Towards Scalable Electric Vehicle (EV) Charging: Hidden Costs of Level 2 AC Charging

[Disclaimer: The views expressed here are personal and meant for information/education purposes only and not representative of his employer directly or indirectly. Any mention of company names are for illustrative examples only. Specifically, this is not intended as a critique of any specific EV charging network - they are doing a great job in the context of early EV penetration. This is an outline of deeper reasons why a next generation EV charging infrastructure is needed, especially to handle scale (in terms of numbers & charging rates) and flexibility: a vision of affordable, ultra-fast, ubiquitous EV charging. ]

This is part of a series of articles on e-mobility: Electric Vehicles & Ride Sharing Economics, Commercial Fleet EVs - Stealth Revolution, Towards Ubiquitous, Affordable, Ultrafast Charging, Electric & Autonomous: Synergies for a TaaS future, Towards Scalable EV Charging Infrastructure (Think Outside the Gas Station Box), Towards Scalable EV Charging: Hidden Costs of Level 2 Charging at Scale.

As the EV revolution takes root both in consumer and commercial segments in major developed markets, it is important to provision sufficient electric vehicle charging infrastructure to "fill them up" with electrons. As articulated in prior articles (see here and here), since Li-Ion batteries take longer to charge vs gas station fill ups (even with fast charging, irrespective of battery size), queuing theory dictates that you need more charging points to refill your EVs with electrons.

For consumer EVs, where you have the convenience of your home, and have a long overnight turnaround time of 8+ hours, having a simple AC charger (L2) like the Tesla charger shown and filling up your "battery tank" full overnight makes a lot of sense and the way to go forward as well.

However, as the EV penetration grows, we will have consumer segments such as multi-user dwellings (MUD) where a large MUD parking lot needs to be electrified.

If folks park their EVs on the street overnight, you need street side charging. There have been innovative ideas of providing such facilities via lampposts and with a mobile kit that does all the billing, metering etc.

If such charging is not available, the alternative is to provision more accessible charging in public places (typically retail locations): eg: grocery stores, malls, fast food chains, restaurants, cafes, hotels etc. Some sites are considering complementing EV charging with solar canopies and local storage as well.

Another key location is in workplaces (i.e. charge your EV at work). If you go to Silicon valley (Bay area, CA) or Norway you can see a number of mall parking lots, workplaces etc with increasing number of EV charging points in line with the growth of EVs in these markets. I think this is an excellent development and broadens visibility of EVs to aid growth: charging and vehicles on road or in your neighborhood, supported by strong EV advocacy by owners, EV OEMs and policies can significantly enhance uptake.

Beyond consumer EVs, there is an early stealth growth of commercial EVs. The "home" for such EVs is often a shared parking location such as a "depot"; and since EVs will have low marginal operations cost (i.e. high contribution margin, especially with autonomy), there will be a desire to operate them as much as possible to maximize productivity and minimize downtime. This will drive the need for more fast charging infrastructure, ideally shared by fleets. For more background, please read this article.

The current trend is to provision simple Level 2 AC chargers in these public or enterprise charging locations as well as private home charging. This is OK for a few charging points per location; but as it scales to tens of charging points or more, I foresee significant hidden costs both for the enterprise and utility. The purpose of this article is to illuminate and quantify some of the hidden cost issues (which may be obvious to electrical engineers, but not to decision makers focused only on capex & installation) but the impact on TCO (total cost of ownership) and electrical limits on scale will ultimately hamper scale of deployments. Some of these issues could be overcome through better engineering (or may be OK to bear at small scale), but for scaling up, a next-generation systems approach and flexible DC charging infrastructure is likely to be far superior, scalable and future proof (scale up average charging speeds easily as well).

The basics first!

What is Level 1 AC, Level 2 AC Charging and Level 3+ DC Charging ?

