Determining most efficient charge rate 2023 M3 RWD LFP

[mknmike]

A) Peak power spikes are bad for grid and add to Peak Load Calculation on my bill. Charging at the max for my vehicle, 32 amps creates spikes. So I want to bring this down.

B) Charging in colder temps requires battery heating, and I believe I read that 300 W is consumed by the car any time the car is “awake” which it is while charging. I’ve also observed the bacon strips on during cold charging times and understand that nothing good happens when the temp is below freezing. So charging under 15 C is likely going to result in power used to heat the battery, especially at higher SoC. This sounds like purely “wasted” energy and I’m thinking that the most efficient winter charging will occur in a warm garage with a friggin blanket over the vehicle.

The above two items conflict with each other and probably have some balance. If power spikes are zero concern, then maybe jamming the charge into the battery as fast as possible is always the way to go. I don’t know.

I think I’ve had my charge rate set at 20 amps recently. I get approximately the number of MPH of range added as the amps I’m using. So 20 MPH is added at 20 amps. That seems to be plenty for an overnight charge.

Now how about max charge? LFP batteries say to charge to 100% once per week, which I assume is for calibration as opposed to actual battery health. It seems to me that the last few percent are quite inefficient for many reasons. One is the time to charge, and probably higher battery temps required. The second is the lack of regen braking at a high SoC. Then there’s also some arguments that you should not leave your car with a high SoC for long periods of time. With all this, I’m keeping my max charge at 80-85% instead of the recommended 100%.

Here’s my after-midnight charge that I believe was 20 amps. You can see it is about double my other max usage for the day. 32 amps would be just that much worse.

I charged at 20 amps again the other night but my hourly usage from that day hasn’t uploaded yet. I will upload it tomorrow.

Considerations?

1) Charge daily and consume electricity and try to jam electrons into a near-full battery?

2) Charge less frequently because there are heating cycles and it takes less energy to get energy into a less full battery?

Savings typically = Being green, and I would like to do both.

 

[GtiMart]

Charging while the battery is already warm, for example returning home after a drive, will charge immediately instead of heating the battery. That's a saving in time and energy.

32A is only 7.5kW of draw. My garage heaters pull that much when they run... I wouldn't call that a huge spike on the grid and wouldn't worry about it. IF you charge slower, you'll charge for longer and thus spend ~250w constantly for longer. That's a loss in energy and time.

All batteries will last longer when kept at a lower charge but I believe this is less important when the battery's cold. LFP are less affected by being kept full (although not immune). The BMS is much happier if you charge the LFP to 100% once in a while, you'll get better estimates of remaining SOC which might reduce anxiety.
I believe shallow charges are better than deep ones so that would mean charging every day.

You need to adapt this to your goals as some are conflicting.
- Someone that wants to simplify their lives would set to 100% and plug every day, charging as they arrive home. Simplest there can be, the battery is always full etc.
- Someone that wants to optimize battery life might keep to 50% and live with consequences of a bad BMS estimate.
- There's a whole spectrum in between those extremes.

Choose what fits you. You won't break the car.

 

[mknmike]

So there’s certainly a balance of convenience and efficiency. I believe that with a lot of things replacing energy with human intervention/energy is going to be the most efficient way, but we can’t spend our whole life trying to figure it out.

For now I’m going to try and record what I’ve been doing. I should probably try to be more scientific about it with temperatures and maybe obtain some of the software that enables me to download the recorded history from the car. But I’ve got what I’ve got right now and might be able to learn from it.

I had left my charge rate setting at 20 amps, and it’s been probably 2+ weeks since I’ve charged to 100%, and my wife asked me to charge. Rather than saving whatever energy it costs by using the free 6 months of supercharging, I decided to plug in and forget it. I think there was more than 100 miles of range left, like ~105, 105/271=0.387. So I think it was at 39%. Charge rate 20 amps. Set to start charging at midnight. I woke and was worried that it wasn’t going to make it to 100% and bumped it up to 32 amps to complete the charge before my wife needed to leave for work, which I was able to do around 7:40 am.

While I THINK my peak usage fee is calculated based on the max hourly usage I have each month, and keeping my usage from having peaks is beneficial for the grid, I also understand that my usage here at home is nothing in comparison to whatever is going on at the fast DC chargers. Maybe the fast DC chargers have massive batteries they rely on to level their usage, but overall, highly utilized fast chargers or businesses using electric in the day must be putting far more strain on the grid. In fact, I put more strain on the grid in the summertime myself I believe. So my 32 amp charger is probably not going to be too much to worry about there. BUT… it does seem worth understanding where the balance is. Is it a full charge 1% to 90% in 15 minutes? Or is it the same over a 24 hour period? Depending on the weather, there’s probably some balance which is perceived as an ideal charge rate, one that doesn’t require much battery temperature management, and is also “healthiest” for the battery too.

I’m looking to learn.

As for the power portion, here is the hourly usage associate most likely with my last post.

That was 20 amps, which seemed to match my peak hourly usage which probably occurs when the double-oven is cranking and there’s maybe a load of wash going, and all the lights in the house are on sometime around dinner time. Maybe every computer in the house is fired up and plugged in too, perhaps phones charging, maybe even a dishwasher running. This is less than summertime when the AC might be running and maybe a pool pump too. (Pool pump may need to be balanced with car charging run at night too.)

