stops is how the charging rate varies with the car’s state of charge. Superchargers charge much faster when your car is close to empty, slowing down significantly above about 40% full, and so you should try to arrange your stops to arrive at the Supercharger as empty as possible: If there are two Superchargers to choose from along your route you should stop at the second one to achieve the shortest charging time. On the other hand, CHAdeMO charges at a more constant rate and, in fact, reach the highest speed at around 75% full: If there are two CHAdeMO stations along your route it is usually better to stop at the first one, or even to split your charging time in two chunks and stop at both.
Supercharging is, in general, much faster than normal AC charging, but because of this it is more likely to run into various limitations during the charge: It is entirely normal for the rate of charge to vary substantially from minute to minute. Therefore the ‘miles per hour’ average charging rate display is close to useless while supercharging, so you must set the display to kW or look at the V and A figures on the display.
When using DC charging the voltage is determined entirely by the battery, and in particular the state of charge: When the battery is empty the voltage is around 350V but when the battery is full the voltage is slightly over 400V. The current is controlled by the Supercharger cabinet (following instructions from the car) and varies according to the factors shown below. Cars with the 60kWh or 70kWh battery have fewer cells and so the voltage is lower; hence at the same current the charging rate is lower.
Another minor difference to normal AC charging is that the measurements on the display are showing the output of the charger rather than the input. Since the charger unavoidably ‘wastes’ a small amount of the energy as it passes through, 1kW at a supercharger is worth about 5% more than 1kW at an AC chargepoint, since the charging losses have already been accounted for (think of it like a price ‘including tax’).
- State of charge. The Supercharging process takes the battery cells close to the maximum rate that they can survive without damage. The current that the cells can safely accept when they are empty is more than 6 times greater than when they are 90% full, so the Supercharger adjusts the charging rate to compensate. This is often referred to as the ‘Supercharger taper’. Supercharging above 80% full is not much faster than standard charging, and therefore to be avoided unless you really need the maximum range (or have time to spare).
- Rating of the cable/connector. Tesla have not published a specification for their connector but it is believed to be limited to around 330A. This is high enough that it currently rarely has any effect, but it may possibly become a limitation for future cars.
- Sharing of available power. Superchargers are normally installed in pairs, with two ‘stalls’ (parking spots) wired to a single cabinet. If two cars are plugged in at once they have to share the available power; the maximum that one cabinet can ever deliver is 145kW, while a single car can use up to 120kW if it is completely empty. If both cars are already at a state of charge where they would be using less than half the available power then there is no effect, and both cars charge as normal. Otherwise, charging speed is reduced.
The sharing scheme gives priority to the first car to plug in. The first car will normally get as much power as it wants, with the second car to arrive getting what is left over (but with a limit so that the second car always gets at least some power, a minimum of about 30kW). After the first car has unplugged and departed, the second car takes over first place and will have priority over any other car that subsequently plugs in. The sharing can’t divide the power into completely arbitrary fractions: It has to work in chunks of about 1/12th of the cabinet’s total power, so the second car will see the power go up in steps as the first car draws less, rather than a smooth increase.The exact impact on charging times is complex to calculate. The first car will normally see little, if any, increase in charge time. The effect on the second car is most severe if it arrives just after the first car and if both cars were empty when they arrived. In this extreme case, the second car will take 20 minutes to get the charge it would have hoped to get in 5 minutes (15 minutes extra, but four times longer than planned). After that, things get better as the size of the second car’s share goes up: After about 45 minutes the first car is no longer having any effect on the second car and the latter has reached about half full (which would normally take 25 minutes). The circumstances for this absolute worst case should be quite rare – more commonly the effect will be slightly smaller.
If you have the choice, you should avoid sharing: Look at the labels on the stalls, which show 1A ,1B, 2A, 2B, 3A, 3B etc., and avoid using 1B if 1A is already in use. If you do find yourself sharing, expect the first part of your charge to be relatively slow but the later stages to go at normal speed, with perhaps an overall penalty of 15 minutes.
- Site power limits. The 145kW per cabinet figure assumes that the power supply to the site has the ideal voltage and unlimited power. Some sites – particularly ones with just two stalls and a single cabinet – are relying on the connection to the power grid that was already there for the existing buildings and can only use as much capacity as is ‘spare’ after the existing users are accounted for. When charging a single car you will see that you don’t quite achieve the highest rates that you expect at the beginning of the charge but otherwise speed is close to normal. But with two cars sharing the effect is more pronounced and you might see up to 30 minutes penalty. Tesla often intends to have a new supply installed to allow the Superchargers to run at maximum rate but, especially in cases where the local power company has to install new wiring, this can take many months; there have been sites where users reported low rates of charge soon after it opened and yet later the performance has improved without any obvious changes on site. It is also rumoured that some sites have power ratings that vary with the time of day – if the grid connection is shared with other users on the site, the Superchargers can run at higher power when the other users are not active.
- Equipment limits. Early Supercharger installations were limited to 120kW per cabinet. There has been at least one major revision to the cabinet design, but the early sites are mostly still using their original equipment. The 120kW units can still charge a single car at full speed, but sharing has more impact at these sites. In a few places there is a mixture of old and new equipment at the same site, and so some stalls may charge faster than others.
- Equipment faults. Superchargers have a modular design internally. It is therefore possible for some modules to be faulty and yet the overall machine to keep on working, just charging more slowly than usual until it can be repaired. If you suspect this is a problem and you are at a site with more than 2 stalls, moving to another stall may get you a higher rate: if you are in 1A or 1B, move to 2A or 2B etc.
