RWD charging on a road trip and SOC accuracy

[galimpic]

I will take my first road trip in March from FL to CA and back (~5,000 mi) in my 2023 M3 RWD. Since it has an LFP battery, it should be sometimes charged to 100% to recalibrate the SOC estimate. However, when fast charging on road trips you should usually go from around 10% to around 80% (i.e. ~30 charges in this case) to keep the stop breaks optimal. With that many partial charge cycles, the SOC may become inaccurate when I need to rely on it to arrive to the next charger with ~10%.

My question is really if someone has taken a long trip with LFP battery and if they had an issue with this. I know the solution is to charge to 100%, but I would really like to know how inaccurate does the estimate become after several partial charging cycles.

 

[DuncanM]

I will take my first road trip in March from FL to CA and back (~5,000 mi) in my 2023 M3 RWD. Since it has an LFP battery, it should be sometimes charged to 100% to recalibrate the SOC estimate. However, when fast charging on road trips you should usually go from around 10% to around 80% (i.e. ~30 charges in this case) to keep the stop breaks optimal. With that many partial charge cycles, the SOC may become inaccurate when I need to rely on it to arrive to the next charger with ~10%.

My question is really if someone has taken a long trip with LFP battery and if they had an issue with this. I know the solution is to charge to 100%, but I would really like to know how inaccurate does the estimate become after several partial charging cycles.

We drove ~2000 miles from BC to SE Arizona, in 4 days, and it wasn't a problem. You'll probably have access to an L1 (120v outlet and your portable charger) or L2 charger and you can use that to charge to 100% at some point. However, the LFP battery will accept a very high charge rate even past 95% SOC, so you can probably charge to 100% on a SC or DCFC (the CCS1 adapter is nice to have...) and it won't take very long.

 

[galimpic]

We drove ~2000 miles from BC to SE Arizona, in 4 days, and it wasn't a problem. You'll probably have access to an L1 (120v outlet and your portable charger) or L2 charger and you can use that to charge to 100% at some point. However, the LFP battery will accept a very high charge rate even past 95% SOC, so you can probably charge to 100% on a SC or DCFC (the CCS1 adapter is nice to have...) and it won't take very long.

Thank you for your answer. I really want to avoid waiting for additional 30 or so minutes on a quick charger for it to go from 80%-100% (and as I understand sometimes superchargers won't even let you). Once I tried, it was stuck at "recalibrating" at 99% for 10-20 minutes until I gave up. I would also like to skip looking for L2 chargers since that would severely limit my overnight stay options. People may think LFP is somehow easier to manage since you don't have to worry about 100%, but I now realize it's easier to deal with the more predictable behavior of the regular battery in the LR.

 

[DuncanM]

Thank you for your answer. I really want to avoid waiting for additional 30 or so minutes on a quick charger for it to go from 80%-100% (and as I understand sometimes superchargers won't even let you). Once I tried, it was stuck at "recalibrating" at 99% for 10-20 minutes until I gave up. I would also like to skip looking for L2 chargers since that would severely limit my overnight stay options. People may think LFP is somehow easier to manage since you don't have to worry about 100%, but I now realize it's easier to deal with the more predictable behavior of the regular battery in the LR.

OK, charge to 95% at a SC/DCFC then plug into a 120v outlet at your hotel (with permission) and let it go to 100% overnight. However, it would seem likely that you could find at least one SC/DCFC where you can charge to 100% whilst shopping or having dinner. It's not like you need to charge to 100% all that often.

The last time we charged whilst shopping, I had to run out after ~30mins because the car was already at 99%, with an initial SOC of ~40%. I wasn'tworried about charging to 100% but wanted to avoid idle fees and blocking a charger.

 

[galimpic]

So, basically, nobody takes road trips with RWD without having to stay once day on a charger that would top it off to 100%? That sounds like a pretty big requirement. I hope there is an alternative, like NOT charging to 100% and still being ably to rely on the SOC within a few percent. But I have no data, since all discussions are about charging at home and how often to go to 100%.

 

[DuncanM]

So, basically, nobody takes road trips with RWD without having to stay once day on a charger that would top it off to 100%? That sounds like a pretty big requirement. I hope there is an alternative, like NOT charging to 100% and still being ably to rely on the SOC within a few percent. But I have no data, since all discussions are about charging at home and how often to go to 100%.

It wasn't a problem for us, not charging to 100%, while driving ~2000 miles over a 4 day period.

How long will you be on the road? A week or two shouldn't present a problem and if it's longer than that, it seems likely that you'll somehow find the few extra minutes needed to charge to 100% at least once.

 

[galimpic]

It wasn't a problem for us, not charging to 100%, while driving ~2000 miles over a 4 day period.

How long will you be on the road? A week or two shouldn't present a problem and if it's longer than that, it seems likely that you'll somehow find the few extra minutes needed to charge to 100% at least once.

It's gonna be 9 days trip, with estimated total 30 fast charges, but if possible all partial (to 80-85%). Not sure how large a problem that would present to the SOC (and memory effect which LFP is prone to).

 

[DuncanM]

It's gonna be 9 days trip, with estimated total 30 fast charges, but if possible all partial (to 80-85%). Not sure how large a problem that would present to the SOC (and memory effect which LFP is prone to).

I doubt it will be a problem. There's no memory effect, but the BMS can lose track of the exact LFP SOC, especially when the battery SOC falls to low levels, say under 10% or so.

 

[galimpic]

I doubt it will be a problem. There's no memory effect, but the BMS can lose track of the exact LFP SOC, especially when the battery SOC falls to low levels, say under 10% or so.

According to research, there is a memory-effect when partially charging, but it seems not to be permanently destructive like in NiCd (if the BMS recalibration when charging to 100% is really doing the proper job of balancing all the cells). There is short info here and in more depth here.

