What kind of variable load does an EV car present to the EVSE? - Page 1
Forum section: electronics / projects

author: 741
author_rank: Frequent Contributor
timestamp: January 06, 2020, 02:22:51 pm
content: What kind of frequency content & phase shifts does the current draw from an EV car have?For the purposes of power calculation is some kind of DSP indicated, or does the car's charge current stay pretty constant (slowly smoothly varying)?I'd like to make a semi-realistic LTSpice model of what one might realistically see during the charge process on an EV car.

author: fcb
author_rank: Super Contributor
timestamp: January 06, 2020, 02:48:07 pm
content: Quote from: 741 on January 06, 2020, 02:22:51 pmLet's assume you have are talking about single phase, 32A (7kW) - probably covers most of the EV's on the market at present.1. The charger circuits built into the cars are actually fairly good when it comes to harmonic content and PF (power factor) - PFC mandatory at those levels. I won't mention the name, but there is one EV i've come across that breaks all these rules when it came to market.2. Charge current can vary hugely, and somewhat arbitarily. The EVSE (charge point on the wall) can also request the car sticks to a maximum limit as well - and the car will ramp down pretty quickly to the requested levels. Power calculation can be done with a DSP easily, or you can roll your own single/three phase meter.3. Bear-in-mind that the typical EVSE is effectively a posh relay and more or less just connects the car to the domestic supply. So modelling this is fairly pointless IMO. If you are talking about DC (rapid charging) then that is a whole heap more complex. All the procedures/protocols are defined in a series of EN documents.4. The most contentious bit now is grounding protection mechanisms and the interpretation of EU rules around them.

author: richard.cs
author_rank: Super Contributor
timestamp: January 06, 2020, 05:52:47 pm
content: Harmonics are definitely controlled, but at least on a Zoe there is substantial ripple current at the switching frequency (a few kHz). Numerous other devices on the same supply can be heard "singing" with the same tone the car makes during charging, presumably ones with piezoelectric filtering capacitors or un-potted chokes.

author: 741
author_rank: Frequent Contributor
timestamp: January 06, 2020, 08:13:40 pm
content: OK Many thanks.ReQuoteSure: But it is "what happens then" - after the relay turns on - I was concerned about. Judging from the reply can I assume a model of a simple pure resistance, hence no need to look at harmonics?...But then I seeQuoteso if this is a significant contribution to power consumption I have to care about it.

author: fcb
author_rank: Super Contributor
timestamp: January 06, 2020, 09:33:23 pm
content: Had a few Zoe’s - the car does sing on charge - but never experienced any secondary effects.If you want to meter the power, then you should definitely build a proper power meter (there are some quite nice signal chip parts if you don’t want to roll your own). The last charger I designed used 3 rowgowski coils for current pickup and we rolled our own energy meter in the main processor, is probably just use a single chip solution if I did it again. What accuracy class do you need to hit??The sequencing for charging and all the voltages/resistances/timings are explained in the EN standards - there is a Wikipedia entry on charging that explains CP/PP and the different connectors etc pretty well.

author: Phoenix
author_rank: Frequent Contributor
timestamp: January 06, 2020, 09:43:50 pm
content: I think you still need to answer if you are talking about AC charging or DC charging.AC charging connects the cars on-board rectifier module to the grid. The EVSE communicates a maximum current/power limit and the on-board charger operates within that range. It is then entirely up to the car manufacturer on the quality of the charger and its waveforms and how the charge cycle is managed.- If you're doing power flow simulations a resistive model here is probably just fine, or slightly more accurate a 50/60Hz constant power load to account for variation in grid voltage. HF component won't contribute anything significant to real power.DC charging uses an external rectifier/charger module which is presented with a direct circuit to the cars batteries. (At least for CCS) the charger continuously provides its maximum charging current (may dynamically vary e.g. temperature) and the car continually requests a suitable charging current (and provides a voltage limit). The standards dictate how fast the EVSE must react to changes in charge level. It is again entirely up to the car manufacturer to manage the charge cycle and power levels (within the EVSE capability) but the external charger is responsible for the AC side power quality.- The car model here will be a cable (resistance) plus a battery (more complex).- The model of the EVSE to the grid will be resistive or a 50/60Hz constant power load.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: January 06, 2020, 10:37:24 pm
content: For AC charging, once the EVSE has turned its output on, the care will ramp up fairly quickly to a maximum level as set by the EVSE, unless the car's BMS is limiting it, e.g. due to high SOC, and stay constant until the battery approaches capacity, at which point it will slowly ramp down.The EVSE has no filtering, so the car's onboard charger shouldn't put out anything outside the applicable EMC standards.

