While safety, reliability, and comfort are all important when choosing a vehicle, with plug-in hybrid electric vehicles (PHEVs) and battery electric vehicles (BEVs) there’s another critical factor to consider; the time taken to charge. While replenishing an internal combustion engine (ICE) vehicle takes just a few minutes, the time needed to replenish the motive battery on PHEVs and BEVs is significantly longer.
Several types of electric power are now available. Hybrid vehicles have both a fossil fuel powered drivetrain and an electric drivetrain which can be organised in different ways. Some types, such as mild hybrid, have a smaller battery and this is charged from a combination of the ICE and a generator. And regenerative braking provides some charge to extend the range while the vehicle is slowing down and the motor acts in reverse as a generator. PHEVs and BEVs include the ability to plug in to the mains grid and recharge the battery from there, while the vehicle is stationary.
The Society of Automobile Engineers (SAE) has defined the primary types of charging available for EVs in its SAE J 1772 standard. The standard contains estimated charging times, however these can only be taken as a rough guide, as there are many influential factors, including the state of charge (SOC) of the battery and the efficiency of the charger.
Figure 2: SAE J1772 defines the various types of charging available for PHEVs and EVs
Primarily, charging can be either AC or DC. The battery requires DC to charge, so, in AC charging, a conversion is needed between the charging socket and the battery, while with DC charging the vehicle is plugged directly into DC that can be fed to the battery. The various ‘levels’ relate to the amount of power that can be delivered, and this, in turn, relates to the power source. The higher the level, the more power that’s available and the shorter the charging time.
The other key difference is where the charger is located. In DC charging, the charger is external to the vehicle and all power conditioning (including rectification) is done outside the vehicle. DC charging tends to have the highest power ratings and is used for many commercial / public charging stations, such as those found on the forecourts of gas stations or beside the highway.
AC charging typically includes an on-board charger that moves around with the vehicle. The AC charger receives the mains via a charging cable and connector and converts this to a DC voltage at the appropriate level.
Charging typically breaks down into three types, based upon the activity / lifestyle of the vehicle user. ‘Main harbour’ charging refers to charging at or near a home or workplace while ‘destination’ charging includes charging when the vehicle is parked somewhere while an activity takes place. Examples include restaurants, shopping malls, sports stadia, etc. Both of these categories typically provide AC power and rely on on-board charging while the final charging type (‘range extension’ charging) uses DC power at very high levels. Range extension charging is similar to fuel stations and allows vehicles to be rapidly recharged during a journey.
The primary role of an on-board charger (OBC) is to manage the flow of electricity from the grid to the battery. This means that the OBC must comply with the requirements of the grid in locations where it will be used. The primary requirement is not to inject reactive power back onto the grid, which is achieved by having a power factor (PF) of >0.9. The OBC must also suit the types of charger available, meaning that it has to support single-phase and 3-phase operation.
There will also be requirements to provide isolation from the power source and a maximum current that the grid can deliver, which must be factored in to the design. As with all power systems, there is a potential to generate electromagnetic interference, so all relevant EMC standards must be complied with. At this power level, the ability to communicate with the grid is also necessary.
Figure 3: A Block diagram of a typical on-board charger
Because the OBC is permanently mounted, the weight must be minimised, to reduce its impact on the range of the vehicle. Efficiency is also important, and there are other benefits to efficiency too, such as requiring less thermal management which will reduce the size, weight and cost of the OBC.
In future, it may be possible to use the vehicle as a portable energy store, using the energy stored in the battery to power the home during times of peak demand or high electricity costs. The battery would then be replenished at times of cheaper electricity. This would save money for the home owner and help the electricity companies by balancing the load on the grid. In order to facilitate this, the OBC would need to return energy to the grid through an inverter.
The benefits of wireless charging could apply equally to vehicles as they do to smaller portable devices such as smartphones or tablets, especially to add a ‘top up’ charge to extend their range. Wireless technology will be particularly appropriate to vehicles that follow pre-determined routes, or often wait at a specific place, because the charging stations will be fixed. This would include buses that follow specific routes stopping at the same bus stop and taxis that wait at a taxi stand (perhaps at an airport or railway station).