I am more worried about SOC calculation at the low percentage, since that is exactly when I need it to be correct at least enough to not let me get stranded on the road when I plan to arrive to the next station at low SOC, so not more than around 8% actual SOC than displayed.

 

[galimpic]

Also, since voltage curve is flat around medium SOC but gets steep at low and high SOC, it seems to me it won't be very inaccurate at very low changes, but it could think it is at, say, 20%, then the voltage starts to drop rapidly and it jumps to 5%, which is accurate, but now I can be in trouble if I counted on 10% more to the next charger.

 

[DuncanM]

I suspect you're over thinking this, but if it's a concern, then take the time to charge to 100% at least once during your trip. There's lots of data now about M3 RWD LFP batteries and old research papers might not be applicable to modern LFP batteries managed by a modern BMS.

During our trip, we considered using the multiple ~10 min charging stop cycle for optimum travel time, but found that 10mins is barely enough time to use the restroom. We found that fewer ~30min charging stops better allowed for meals, coffee and restroom stops. We drove from Ferndale WA to Yreka CA in ~11 hours (~600 miles), including 3 - 30min stops (timed for breakfast and lunch), and then a final ~30min charge at Yreka, as we had dinner after checking in at our hotel.

 

[galimpic]

As I said, I am not worried about the memory effect, I just wanted to put some documented response to "no battery effect" comment. We can forget about that. The question really is "how inaccurate will M3 RWD get at low SOC after repeated partial charges". I get the "just charge to 100%" solution, and while I understand it and accept it, I am more interested in your (and others') experience when you were at low SOC during the road trip - was it still accurate? Did you arrive at chargers with ~10% after not charging to 100% several times in a row without issues, or did you charge to 100% to reset it at every opportunity during the trip and THAT is the perhaps the reason it wasn't an issue for you?

 

[DuncanM]

As I said, I am not worried about the memory effect, I just wanted to put some documented response to "no battery effect" comment. We can forget about that. The question really is "how inaccurate will M3 RWD get at low SOC after repeated partial charges". I get the "just charge to 100%" solution, and while I understand it and accept it, I am more interested in your (and others') experience when you were at low SOC during the road trip - was it still accurate? Did you arrive at chargers with ~10% after not charging to 100% several times in a row without issues, or did you charge to 100% to reset it at every opportunity during the trip and THAT is the perhaps the reason it wasn't an issue for you?

If you charge and immediately drive then the BMS can accurately track consumption and give an accurate SOC estimate. It's when the car sits for a while that the BMS can lose track. I've charged to ~90% and then drove until I was down to 8% before charging again, and the resulting charge time and kws added confirmed that the BMS was accurate. Consequently I had no worries during our drive down to SE Arizona. I was careful in northern CA to not let the SOC drop too far before our first charging stop, on the 2nd morning of our trip as the temperatures fell into the mid 20sF overnight in Yreka.

 

[rudythecar]

As I said, I am not worried about the memory effect, I just wanted to put some documented response to "no battery effect" comment. We can forget about that. The question really is "how inaccurate will M3 RWD get at low SOC after repeated partial charges". I get the "just charge to 100%" solution, and while I understand it and accept it, I am more interested in your (and others') experience when you were at low SOC during the road trip - was it still accurate? Did you arrive at chargers with ~10% after not charging to 100% several times in a row without issues, or did you charge to 100% to reset it at every opportunity during the trip and THAT is the perhaps the reason it wasn't an issue for you?

Check out Bjorn's video on the model 3 lfp being driven until the battery died. I think he drove it for 50km AFTER SOC showed 0%. So there is some buffer (few kWh) that seems to get bigger as the BMS gets less and less sure about the actual SOC. So it seems that they have already designed a workaround that will make it safe for you to trust the % SOC. In other words as long as you pay attention to SOC you won't get stranded.

 

[galimpic]

Check out Bjorn's video on the model 3 lfp being driven until the battery died. I think he drove it for 50km AFTER SOC showed 0%. So there is some buffer (few kWh) that seems to get bigger as the BMS gets less and less sure about the actual SOC. So it seems that they have already designed a workaround that will make it safe for you to trust the % SOC. In other words as long as you pay attention to SOC you won't get stranded.

That would actually make sense - to err on the side of caution. In that case, the worst that can happen is you arrive at the charger with a little more than you anticipated, which is not a big deal. Thanks!

 

[Despized]

The worse I have ever worked with was arriving at a Supercharger at 1.9% battery on a 2023 Model 3 RWD. I would agree with a lot of the statements above, I have 3900 miles on it since 12/03/2022 when I picked it up. I did 1800 miles in the first 7 days of ownership in North Dakota/MN winter weather. If you are worried about the calculation of BMS for me once a week going to 100% has been pretty accurate. Though I have not gone below the 1.9% mentioned above. I have had quite a few 2% arrivals due to 140+ miles between chargers in -20F weather.

I would recommend just for the first couple points keep the battery usage/trip information up periodically to watch it and get a "feel" for the vehicle and how to maximize your efficiency. Once you get this locked down. In my experience this completely wiped away any worry with the vehicle. The first week of 1800 miles was me taking the car on routine family visit trips that I do multiple times a year to build confidence in the car beyond AbetterRoutePlanner calculations, and estimates from various YouTube videos I watched.

 

[galimpic]

One interesting note: as I am preparing for the trip and enter it in ABRP, it shows a difference for RWD vs LR of some 3 hrs of travel time and 3 more charges total, for a trip of 2,500 miles and 40-43 hrs of travel. Assuming it's done in 3 days, I will spend the same time driving, but 1 hr more charging and stop at 1 charger mode daily with RWD. I'd rather have a LR for this trip, but it's once a year so not a big deal.