Stopping and starting Home Wall Connector

[GedMelbAU]

I am writing an app that controls the home wall charger (running at 32amp/7kw) based on electricity prices. If the price is low enough the charger will start.

My concern is that if prices fluctuate up and down heavily, this can cause the app to stop and start the charger numerous times (I check prices every 15 minutes and the app could turn the charger off after 15 minutes, wait 15 minutes and then turn it back on again, cycling numerous times.

Would this be an issue for the car or the wall charger? Could this type of charging behaviour cause issues for the car or charger if done on a regular basis?

I appreciate any thoughts/insights.

 

[Rocky_H]

For a much less harsh way to do this, you should control it from the car side instead. There are external programs that interface with the car's systems and can control it that way, with telling the car to start and stop charging. There are some scripting interfaces for those programs, so you could tell the car to start and stop charging as you want that way, and it's probably smoother.

I don't use any of those programs, so I don't remember the names, but TeslaFi is one, and there are several others. I'm sure people will chime in here with their recommendations.

Also, I don't know if they have gotten this bug/behavior fixed, but there were issues with trying to do this on some kinds of systems that cut power to the external charging unit. If it got turned off, and the car went to sleep, it would stop detecting for power. And when the external unit got power turned back on, the car wouldn't notice and would just continue sleeping instead of starting to charge. So that's another reason to control this by telling the car to start charging.

 

[camalaio]

Outboard chargers (of any kind) usually have relays. Relays are limited in cycle life. That's pretty much the only part that would theoretically have significantly more wear, since you'd potentially be putting a lot of extra cycles on it.

But from a cost and total energy standpoint, this might not be a win anyways. If you do something like on for 15, off for 15, on for 15, the car might be "awake" in the 15 minutes in between (charging wakes it, and it stays awake a bit after charging). By increasing the total time awake by this cycling (instead of just once at the end of a usual all-at-once charge), you're wasting 100-300W during those increased wake times (the car's computers, coolant pumps, etc.). You could balance that in your charge logic, but you're decidedly in the high-effort-low-reward end of the project then! Maybe that's just an extra interesting thing for you to work with though. Have fun either way!

 

[Rocky_H]

And to second that point, since you're going for low prices, I would recommend just using the basic setting in the car for a daily scheduled start time. Your prices are always going to be low at night, not during the day time, so just set it for something like 1:00 AM and never think about it again. If the car doesn't happen to be plugged in at 1:00 AM, but maybe you get home late at 1:17, the car still watches for the next several hours (6?) after the scheduled time and will then start as soon as it's plugged in. Much simpler, and then your wall connector relays are only getting worked once per day at most.

 

[RB88]

What kind of power plan do you have where prices vary as often as every 15mins? I’ve only ever heard of off and on peak plans.

 

[GedMelbAU]

For a much less harsh way to do this, you should control it from the car side instead.

I realise after rereading my post I had not explained properly. I am actually controlling the charging from the car using api and not the power to the home charger on the wall.

I buy electricity at the current market rate so this means at times it can fluctuate quite a bit. Practically, it is unlikely to fluctuate wildly and cause the effect I'm concerned about, but it could.

If I'm controlling enabling and disabling the charging from car does doing this frequently have an impact on the car or tesla home wall charger and what sort of cycling frequency would make it an issue.

 

[Rocky_H]

Short term? No, it probably won't make any difference at all. Having the car activate or deactivate charging is doing exactly what it is meant to do. But as @camalaio is talking about, this is more a question of using up the eventual mechanical lifecycle of pieces of equipment. Let's look at the example of the solenoid that raises the locking pin in the charge port, or the tiny motor that opens and closes the charge port. How many tens of thousands of cycles of use do you think those have before they would break?

So let's say you plug in once a day. That's 365 times a year for 10 years is 3,650 times that pin operates. Probably fine, yeah? But I do hear people talk about how they want to make sure they build the "good" habit to always plug in every single time they pull the car back in the garage. But what if you're going out a few times each day? You put that plug in the port and activate that locking pin 3 or 4 times a day. Now you triple or quadruple the amount of cycles on that pin mechanism. Now will it last 10 years without breaking failing? Who knows, but it's pushing the chances. So closing and opening the relay for the connection in the wall connector from activating and deactivating charging a few times a day would be like that too.