The simplest model is to plug your EV to the wall socket, draw ~15 amps at 110-120 V (US) or 220-240 V (UK) single phase supply, and use the on-board power converter to charge the battery pack. This is called Level 1 AC charging. This technique comes standard with all Battery EVs (BEVs) and plug-in hybrid (PHEVs) with small battery packs (<15 kWh). At a charging rate of ~1.5 kW, it takes a super-long time (15+ hours) to charge even small BEV battery packs (24 kWh+). EVs made in 2019+ are coming out with packs of 50 kWh+ for 200+ mile range (which would need 30+ hours to charge at L1 rates).

Level 2 charging is also AC charging where you increase the voltage and current limits by ~2X each. This allows ~8x faster charging (i.e. power delivert) compared to Level 1, but it depends a lot upon the specific voltage and current limits of the L2 charger, breaker etc.

Level 3+ or DC fast/ultra-fast charging involves AC/DC conversion and feeding DC directly to the vehicle based upon negotiation with the vehicle battery management system (BMS) using an interface protocol such as CCS or CHADEMO. Today level 3 chargers have power conversion co-located with the dispenser/HMI etc; but that is not a requirement.

Level 2 Charging: A Closer Look

Coming back to Level 2 charging, the graphic below is taken from Tesla's web site. Tesla Model 3 standard range/mid-range charge at 7.7 kW (32 Amps @ 240V = 7680 W); while the bigger pack vehicles / performance versions charge at 11.5 kW (48 Amps @ 240 V = 11520 W).

The term Level 2 charging is a bit misleading, and often it is termed as EVSE or Electric Vehicle Supply Equipment. The wall connector shown above is not technically a full charging system. The connector takes a higher voltage 240 V connection (between two hot wires of a standard split phase supply, see below), with a higher current rating 32A - 80A. It has some control and signaling logic blocks that tells an on-board charger in the vehicle how much power (voltage, current) is available for charging. The actual power conversion is done on-board the vehicle by a AC/DC converter {comprising a full wave rectifier, filter and some power factor correction}. This why the capital "cost" of the Level 2 "charger" appears lower even when normalized by power delivered compared to Level 3 chargers etc (which have an off-board power converter). The trenching/construction costs for cabling would be similar, though cabling and equipment complexity is higher in L3 / DC charging.

The picture above (from Gilbert Masters book) shows a typical residential single-phase 110-120 V electrical connection. The local distribution line is at ~4 kV, and a utility pole transformer drops it down to 120V / 240 V via a center-tapped transformer and gives a so-called 3-wire single-phase drop to the home.

There are two "hot" wires (which are 240V apart, also called "split phase"), and a grounded neutral return circuit which is 120 V from each hot wire. Loads are distributed between the neutral and each hot wire to be approximately balanced to reduce the current back through the neutral. The two-pin plugs you see at homes at between hot-and-neutral and connected to 20A breakers at the main panel of the home (i.e. up to 16 A available after applying 80% National Electrical Code (NEC) standard overcurrent safety margin).

Note that the normal circuit breaker occupies one slot in the main panel and is rated at 20 A. To get to 240 V and 30A (or potentially upto 80A), you first need to install a larger current breaker (occupying two-or-more slots in the main panel, and rated at 40A for a 32 A available load current). The voltage is now taken from hot-to-hot as shown above (i.e. 240V instead of 120V hot-to-neutral). If you run out of slots in the main panel - there is more electrical work needed. The home main breaker may typically be rated around 100A. If the sum of other loads in the home does not allow you to draw 30A or more for charging, the main breaker needs to be upsized as well (and the utility may need to be notified since the wires from the transformer to the main breaker may be sized for that).

The wall sockets for Level 2 chargers are also different for safety reasons. These are specified by NEMA. The Level 1 standard socket is NEMA 5-15R (15Amp). As indicated in the picture (right) above, the sockets for 30A Tesla Level 2 chargers are usually NEMA 14-50 (50Amp breaker).

The connection to the car (vehicle inlet) is via a standard such as the CCS (Combined Charging System) Type 1 (North America) and Type 2 (Europe), or Chademo (Japan, France etc). In North America only single phase 240V is allowed for AC charging (3 pins); but in Europe 3-phase input is possible with four additional pins in Type 2 CCS (as shown above). Mode 3 is a common mode, at 32 A, 3-phase available in Europe public AC charging.