- High battery temperature. High speed charging causes the battery to become warm, so on a hot day you will notice the car’s cooling fans running at high speed. If the battery is too warm then the charging rate will be reduced automatically to prevent overheating. Usually, the car is able to keep the temperature under control, but if you arrive at the Supercharger with the battery already warm (perhaps as the result of some spirited driving) then you may miss out on the first few minutes of highest charging rates. Minor faults on the car can also impact cooling: If the radiators are blocked, or the air conditioning system has lost refrigerant, then this may show up as slower Supercharging on a hot day before you notice it in any other way.
- Low battery temperature. This is seen less often, but can have a huge impact under specific circumstances. You will usually arrive at a Supercharger after a long drive, and that will have kept the battery warm enough, even in winter. However, if you have allowed the battery to cool down – perhaps you stopped overnight at a hotel close to the Supercharger and planned to Supercharge in the morning – it must be heated before charging can start. Although the Supercharger has a huge amount of power available, the only way to heat the battery is with the car’s own heater, which is relatively small. If the battery is only moderately cold (say around freezing) the heater will run and charging will start slowly, but the combined effect of the heater and the charging itself will quickly allow the rate to pick up. If the battery is well below freezing then charging can’t start until the heater has warmed it, which could take a long time (up to an hour). So, if you have a very cold battery and a moderate amount of charge still in it, you may do better to drive for a few minutes before plugging in to the Supercharger. The best thing would have been to charge when you first arrived, before it had time to cool down.
- Battery balancing. As with AC charging, when attempting to charge to 100% you may see the display sit at 99% for a long time before the charge is completely finished. If you are watching the miles-per-hour charging display you may be fooled into thinking that charging is still going on, when in fact the car is doing battery balancing and the charging rate has slowed to a trickle. There is no point in waiting for this to finish before driving on.
- Irrelevant factors. Unlike AC charging, running the heating or air conditioning will have no noticeable impact on the charge rate and so it is well worth getting the cabin to a comfortable temperature before departing. There is also no manual control of charge rate: The charge rate dial on the charging screen disappears when plugged in to a Supercharger.
- State of charge (current limit). As with Supercharging, the current has to be limited to avoid damaging the battery, and the limit is much higher when the battery is empty than when it is close to full. However, the low power of CHAdeMO stations means that for much of the time they can’t reach this limit, and so it only has an effect above about 75% full at the best CHAdeMO stations (or above 80% for weaker ones).
- State of charge (voltage). The battery voltage varies with state of charge, from 350V (empty) to 400V (full) approximately for an 85kWh Model S. Power (charging speed) is battery voltage (V) multiplied by current (A). Often, at a CHAdeMO station the maximum current will be limited by one of the other factors and so the power will gradually rise during charging as the voltage increases – the opposite of Supercharger behaviour.
- Rating of the cable/connector. The CHAdeMO specification limits the current to 125A; this is often the dominant effect when charging a Model S at a CHAdeMO station. It is particularly disadvantageous to cars with the 60kWh pack.
- Sharing of available power. CHAdeMO connectors are often installed as one of the outputs of a multi-standard charger, usually styled like a fuel pump with multiple ‘hoses’. Sharing here is normally ‘all or nothing’, so if one ‘hose’ is in use then the others cannot be used. Proportional sharing, as at Superchargers, is theoretically possible and may be seen in the future.
- Site power limits. CHAdeMO equipment is available in a variety of power ratings, so sites with less power available will normally just install lower power equipment. However, you may occasionally find high-power equipment with its output ‘turned down’ to a lower level.
- Equipment limits. There are a number of different manufacturers of CHAdeMO equipment, each with several models. Full power CHAdeMO stations are usually branded as ’50kW’, with ’25kW’ models available for lower power sites (and units with even smaller ratings can be found in Japan). However, these ratings can be misleading: No CHAdeMO charger will ever deliver 50kW to a Model S, since the connector limit of 125A requires a voltage of 400V or more to deliver the full 50kW, and the Model S is already fully charged by that point. An ideal CHAdeMO unit would deliver 125A at any voltage and so give a maximum with the Model S of about 48kW at around 385V (70% full). However, some ’50kW’ models have a maximum current of only 120A or less, or interpret the figure to mean 50kW at the input to the charger and can actually deliver only about 45kW to the car. Nominal 25kW units have similarly variable specifications.
- Equipment faults. CHAdeMO units currently have a poor overall reputation for reliability, with problems including air filters becoming clogged and the units shutting down due to overheating. Most CHAdeMO points require payment or a membership card for access, so there is a whole category of communication and authorization problems that Superchargers don’t suffer from. Most of these faults seem to result in the charger becoming inoperable, rather than a reduced charging rate. A different kind of fault affects a large number of CHAdeMO stations at the time of writing (mid 2015): These units have a design fault which means they become unreliable if used at full power for long periods. Tesla have implemented a work-around whereby the car detects these stations and only charges at full speed for up to 30 minutes; if left connected for longer, the charging speed is reduced.
- High battery temperature. In theory this can affect charge rate, but is unlikely to be significant at the lower charging rates seen at CHAdeMO stations.
- Low battery temperature. Where applicable, this has exactly the same effect as at Superchargers – see discussion above.
- Battery balancing. As with AC or Supercharging, charging may ‘stick’ at 99% for a long time.
- Minor factors. Since CHAdeMO is normally limited by the charging equipment rather than the battery, running the heating or air conditioning will have a modest effect on the charge rate, using up some of the power that would otherwise have gone to the battery. It is still better to get the cabin to a comfortable temperature before departing although, if doing a long charge, you might consider leaving the heating/cooling until the end, when the battery might be limiting the charge rate and there is power to spare. As with Supercharging, there is no manual control of charge rate: The charge rate dial on the charging screen disappears when the CHAdeMO adaper is connected.