So I kind of think of those with a bit of a wear lifetime, and if I don't need to overwork them, I don't. I'll go for plugging in about every other day. I don't consider it worth the wear on that stuff just to refill some tiny amount like 20 miles or less.

 

[Craig 128]

I wouldn't expect too much of an impact. Most items should be well and truly overrated.

For example my Tesla Wall Connector (TWC) version 2 has an ABB ESB63-40-DC-B contactor. This is what will switch on and off the power.
ABB ESB63-40-DC-B

It has an rated Electrical Durability for AC-1 of 150,000 cycles. (AC-1 is resistive loads where the power factor is >0.95, which is what you expect from the Model 3 built-in charger)

If you work backwards, lets say we expect the TWC to last 20 years. At 150,000 cycles, that is 20 cycles a day.

I have exposure to the wholesale price through Amber and have been doing a similar thing. But from what I have seen, it hasn't been that volatile. And if it does, you could put a little hysteresis in such as turn on when the price falls below 20c/kwh, but not switch off until it exceeds 23c/kwh.

I'm curious with the 15 minute cycle? In Australia, I thought it was 5 minute bids/offers and 30 minute settlements.

 

[camalaio]

I wouldn't expect too much of an impact. Most items should be well and truly overrated.

For example my Tesla Wall Connector (TWC) version 2 has an ABB ESB63-40-DC-B contactor. This is what will switch on and off the power.
ABB ESB63-40-DC-B

It has an rated Electrical Durability for AC-1 of 150,000 cycles. (AC-1 is resistive loads where the power factor is >0.95, which is what you expect from the Model 3 built-in charger)

If you work backwards, lets say we expect the TWC to last 20 years. At 150,000 cycles, that is 20 cycles a day.

I have exposure to the wholesale price through Amber and have been doing a similar thing. But from what I have seen, it hasn't been that volatile. And if it does, you could put a little hysteresis in such as turn on when the price falls below 20c/kwh, but not switch off until it exceeds 23c/kwh.

I'm curious with the 15 minute cycle? In Australia, I thought it was 5 minute bids/offers and 30 minute settlements.

Bolded part is my emphasis. I'm not so sure I'd naturally make the assumption that the charger has a near-unity power factor as it's much like a switched-mode power supply. At this tier there's surely some power factor correction built in, but measuring this would be neat (I have nothing that can do this, but now I'm very curious).

As consumer devices drawing large amounts of power continuosly, they probably are fairly close to 1.0 I guess.

Anyhow, for some reason when they said "wall charger" I assumed that was the UMC for some reason, whoops. I'm guessing that has slightly less good stuff inside it, though a quick look at a teardown wasn't deterministic.

 

[GedMelbAU]

What kind of power plan do you have where prices vary as often as every 15mins? I’ve only ever heard of off and on peak plans.

I am with Amber Electric in Australia.

 

[GedMelbAU]

I'm curious with the 15 minute cycle? In Australia, I thought it was 5 minute bids/offers and 30 minute settlements.

Yep you are correct, currently they settle every 30 minutes. The plan in 2021 is to move to 5 min settlements.

I've added a % over factor into my test to allow for a bit more smoothing. The 15 min cycles also helps with this. You are right, the volatility of price is typically not high though.

I assumed that was the UMC for some reason, whoops.

Sorry for my ignorance, I had never heard the term UMC before, is this the plugin charger?

 

[Rocky_H]

Sorry for my ignorance, I had never heard the term UMC before, is this the plugin charger?

I see that is an acronym that has carried over, but the term has changed. For many years, Tesla used to call it the Universal Mobile Connector. Now, I see in the online store, it is referred to as the Gen2 Mobile Connector Bundle. It is the mobile charging cord that has the changeable adapter plugs.

 

[rrolsbe]

I charge at 240V 24A and in the winter time at least some of the power goes to heat the battery until the battery temp reaches 56F, if I remember correctly. If the battery is cold soaked to around 40F, ALL of the power or at least 4KW is used to heat the battery and this can be for several minutes (no traction battery charging). By starting and stopping the charging, seems you would not be as power efficient but maybe the lower cost would make up difference?

Note I own RWD Model 3