The line-to-line voltage in USA/North America is 208V; EU is 380V; UK is 415V; Australia & India is 400V. Note that this is sqrt(3) * line-to-neutral voltages of 120V, 220V, 240V and 230V respectively.

The wiring from the main panel to the load (i.e. our EV) also needs to be sized to carry the higher current load. Note that we are doubling BOTH the voltage (120V to 240V) AND current (15A to 30A). Since losses are proportional to the square of the current (I^2 R effect), to lower the resistance (R), a thicker cable needs to be used. Notice that the resistance needs to overcome the square of the current to keep the losses the same.

Every lower gauge of cable (eg: AWG 11 vs AWG 12) has 26% lower resistance due to 26% larger area (and resistance halves every three gauges of cable), but you also need 26% more cost (due to copper). The picture above shows a calculation for an AWG 12 cable for 30A and illustrates that you can have 9% power loss just in the 100 ft cable. So, a AWG 10 cable is preferred to cut the cable losses to ~5-6%. For longer run lengths, thicker cables are required, since resistance linearly increases with run length. Also, notice that for every charger, you need at least three cables from the main breaker (two hot wires, one neutral, and one ground wire), and each of them becomes thicker => more copper costs.

One of the hidden costs of level 1 / level 2 charging is therefore cable / copper costs (x3 for three cables), and 5-6% power losses just on the cable due to the lower voltages (120-240V at which the cables are operated at). 5-6% may seem to be a small number, but it is not (we will see this later), especially when coupled with on-board power converter losses.

Roughly speaking if you double the current, the losses can quadruple on the same cable; so to completely eliminate this loss, you need 4x more copper (and 6 more gauges). Thicker cables are also harder to work with. Practically there is a tradeoff between cable thickness, copper cost and distance of wiring, and Level 2 chargers tend to accept more wiring cable losses. In a home this is fine, but in a commercial setup (eg: parking lot), the cable run lengths are longer; and Level 2 charging wiring losses can be in the range of 5-10% !

Ideally, it would be good to raise the voltage even further (to be equal to the voltage of the EV battery pack (typically 400V-dc - 750V-dc) and transmit the same power with fewer DC cables (two cables + and -), and with less current.

Beyond split single phase, the alternative in AC is to tap power from each of the three AC phases (similar to how an electric motor taps power, in a Wye or Delta configuration) and rectify (half- or full-wave) / choke filter / sum up each of the converted DC waveforms. The full wave rectification of three phase is smoother with lower ripples compared to half-wave (or single-phase), with higher average voltage.

This three-phase AC Level 2 charging is available in Europe, and can deliver upto 1.732 (i.e. sqrt(3)) times the product of the line voltage and line current. With the higher 3-phase voltages (380-415V) in EU/UK, Australia/India compared to 208V in US/North America, this also means more power transferred for a given level of current flow. Level 2 charging is potentially much faster in Europe than in North America (upto 43 kW = 1.732 x 400V x 63A)! However, in practice most line currents are limited to 32A, so at 400V line voltage, this corresponds to 22 kW which is still three times the single phase speeds in USA. This does require the on-board AC/DC converter (sometimes called the "dual car charger") to be capable of three-phase AC conversion. A summary of these charging possibilities (for Australia) for Tesla is shown below:

For a commercial installation, the supply in North America is typically 480Vac three phase, or 277V line-to-neutral supply (the supply schematic is shown below). The principles for power conversion are similar; however; since the CCS type 1 plugs and on-board converters do not directly take 480 V-ac 3-phase input, it has to be transformed down to 240V split phase. What this means is that ideally the transformer from 480/120-240V should be kept as close as possible to the EVSE as possible (or the set of EVSEs) otherwise there can be a lot of wire level I^2 R losses due to step down of voltage in North America.