Since 20 amps isn’t enough in freezing temps (frost on the ground this morning and lows around 25-28 this week) to get the car from 39-100%, I am going to want to find the ideal times to plug it in. I know Tesla recommends keeping it plugged in all the time possible. We may end up doing that, but until we have a charger out front of our house, I typically only back it all the way in the driveway when I decide to charge. So, do I charge any time it drops below 50% and keep my 20 amp setting? Or let it get lower and bump up the amps? I don’t think with the current setup we will be charging every day. My wife is really bad at driving backwards, and I back the car in when it needs charging.

I will figure out the optimal settings some day. I really think I will.

 

[mknmike]

So with the post above, you can see that at 20 amps I can keep the consumption rate below 6 KWh/h (my approximate winter usage limit) but when I bump it up to 32 amps you can see in this picture for my usage Wednesday, there’s a clear peak that is higher than any of my other usage during the month. It makes sense that usage would jump to nearly 9 KWh/h. In my scenario this MIGHT cause my Peak load calculation to bump up. I don’t know if Delmarva Power / PEPCo only counts peak loads during peak usage, or if they count it even if the peak usage is off-peak. I can probably ask Delmarva.

 

[Big Earl]

Can you share the details of your electrical rate details? I'm very curious, since at my last house, I was on a complicated time-of-use plan with demand charges that I was really able to take advantage of.

I was on Dominion and they only calculated demand charges during peak times of use. You could use as much as you wanted during off-peak.

 

[mknmike]

Can you share the details of your electrical rate details? I'm very curious, since at my last house, I was on a complicated time-of-use plan with demand charges that I was really able to take advantage of.

I was on Dominion and they only calculated demand charges during peak times of use. You could use as much as you wanted during off-peak.

I researched more after posting yesterday and, while I don’t know the exact peak load times, determined that peak load charges are only calculated during the peak load times for the poco.

So it seems that my peaks after midnight are NOT contributing to increased bills. So between midnight and 7-8 am, I should charge as fast as I can to minimize battery heating. And set to have it charged and warm when departing in the morning.

That took me a while to get to a very simple conclusion that I’m sure others have told me a long time ago, but it’s good to feel like I truly understand.

So for the Jan 13 to Feb 12 period, somehow Delmarva calculated that 6.40 kw was my peak load contribution/draw. I found several peaks that were much higher during that period but outside the ~12-8 pm weekday window that I peak are the peak demand times. Having just bought the car in late November, this is really only the third bill that I’d have an opportunity to create peaks outside the pick’s peak demand times. I guess I didn’t catch the verbiage or other cases with higher peaks before.

Here’s one from that billing way above 6.4 kw, but clearly off-peak, and therefore didn’t contribute to my PLC peak load contribution.

 

[mknmike]

One other thing I “verified” is that it’s ok (for now) to use ~$0.19/KWh as my cost. Since there are so many parts to my electrical bill, some fixed, some a direct KWh charge, I believe my actual cost per kWh is less, but if you just divide my total monthly cost for electricity by the number of KWh used, it’s less than $0.19. I think summer rates will be a little higher, but I can calculate that later.

 

[mknmike]

Around 8:30 pm I think I set the max charge to 90% and set it for a departure time of 7:30 am and also put that the end of my off-peak period was 8 am. I did not set the car to precondition. I got in at like 7:31 without checking anything, and the car was at 87%. I have yet to try and download anything, but did leave the charge rate at 32 amp. I think the car had about 33% when I parked it around 8:30 pm. I had the schedule turned off, but the start time had been 12 am. When I looked at the app around 8:35 pm or so, I believe it said "parked" not charging. It did show the cable connected. Right now, my wife has it at work. So I'm not seeing all details right now.

Now seeing that I pay about $0.19/kwh or less. And I see that most full charges seem to take less than 60 kwh, That would be $11.40 per charge if that's the case. So I guess it's just not worth going to a supercharger even though it's free for the first 6 months. I figure that's my time, plus beating on the battery when I do it. I don't know if supercharging is as hard on LFP batteries or not, but when it's less than $10-11, the time becomes more of an issue.

Edit: got stats off my app even though the car is not here.

 

[Lexo]

Brand new owner here.
I just switched to charging from 50 or 55% to 75%, instead or doing 30% to 80%. I found when at 80, I would immediately lose a couple % even when not using. I will test if have that issue with 75%.
I charge at home, fixed monthly rate of $30. Turned amps down to 10 for overnight, slower charge rate is better for battery I think.
Still learning, it's only been 2 weeks!

 

[mknmike]

Brand new owner here.
I just switched to charging from 50 or 55% to 75%, instead or doing 30% to 80%. I found when at 80, I would immediately lose a couple % even when not using. I will test if have that issue with 75%.
I charge at home, fixed monthly rate of $30. Turned amps down to 10 for overnight, slower charge rate is better for battery I think.
Still learning, it's only been 2 weeks!

I was doing this but realized that the 300W used by the car being awake, and battery heating when cold makes it more efficient to charge as fast as possible.

I was worried about causing peaks in my load with higher amps and only just realized that the peaks are only measured if they are during my power company's peak usage time, typically noon-8pm. So I am now charging starting around 11 pm or midnight (nicer to see all show up on one day) by setting the scheduled departure time. I figure since we won't charge as soon as the car gets home in the evening (still at peak times) it makes sense to charge before we drive to have the charge end right before we drive.

All that said, I only have to charge once or maybe twice per week and appear to be driving about 1000 miles per month. LFP RWD 2023 base model. White interior is the only option I paid for.