Charging Terminology

Last modified: 20/08/2017
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Article adapted from http://teslapedia.org/model-s/tesla-driver/charging-terminology/

There’s a number of terms that you need to be familiar with in order to understand your charging options. This article gives more detailed, technical descriptions than those in ‘Charging Basics’.

kWh (“kilowatt-hours”)

kWh is a unit of energy. It’s what your home electric meter counts to show how much you have used, and is used to measure the capacity of your car’s battery (85kWh or 60kWh in the case of the Model S) in the same way that you might measure a conventional fuel tank in gallons or litres.

1 kWh is enough energy to drive about three miles.

Sometimes a smaller unit is more convenient, so we use Wh (watt-hours), where 1000 Wh = 1 kWh.

Miles (or Km)

Often, it is more useful to think of the amount of charge in terms of how far it will take you. The car displays the remaining charge in miles, and this is calculated by simply multiplying the value in kWh by a fixed number (although the exact conversion factor depends on your car model and whether you have set it to display ‘rated’, ‘typical’ or ‘ideal’ range).  All cars have two options, one describing a ‘best case’ that will rarely be achieved in practice, and another giving a more realistic figure that can readily be achieved in normal driving.  However, the names for the two modes are confusing, because ‘rated’ has different meanings in different countries:

  • Rated (as used in North America): Based on the Government’s EPA 5-cycle test.  This gives a figure that can be achieved in gentle everyday driving.  The specific test results are used for each car model, so a full charge on an 85kWh battery gives 265 rated miles for the standard 85, 270 rated miles for the 85D and 253 rated miles for the P85D.
  • Rated (outside North America): Based on the NEDC test, this is a very optimistic figure.  A full charge on the 85kWh battery gives 310 rated miles.
  • Ideal (North America only): Based on constant speed driving at 55MPH, and therefore optimistic for normal driving.  A full charge on the 85 gives 300 rated miles (with slightly higher numbers for the ‘D’ models).
  • Typical (outside North America): Designed to reflect normal driving, can be achieved at constant speed of 70mph or in moderate city driving.  A full charge on the 85 gives 245 typical miles.

Note that none of these are affected by your driving history – ‘typical’ means ‘typical for the car’, not typical for your personal driving.  The car does calculate remaining range based on your recent driving, ‘Predicted range’, but this is only shown on the Energy graph and is not an option that you can select for the other displays (most people don’t in fact find the Predicted range number to be very useful).

The setting giving the lower number (‘Rated’ in N.America, ‘Typical’ in Europe) is most commonly used, but take care when comparing with other drivers, or if you pick up a Tesla loan car, as some people prefer the other setting.

kW (“kilowatts”)

kW is a unit of power – the rate at which energy is being used (or transferred). The power meter on the Model S dash is marked in kW, and the most useful way of describing the rate of charging is in kW. The relationship between kW and kWh is straightforward – charging at 1 kW for one hour will deliver 1 kWh to the battery, so would take 85 hours to fill an 85kWh battery from completely empty to completely full. Charging at a more reasonable speed of 8.5kW would take 10 hours.

kW and horsepower are both measures of power – 1kW = 1.35 hp.

Again there is a smaller unit sometimes used: 1000 W = 1 kW.

Amps (A) and Volts (V)

Volts are a measure of the strength of the electricity supply, and Amps are a measure of the current – how fast the electricity is flowing at this moment.

Both are important for charging – the speed of charging (in W) is simply the voltage (V) multiplied by the current (A)

Power = V * A

kW = (V * A) / 1000

For charging we are usually only interested in the total power (kW), so having chargepoints labelled with amps or volts is a nuisance, needing us to do arithmetic. The reason that it is done is that the V and A are often controlled by different things:

For ordinary (AC) charging the maximum current (A) is mainly determined by the thickness of the wires and other parts in the chargepoint (and supporting wiring), while the voltage depends on the power company.

For example, a charging equipment manufacturer will sell a chargepoint rated at ’30A’ based on their design, but can only tell you the actual power if they guess what the voltage will be in the place where you install it.

This is particularly relevant for North America, where three different voltages are widely used: 120V (normal wall outlets), 240V (domestic and small business) and 208V (larger commercial/industrial). In Europe, the notional voltage is 230V everywhere, although there are minor regional deviations for historic reasons.

Things are slightly different when supercharging – the voltage is fixed by the battery, and the current is controlled by the charging equipment – but it is still true that Power = volts * Amps.

Three-phase

Three-phase is a method used to reduce the cost of wiring in the electrical supply system. Normally, if you split a high-power circuit (two wires) into three smaller ones (six wires of 1/3 the size) this would use exactly the same amount of copper in the wires. However, with AC power and clever arrangement of the phase in the three smaller circuits, you can make the currents cancel out and so you only need four wires instead of six – a saving of 33%.

This saving means that three-phase is used almost everywhere for high power distribution, but the inconvenience of using 4 wires rather than 2 means that it is not used for very small appliances. In Europe, three-phase is more widely used than in North America, hence the Tesla vehicles sold in these places have different charging connectors: the European one has more pins to allow use of three-phase.