Hidden Costs of Level 2 Charging: A Compilation

We have already seen that for USA/North America, the 240V, 32A delivery over 100ft or more in public charging/workplace charging can lead to 5-6% (and upto 9-10% if under-sized) line losses. Since it is the amperage that determines the losses; and higher average voltage in Europe raising the average power delivered, the percentage line losses for 3-phase power can be somewhat lower (4-5%).

The next source of losses is in the on-board AC/DC converter itself. Typically these circuits are built out of Si-MOSFETs as the power-switches which are cheaper and offer high switching rates. However for high currents (>15A), they can be quite lossy in the on-state, losing 1.25-3.75% in each MOSFET along the circuit. This means that as you try to push more current through the MOSFETs (eg: 32A or 63A), there will be more I^R losses which gets converted to heat. This in turn increases the temperature of the device, which in turn increases the on-state resistance and losses. Strong thermal management and cooling is necessary to conduct and convect the heat away (which in turn requires power). Assuming at least two MOSFETs on the current path (for PFC - power factor correction) and AC/DC power conversion, and each consuming 2.5% losses (inclusive of thermal management costs), a conservative estimate is 5% power loss at current levels of 30A (or 7.7 kW) in the MOSFETs alone. The rest of the circuits may contribute about 3-4% loss which implies a 8-9% aggregate power loss in the power converter. Together with the wire line losses, the total losses are of the order of 15-20%, in other words, the efficiency of the end-to-end system is 80-85%.

In addition to active power losses, the diode front end (DFE) rectifier circuits also lead to a combination of power factor (i.e. phase distortion of incoming AC power) and residual harmonic power quality distortion of upto 35%. A quick introduction to power factor is in the graphic below (courtesy: Engineering Mindset, youtube).

Power factor correction (PFC) circuitry in the charger corrects the power factor to around 95%; which implies a 5% loss w.r.t apparent power. Combined with the active power losses, this implies a end-to-end efficiency of 75-80%.

Harmonic distortion (see youtube intro, KatKim Show, or here (brain amplifier)) is a function of higher order frequencies (especially triplen odd harmonics), which wrecks havoc in neutral lines of three-phase circuits (in a Wye configuration) or leads to circulating currents (in 3-phase delta configuration), eddy current and hysteresis power losses in transformers, can affect neighboring circuits as noise, and could confuse relays/circuit breaker logic in the worst case. Neutral wires expect zero current with balanced loads. However with harmonic distortion, the neutral wire current can be significant, and sometimes more than the line current itself! This also means that there will be fire risks, and I^R losses in the neutral return path due to harmonics. If residual THD (total harmonic distortion) in mains is 35%, it can cause neutral current of 35-50% of main current at least; which means additional I^R losses of 2-4% on the neutral return path in three phase circuits. A short summary of eddy currents and hysteresis is shown in the graphic below (Source: Gilbert Masters book).

In transformers, power loss due to hysteresis is a function of the frequency; and the power loss due to eddy currents is a function of the square of the frequency. Despite mitigation techniques, this can lead to power losses in every transformer on the path (isolation transformers, step down transformers as we saw in 480V / 120-240 V in commercial installations). A conservative estimate is 1-2% losses in transformers and heating. But the effects on heating, and life of the transformer is also non-trivial. If the THD adds up significantly the commercial establishment may need to invest in additional harmonic filters to avoid propagation of harmonics into the mains/utility. A large number of Level 2 chargers can exacerbate the situation for the commercial establishment and/or utility.

Next, as discussed in a prior article, every commercial location has a electric grid connection with a local utility called a "connected load" which determines the site's commercial tariff class, and may be a constraint based upon available substation capacity of the local utility. The way a utility computes the "connected load" is to sum up all the loads based upon their RATED capacity (or breaker amperage, eg: 40-50A breakers for 30A circuits). In a gas pump like model of EV charging, this means summing up the rated capacity of the EVSE (EV supply equipment) AC capacity, adjusted for power factor, scaled up by 25% or more and comparing it with the connected load allowance at the site. For example, if you want to scale up a site to have 100 active L2 chargers (at 11 kW rating each), this means you need 1.1 MVA (Mega watt) of site level connected load capacity BEYOND all other commercial site uses. In fact due to NEC norms, this could be at least 25% higher, i.e. 1.375MVA. This could be a problem since utilities tend to bump up commercial sites to higher levels of tariffs at thresholds such as 250 kVA or 500 kVA. Alternatively, the utility may just have a local constraint and will not approve a higher connected load, even if the customer may be willing to pay for it, i.e. it may be a site-specific constraint to consider.