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 07, 2020, 12:08:48 am
content: Quote from: fcb on January 06, 2020, 09:33:23 pmIf you're building just a few, an easy way to get a power meter is to take apart a cheap smart plug with energy monitoring. Most of those just have an 8 pin power measurement ASIC connected to an ESP8266, which of course can be reprogrammed to integrate into your design. Remove the existing shunt and connect the current sense pins to a shunt of whatever size you need. Beware that the logic is not isolated.

author: 741
author_rank: Frequent Contributor
timestamp: January 07, 2020, 09:28:19 am
content: It's AC charging.QuoteSounds like a useful question I have to find the answer to.Most replies above suggest harmonics can be ignored. I did find a report on this subjecthttps://strathprints.strath.ac.uk/39621/

author: fcb
author_rank: Super Contributor
timestamp: January 07, 2020, 01:44:36 pm
content: Quote from: 741 on January 07, 2020, 09:28:19 amI haven't yet see an EVSE that filters the mains for the vehicle. That would cost a fair bit to do and not your responsibility.If you are metering energy use inorder to charge £$ for the service, you are about to enter a whole world of s***.

author: richard.cs
author_rank: Super Contributor
timestamp: January 07, 2020, 01:52:08 pm
content: Quote from: fcb on January 07, 2020, 01:44:36 pmI haven't looked up the rules for this, but it's conceivable that you could sidestep some regulatory complexity by designing it to always under-read by more than your worst-case error budget. It then becomes trivial to show that errors are always in the customers favour (electricity is cheap anyway, and you can just claw it back in the price markup).

author: max_torque
author_rank: Super Contributor
timestamp: January 07, 2020, 02:14:05 pm
content: There will be very little dynamic load tapering on single phase AC charging because the charging power/rate is so low. Even on a 7kW system,which tends to be the maximum power rating of a single phase OBC, that is, relative to the battery of a modern EV, trivial to handle (where the battery is rated to deal with hundreds of kW during useage). Hence, connect the EVSE, the car signals its presence, the EVSE closes it's contactors, and the OBC will pretty much straight away ramp to the requested charge current and hold that untill 100% SoC is reached in most cases (or the OBC charge scheduler otherwise terminates the charging). Even with a hot/cold battery, just 7kW is unlikely to cause an issue with cell balancing or cell thermal effects.Once you get to dc fast charging, at powers between 50 and 250 kW then yes, the real time dynamic capability of the battery cells starts to matter, and charge rates will vary with SoC for example

author: richard.cs
author_rank: Super Contributor
timestamp: January 07, 2020, 03:02:30 pm
content: The 22 kWh Zoe tapers off the charging in the last 1-2% of SOC, and then continues to draw low power around, 2 kW, for quite a long period of time (half an hour or more) as cell balancing or similar takes place. On connection it does a few tests taking 10-20 seconds, at least one of which is earth impedance, and then ramps up the power smoothly over around 5 seconds.

author: DBecker
author_rank: Frequent Contributor
timestamp: January 07, 2020, 07:32:12 pm
content: The EVSE typically has an incidental bit of line filtering as a side effect of the GFI/RCI sensing coil.Power monitoring is either not done, done with current sense coils when the point is informational, or done with revenue-grade shunt resistors (e.g. Vishay WSL3637) when the result might be used to bill a customer. A typical approach is using the CS5463 Cirrus Logic power monitor.Every type I've examined has the power monitoring separated from the GFI safety circuit.

author: richard.cs
author_rank: Super Contributor
timestamp: January 07, 2020, 10:50:25 pm
content: Quote from: barycentric on January 07, 2020, 10:34:35 pmHe did suggest changing the shunt, that would allow freely rescaling the current range.

author: coppice
author_rank: Super Contributor
timestamp: January 07, 2020, 10:58:35 pm
content: Quote from: fcb on January 07, 2020, 01:44:36 pmNot really. WELMEC, OIML, and ANSI set the standards for most legal metrology around the globe, including charging for electrical energy. The specs are straightforward, and not difficult to comply with. They include the harmonic content you need to deal with, although many utilities require the meters they buy to go beyond those requirements. There are a number of electrical energy measurement chips designed specifically to meet these standards.

author: fcb
author_rank: Super Contributor
timestamp: January 07, 2020, 11:28:11 pm
content: There are no half-measures when it comes to metering for revenue purposes. It is either fully tested and compliant or not. Building a metering system is not hard, paying for the testing is. Just try getting a price out of test house.

author: ejeffrey
author_rank: Super Contributor
timestamp: January 08, 2020, 12:40:02 am
content: Quote from: coppice on January 07, 2020, 10:58:35 pmNot only what fcb said about testing and verification, but if you are actually deploying these (rather than just manufacturing them) you have to worry about the laws that govern utilities. As usual, the US has taken many approaches here. So for instance in some states only utility companies can sell electricity. Therefore public EV chargers cannot charge metered power unless they are owned by a utility company. On the other hand California recently ruled that chargers are de-facto selling electricity and therefore it must be metered -- charging per minute will no longer be allowed. At the same time California also ruled that the charger must have an easily visible real-time readout of the rate and total kWhr (basically like a gas pump) and that relying on the screen in a customers vehicle or cell phone is not sufficient.

