EV Charging - Is it realistic to simulate a battery? - Page 1
Forum section: electronics / projects

author: 741
author_rank: Frequent Contributor
timestamp: September 26, 2019, 04:06:01 pm
content: So far I've had just a brief glimpse of the specs a possible project, an EV charging station. It might well not happen. So this just testing the water; I am not knowledgeable about the topic.I gather that a test facility can use a standard circuit to evaluate 'domestic' EV charging stations. The signal side of this circuit is simple: Basically some switches, a diode, and resistors. The switches are twiddled to various positions to simulate what a car's charging unit would do (implementing the basic analog 'protocol'). The charging station responds accordingly according to some state diagrams. (An optional digital interface exists too, but I'm ignoring that here).It would be ideal to do what a test facility would do before sending a new design off for verification.The power side of the test circuit seems to be basically a variable load. The EV unit allows mains power (or not), via a relay - taking into account how many amps the EV unit states it wants (if the EV car wants 32A then the charger can 'refuse' for example). It also monitors the ground lead's integrity.(1) Is it all feasible to design & build some circuit which dumps power into say two parallel domestic heaters in such a way that this looks realistic from the charge station's stance?(2) Would it be worth doing, or would little information be gained?

author: MagicSmoker
author_rank: Super Contributor
timestamp: September 26, 2019, 04:44:42 pm
content: An EV charging station is basically a glorified power cord - supplies mains AC to an onboard battery charger; is that what you are designing, or the charger itself?

author: DBecker
author_rank: Frequent Contributor
timestamp: September 26, 2019, 05:42:41 pm
content: A basic EV charging control system is very simple. It's documented all over.The supply side is basically a GFCI, with a relay that connects the 120-240V supply when the right diode-resistor pair is connected to the "pilot" sense line.A key point that you won't be testing with a simple setup is that the pilot waveform is strictly symmetrical, both in voltage+impedance and pulse width. You should be able to drop the output in a bucket of water and have no DC bias. This should be true over the full range of conductivity. This is important for long-term corrosion resistance. You should be able to simulate that draw and measure the bias.And of course be able to simulate the various types of ground faults, but that's a little more obvious.

author: 741
author_rank: Frequent Contributor
timestamp: September 27, 2019, 07:29:04 am
content: Just the "glorified power cord" I think.Thanks for the note about PWM symmetry: PWM is not mandatory, the charger is permitted to output DC instead of PWM.

author: Kilrah
author_rank: Unknown
timestamp: September 27, 2019, 08:06:53 am
content: Those glorified power cords are pretty boring, they're common and cheap and just do a few basic measurements, some communication and fault detection that ends up switching a relay. Not very interesting as a project.Lots of detail about those here:

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 27, 2019, 01:58:48 pm
content: Simulate in SPICE, sure -- I wonder actually if you could use a PN diode with a preposterously large transit time, the physics is very similar; but more likely, you're going to want to be able to set the state of charge, and so you'll need a dependent capacitor. In series with that, some RCs to model ionic diffusion (as applicable; this is responsible for voltage recovery on discharge, and for current tailing off during a float/finishing charge, as used with lead-acid and lithium-ion types).IRL -- can't really imagine it's worth it, compared to a real battery. An electronic load of adequate size is going to be pretty pricey. If you just want to test load capacity, yeah, a bunch of heaters would be something.Tim

author: max_torque
author_rank: Super Contributor
timestamp: September 27, 2019, 10:01:04 pm
content: But you're not simulating a battery, you are simulating an On Board Charger (OBC)The AC charging interface (single or 3 phase) is handled on the EV by this OnBoard charger, and the EVSE is just a switch to grant it power. The charge rate (current drawn) is entirely regulated by the OBC and not the EVSE. The EVSE can signal the maximum current it can supply, and the charging cable includes an identifying resistor (to prevent the charger trying to pull 30 amps through a 12 amp cable...) but in all cases, the actual charge control is done by the OBC acting as a rectifier, and outputting a DC voltage of suitable magnitude to create a current draw that it is targeting, within the limits that the EVSE and cable can supply.

author: 741
author_rank: Frequent Contributor
timestamp: September 29, 2019, 01:38:23 pm
content: Well, yes I agree I deal only with the charger, but then I ask why the standard test circuit (as in figure A.8 on the documents) includes a variable load?Presumably the charger itself presents a variable load, but I don't get why the load is shown as variable if this variability is of zero concern to the 'power cord'.

author: MagicSmoker
author_rank: Super Contributor
timestamp: September 29, 2019, 02:50:33 pm
content: Quote from: 741 on September 29, 2019, 01:38:23 pmSome reasons charging stations might monitor load current: bill for energy used; protect against overload; negotiate maximum power if the onboard charger exceeds the ampacity available; notify driver when battery is charged; etc.In all cases you need to be able to vary the load and the charger needs to be able to monitor the current.

author: Kilrah
author_rank: Unknown
timestamp: September 29, 2019, 05:51:24 pm
content: We still don't know what document you're talking of, but if it's for testing the EVSE as there is negotiation going on with the EVSE telling the charger "you may draw X", you might want a variable load to for example try drawing more than X and confirm the EVSE disconnects power.

author: 741
author_rank: Frequent Contributor
timestamp: September 30, 2019, 07:55:30 am
content: The diagram A.8 is in a standard called 61851-1. You might be right about the purpose of the variable load, makes sense to me anyway.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: September 30, 2019, 09:04:16 am
content: Quote from: 741 on September 26, 2019, 04:06:01 pmThat's not how it works - the EVSE tells the car how much it's allowed to draw via the PWM signal on the CP pin. The car should never draw more than this.And EVSE does not actually need to monitor the car's current draw at all.It will also not usually monitor ground integrity, just provide residual current protection for AC, and ideally DC faults.However due to the earthing requirements of the latest UK wiring regs, which sometimes require a local grounding rod, some EVSEs get around this by being able to isolate the earth connection. In this case it should have some monitoring to ensure that the earth is connected before applying power, i.e. the earth isolation relay hasn't failed open.