Finally, commercial sites incur tariff plans that are a combination of power charges (i.e. $ / kW / month, also called "demand charges") and energy charges (i.e. $ / kWh / month). In addition they may also have monthly charges for power factor (which is a function of power quality of your power conversion equipment for EV charging) and total connected load. As consumers, we tend to pay only energy charges, but commercial sites may have power charges that are as high as (or higher than) energy charges, which can double their monthly bills. The way power charges are computed is by observing the worst-case 15 min energy consumption (in kWh) and divide that by 1/4 (15 min = 1/4 of an hour). 

In other words, the indiscretion of the "peak" or worst 15 min (compared to "average" power consumption) can cause your monthly bill to be significantly higher than it other wise would be. Peak-to-average ratios of power is often far greater than 2:1! A "gas station" model of EV charging means that several tens of EV chargers (and larger with scale) have to be closely coordinated via software schemes to minimize demand charges, and often not done in practice. However this is very difficult to do with tens of Level 2 chargers at a site, since they charge slowly and tend to operate at peak capacity for a long time. Even if were possible via OCPP 1.6 (or 2.0) software controls to throttle down a charger; if there is a persistent demand of a number of EVs with low SOCs (state of charge) that need to be charged up, customer demands may drive up the aggregate power draw to be higher.

Further more the 20-25% end-to-end power loss discussed earlier is a huge problem now - it not only contributes to wasted energy charges, but also forces your hand on demand charges or not meeting customer charging expectations. Wasted energy also increases the (turnaround) time for charging a given battery pack. Demand charges in California can range from $10/kW/month to $20/kW/month; and energy charges can vary from 5-10c/kWh (off peak) to 15-30c+ / kWh peak (eg: summer, evening when solar goes off and demand comes in).

We have focused on direct and indirect costs (operational) so far. There is also an important opportunity cost of slower charging (especially in the US Type 1, Level 2 AC charging standards). A number of commercial fleet owners are considering buying light / medium duty EV fleets right now and it will only increase (a stealth revolution beginning!). As a default many of them assume that Level 2 charging is the right solution since it is "cheap". In addition to the hidden costs already discussed, there is a huge impact on asset productivity due to the slow charging (and 80% efficiency) of Level 2 systems.

Specifically, fleet owners who need their fleets to be productive for more hours of the day (rather than the 8 hours+ overnight charging model) or drive longer distances than a few tens of km will either have to invest in fast charging infrastructure, or will have to purchase MORE EVs to compensate for the EVs stuck in slow L2 AC charging. Eg: think of a Uber fleet owner who invests in an EV - will necessarily have to idle their vehicle; or their drivers during charging periods -- they would prefer their fleets to be in operation 24x7. The same is true for port / airport vehicles that go electric; or in a number of other logistics use cases. In other words the replacement of EVs vs ICE fleet vehicles could be greater than 1:1. This additional fleet procurement especially when battery packs are costlier in the early phase of EV growth is a huge inefficiency which can be overcome with more economical fast charging depot models. This unnecessary over-investment in excess EV procurement can be avoided by appropriate shared / smart DC fast charging infrastructure, coupled with predictive data from fleet tracking/management systems to keep the EVs back in service with quick turnaround times.

Another aspect of opportunity costs is in installation and expansion. Provisioning low speed and inefficient (L2) chargers not only means longer dwell times of charging per charger today for users, but these chargers may need to be ripped and replaced with faster DC charging in the future {not even counting the hidden costs of L2 charging mentioned above}. It is strategically better to provision cabling, trenching, wiring to allow peak-charging capacities to be 200A dc or above (i.e. 80-150kW-dc) one time rather than having to rip / replace in a few years at double the installation & labor cost. This will be especially so with the growth of commercial EV fleet use cases that require the EV asset to be sweated as much as possible, in part, enabled by new autonomous technologies.