One complication with three-phase is that you can measure two different voltages depending on which of the four wires you choose to measure – one will be 1.73 times greater than the other.

For most of Europe, there is only one supply voltage – you just need to be aware that that the same supply can be described as 230V when talking about one phase or 400V when talking about three-phase. If you find a socket labelled 400V, it will still show as 230V on the car’s display.  Current for three-phase charging is normally specified ‘per phase’, so you need to multiply the current by three:

Power (W) = Volts (V) * Amps (A) * number-of-phases

In Norway, there are some homes with a different form of three-phase which uses only three wires. This can not be used to charge a Model S in three-phase mode: you can only use one phase from such a supply and therefore charging will be relatively slow. Elsewhere in Norway, standard European three-phase is used.

In North America, all charging is single phase but the voltage available varies depending on the type of supply. Homes have both 240V and 120V available, but commercial premises may have 208V and 120V, due to the 208V being derived from a three-phase supply. Since the voltage affects the speed of charging, this explains why some apparently identical chargepoints are faster than others.

Charger and EVSE

These two terms cause much confusion, mainly because EVSE is jargon that nobody likes to use but the more friendly alternatives are ambiguous.

A charger is a piece of electronics which converts the voltage of the mains supply to precisely match that of the battery. It carefully controls the current to avoid overcharging or overheating of the battery. Chargers are fairly large and expensive, with size and weight increasing in proportion to the charger power (and hence speed of charging).

An EVSE (‘Electric Vehicle Supply Equipment’) is much simpler than a charger: it just contains some safety components which protect the user when plugging and unplugging cables outdoors, plus a means to tell the car how much power it is allowed to use without overloading the cables, fuses etc. Tesla’s UMC and HPWC are examples of types of EVSE.

The Model S has at least one charger fitted inside it (a second one can be fitted as an extra-cost option) but the two chargers work together to give the effect of a single larger charger. When using the internal charger, you need an EVSE – typically a box mounted on the wall or a ‘lump in the cable’ – to connect it to the mains safely.

Alternatively, the internal charger can be bypassed and you can use a separate charger; such as when using a Tesla Supercharger or CHAdeMO public charge points. In this case, the safety equipment is combined with the charger.

AC vs DC

AC and DC are different types of electrical current: batteries naturally produce DC, while rotating machines (motors/generators) naturally produce AC. Since the mains electrical grid is always AC and we want to charge a battery in the car, charging will always involve a combination of AC and DC.

However, the rather meaningless terms “AC charging” and “DC charging” have come to be used as shorthand for “using a charger built into the car” and “using a charger in a separate cabinet at the side of the road” respectively. Since the chargers carried around by cars tend to be smaller (and hence lower power) than you can put in a large cabinet, “AC charging” tends to mean “slow” and “DC charging” means “fast”; though that is not always the case.

In the case of the Model S, “DC” covers Supercharging and use of the CHAdeMO adapter with public fast charging points, and “AC” is everything else.

CHAdeMO and CCS

CHAdeMO is a fast charging system designed by Japanese manufacturers (the name is a pun between English and Japanese languages). It works on exactly the same principles as Tesla’s Supercharger, with some minor technical differences. The two major differences are:

  • Power rating. CHAdeMO is limited to 125A, which means a maximum of about 48kW when charging a Model S. Superchargers can deliver more than twice this rate.
  • Connector. CHAdeMO uses a complex, bulky, connector – the earliest models have a connector that is particluarly awkward to handle but newer ones are a bit less unwieldy. Also, since the CHAdeMO connector is only intended for DC charging, cars such as the Nissan Leaf or Citroen C0 have to have two sockets; one for CHAdeMO and one for ordinary AC charging. In comparison, Tesla uses a single connector for both AC and Supercharger.

Tesla offer an optional adapter to go between the car and a CHAdeMO charge point, making the car behave as if it were connected to a Supercharger.

CCS is another similar charging scheme designed by a more international consortium. It is currently favoured in particular by BMW and Volkswagen. It has yet another shape of connector and offers a theoretical power rating somewhere between CHAdeMO and Supercharger, though in practice almost all CCS chargepoints at present are limited to 50kW or less; the same as CHAdeMO.

There is no means at present to use a CCS chargepoint to charge Tesla vehicles, although an adapter like the CHAdeMO one is a theoretical possibility for the future. There is little need for it yet since nearly all public CCS chargepoints installed so far are ‘dual head’ units with both a CCS and a CHAdeMO connector and so the Tesla driver can simply use the CHAdeMO head.

Slow, Fast and Rapid chargepoints

Unfortunately, these terms have been so widely abused as to have little meaning. ‘Rapid’ often means a CHAdeMO point – particularly when talking to Nissan drivers.

In practice, you need to look at the type of connector and the rating, rather than the owner’s description as ‘fast’ or ‘slow’.

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