author: coppice
author_rank: Super Contributor
timestamp: January 08, 2020, 01:27:18 am
content: Quote from: ejeffrey on January 08, 2020, 12:40:02 amThe testing for a meter isn't really that much worse than most other products. Testing anything gets expensive when you add up EMI/EMC testing. safety testing, and any functionality tests. Testing a meter is a lot cheaper than testing, say, a radio device.I thought the original poster was trying to develop a product. If they are trying to deploy a service things may get a lot more complex. In some places landlords can sub-meter their tenants. The regulations for that could well be applicable to metering charging stations, but these things are going to vary with time and location.If California is banning timed charges, is that going to impact the charging networks which charge large per minute penalties for not freeing up the charging station soon after the charging operation is complete?

author: rstofer
author_rank: Super Contributor
timestamp: January 08, 2020, 02:43:14 am
content: If you just want to meter the circuit serving a charger, buy a revenue meter. They are already approved for the application, their tolerances are known and they will likely match what the local utility is using. This is just the first one I came acrosshttps://www.gordonelectricsupply.com/p/Milbank-Cl200-D-200A-Watt-Hour-Meter/5952665Applications will vary.

author: richard.cs
author_rank: Super Contributor
timestamp: January 08, 2020, 10:05:12 am
content: Another approach used for EV charging in the UK is not to meter power usage at all. Instead they just say this is a x kW nominal charger and it costs £0.yy per minute to use this service. If your car can't charge that fast or is nearly full then tough. Possibly this is due to a desire to avoid metering related regulations, but it also gives users a financial incentive to terminate the charge when it begins tapering off, freeing up the charge point for other users.

author: 741
author_rank: Frequent Contributor
timestamp: January 08, 2020, 11:21:24 am
content: The charger is 3-phase capable, my involvement is very late-stage. I have had no input to the hw design. It's just evaluating the viability of what has been already done. I do not yet know how the client wants to use metering.

author: 741
author_rank: Frequent Contributor
timestamp: January 10, 2020, 01:56:36 pm
content: Is there a way to query the car's existing energy state (how full the 'tank' is) ?.Also, do most EVSE & most cars use the digital protocol? I guess it's "analog alone" or "analog & digital" (but never just digital), analog being the fall-back.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: January 10, 2020, 02:12:02 pm
content: The only status the evse knows is charge current (though it's not actually essential to.measure it, so may not even have that), so no way to know state of charge, other than the tapering off when close to 100%

author: richard.cs
author_rank: Super Contributor
timestamp: January 10, 2020, 02:20:40 pm
content: So far as I am aware most of the AC EVSEs use the variable duty cycle square wave, I assume that's what you mean by the "analog protocol."

author: fcb
author_rank: Super Contributor
timestamp: January 10, 2020, 02:41:46 pm
content: Quote from: 741 on January 10, 2020, 01:56:36 pmNope.AC. All EV's with the exception possibly of Tesla, are solely 'analog' as far as I'm aware. So no SOC from the vehicle. As Mike said, you could detect the taper-off, but this will vary depending on how the vehicle is setup, temperature, initial SOC, etc...There is a PLC (power line comms) protocol for V2G though, but largely irrelvant as no car has production V2G as far as I'm aware.If you want to tap into OLEV grants then their are a whole heap of other things you should do - OLEV distorts the market..

author: DBecker
author_rank: Frequent Contributor
timestamp: January 10, 2020, 11:37:38 pm
content: Quote from: 741 on January 06, 2020, 02:22:51 pmThere are a wide range of chargers. There are few general statements that you can make.At this power level you can expect that the in-vehicle chargers are high quality with good power factor correction and filtering. Even the conversion market from a decade ago had reasonably good designs.Are you planning to calculate real and apparent power in your EVSE "by hand"? Rather than using a proven solution such as a CS5463? You are going to need quite a bit more in-depth expertise than you'll get from a few questions on a message forum.Charge strategy varies. No EV charges to 100% capacity, or even 100% of nominal capacity. They typically terminate far enough away from fully charged that the battery charge acceptance rate is easily above the EVSE Level 2 maximum. So it wouldn't be unusual for charging to end with no taper down.

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 11, 2020, 12:55:34 am
content: Many (if not most) modern EVs have networking, is there a standard way to query the current charge level?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: January 11, 2020, 02:59:10 am
content: Quote from: NiHaoMike on January 11, 2020, 12:55:34 amPretty sure each manufacturer does their own thing

author: 741
author_rank: Frequent Contributor
timestamp: January 17, 2020, 11:41:16 am
content: QuoteDo you mean things like DSP?