Finally, almost all Level 2 charging is unidirectional, and do not allow bi-directional flows (eg: V2G) or other use cases for sophisticated smart grid services. It can be employed via OCPP software control in demand response though.

Summary

Level 2 charging is great for home charging, but as you deploy many Level 2 chargers in the context of commercial charging, large parking lots, workplaces etc, a number of hidden costs of Level 2 charging come to the fore. We discussed I^R losses in the conductor (due to lower voltage, high current, conductor sizing, run lengths), I^R losses in the power conversion circuits on-board the vehicle (due to Si Power MOSFET fundamentals), power factor effects (kVAR or kVA charges), harmonic distortion effects (and knock on effect on three-phase neutral lines & transformers), connected load constraints of the utility which limit scale (especially magnified by the power losses, and NEC safety margins), and impact on opex due to the wasted energy/power and due to the need to persistently charge (additional demand charges, kWh charges).

A simple calculation valuing the wasted energy, linear impact on incremental power (or demand) charges over a period of 5-7 years is of the order of several thousands of dollars per Level 2 EVSE. In other words, it may pay to invest in a scalable, highly efficiency DC fast charging system rather than bleeding money through wasted energy and power charges. It is also important to decouple the AC power constraint from the power dispensed to each vehicle, and the greater the flexibility in peak charging capacity, the more easier it is manage this tradeoff.

Again to re-emphasize, these hidden cost effects do not manifest in home Level 2 chargers (one unit per home) or in small installations (5-10 Level 2 chargers). These effects are magnified at scale with Level 2 chargers. It is therefore important to consider alternate architectures at scale for fleet charging, and scalable multi-terminal charging which deliver superior economics, avoid the hidden costs, and give outstanding flexibilities to both the commercial enterprise and the utility.

LinkedIn: Shivkumar Kalyanaraman 

Disclaimer: The views expressed here are personal and meant for information/education purposes only and not representative of his employer directly or indirectly. Any mention of company names are for illustrative examples only.

Twitter: @shivkuma_k

If you like this article, please check out these articles: Towards Scalable EV Charging: Hidden Costs of Level 2 Charging at Scale, "Towards Scalable EV Charging Infrastructure (Think Outside the Gas Station Box)", "Commercial Electric Vehicles (EV) Fleets: The Stealth Growth", "Towards Affordable, Ubiquitous, Ultra-Fast EV Charging: Part 1: Need & Battery Issues", "EV Taxi Fleets & Ride Sharing: Poised for Huge Growth", "Shared EV Transportation in India", "Understanding the Rs. 3/kWh bids in India in 2017", "Distributed / Rooftop Solar in India: A Gentle Introduction: Part 1","Rooftop Solar in India: Part 2 {Shadowing, Soiling, Diesel Offset}", "Rooftop Solar in India: Part 3: Policy Tools... Net Metering etc..." "Solar Economics 101: Introduction to LCOE and Grid Parity" , "Solar will get cheaper than coal power much faster than you think..", "Understanding Recent Solar Tariffs in India", "How Electric Scooters,... can spur adoption of Distributed Solar in India," "Solar + Ola! = Sola! ... The Coming Energy-Transportation Nexus in India", "UDAY: Quietly Disentangling India's Power Distribution Sector", "Understanding Solar Finance in India: Part 1", "Back to the Future: The Coming Internet of Energy Networks...", "Tesla Model 3: More than Yet-Another-Car: Ushering in the Energy-Transportation Nexus", "Understanding Solar Finance in India: Part 2 (Project Finance)", "Ola! e-Rickshaws: the dawn of electric mobility in India", "Understanding Solar Finance in India: Part 3 (Solar Business Models)" , "Meet Olli: Fusion of Autonomous Electric Transport, Watson IoT and 3D Printing".

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